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season. At this time the Mediterranean, which is com- were led, independently of each other, to form an opinion pletely shut in by land, and the Atlantic, which is bounded that the daily barometric oscillations were due to the by two great continents, show a much smaller oscillation magneto-electric influence of the sun. It admits of no than prevails over the land adjoining them, and the lines doubt, looking at the facts of the case so far as they have of equal oscillation now attain their annual maximum. On been disclosed, that the daily barometric oscillations the other hand, in January, when the sun's rays fall per originate with the sun, and that more than the sun's pendicularly over the most uniform surface, or over the influence as exerted on the diurnal march of the tempermaximum extent of ocean, the lines are almost everywhere ature and humidity of the atmosphere is concerned in parallel with the parallels of latitude. •

bringing them about. But from the facts adduced, it is Again, on advancing inland from the Atlantic, the effects equally certain that, be the originating cause what it may, of comparatively local influences are very striking, as the its effects are enormously modified by the distribution of following mean July oscillations, from places situated in land and water over the globe, by the wind, and by the lines running in different directions, show :-Dublin, 0:012; absolute and relative humidity of the atmosphere. The Oxford, 0·022; Ostend, 0·009; Brussels, 0019; Vienna, smallness of the amount of the summer oscillation from the 0-049; 'Odessa, 0·024; and Tiflis, 0.077 ; Limerick, 0·010; forenoon maximum to the afternoon minimum over the Helston, 0-007 ; Paris, 0·020; Geneva, 0·045; Turin, 0.052; North Atlantic as far south as lat. 30°, and its diminished Rome, 0-036; Palermo, 0.008; and Malta, 0.020. But the amount, as far south at least as the equator, will no doubt most remarkable illustration is the following, the places play an important part in the unravelling of this difficulty. being all situated between 38° and 42° N. lat. : San Fran- One of the most important steps that could be taken cisco, 0.068; Fort Churchhill, 0.091; Washington, 0.063; would be an extensive series of observations from such Angra do Heroisma, 0·006; Lisbon, 0·030; Campo Maior, countries as India, which offers such splendid contrasts of 0-054; Palermo, 0·008; Tiflis, 0.077; and Peking, 0.060. climate at all seasons, has a surface covered at one place

It follows from what has been stated that much which with the richest vegetation, and at others with vast stretches has been written regarding these fluctuations, and in ex- of sandy deserts, and presents extensive plateaus and sharp planation of them, does not rest on facts; and nearly every- ascending peaks—all which conditions are indispensable in thing yet requires to be done in the way of collecting data collecting the data required for the solution of this vital towards the representation and explanation of the daily oscil- problem of atmospheric physics. lations of atmospheric pressure which are, as regards two- The ancients thought that air was one of the four elethirds of the globe, perhaps, as already stated, the most ments from which all things originated, and this doctrine regular of recurring phenomena, and an explanation of continued to prevail till 1774, when Priestley discovered which cannot but throw much light on many of the more oxygen gas, and showed it to be a constituent part of air. important and difficult problems of the atmosphere. The Nitrogen, the other constituent of air, first called azote, was data chiefly required are— barometric data from which the discovered soon after, and the marked differences between amplitude of the four daily oscillations can be represented these two gases could not fail to strike the most careless obin their distribution and times of 'occurrence for each of server. It is remarkable that Scheele independently disthe months; temperature data, comparable inter se, from covered both oxygen and nitrogen, and was the first to which the diurnal march of temperature for each month enunciate the opinion that air consists essentially of a can be ascertained; hygrometric data for hourly values; rain mixture of these two gases. From experiments made by data also for the hours; wind observations conducted on a him to ascertain their relative volumes he concluded that satisfactory and uniform plan; together with magnetic and the proportions are 27 volumes of oxygen and 73 volumes electrical observations. It is singularly unfortunate that of nitrogen. It was left to Cavendish to show from 500 the disposition of meteorologists of recent years has been analyses that the relative proportions were practically conto recommend as hours of observations for places which ob- stant, and that the proportion is 20·833 per cent. of oxygen. serve only twice or thrice daily, hours which do not cor- The results obtained by Cavendish, though not attended to respond with the times when the great barometric and for many years after they were published, have been shown thermometric daily phases occur; hence these phases can- by recent and more refined analyses to be wonderfully exact. not be noted except at the great observatories, which are The most recent analyses of specimens of air collected too few and far apart to give sufficient data for the proper under circumstances which ensure that it is of average discussion of many of those questions.

purity, give as a mean result the following :Since the two maxima of daily pressure occur when the temperature is about the mean of the day, and the two Oxygen

20.96 per cent. minima when it is at its highest and lowest respectively, Nitrogen

79.00 there is thus suggested a connection between the daily bar

Carbonic acid.... ometric oscillations and the daily march of temperature;

100.00 and similarly a connection with the daily march of the The circumstances under which these proportions vary, amount of vapor and humidity of the air. The view en- and the other gases and substances which are found in the tertained by many of the causes of the daily oscillations may air, will be afterwards adverted to. be thus stated :-The forenoon maximum is conceived to be Besides these three constituents of air, there is a fourth, due to the rapidly increasing temperature, and the rapid viz., the vapor of water, from which no air, even at the evaporation owing to the great dryness of the air at this lowest temperatures yet observed, is wholly free, so that time of the day, and to the increased elasticity of the low- absolutely dry air does not exist in the free atmosphere. ermost stratum of air which results therefrom, until a The dry air of the atmosphere-oxygen (inclusive of ozone), steady ascending current has set in. As the day advances, nitrogen, and carbonic acid—is always a gas, and its quanthe vapor becomes more equally diffused upwards through tity is constant from year to year; but the vapor of water the air, an ascending current, more or less strong and does not always remain in the gaseous state, and the quansteady, is set in motion, a diminution of elasticity follows, tity present in the atmosphere is, by the processes of evapand the pressure falls to the afternoon minimum. From oration and condensation, varying every instant. Water this point the temperature declines, a system of descend- evaporates at all temperatures, even the lowest, and rises ing currents set in, and the air of the lowermost stratum into the air in the form of an invisible elastic gas called approaches more nearly the point of saturation, and from aqueous vapor. The elasticity, of vapor varies with the the increased elasticity, the pressure rises to the evening temperature. At 0° Fahr. it'is capable of sustaining a maximum. As the deposition of dew proceeds, and the fall pressure equal to 0:044 inch of the mercurial barometer, as of temperature and consequent downward movement of calculated from Regnault's experiments; at 32° (freezing), the air are arrested, the elasticity is again diminished, and 0.181 inch; at 60°, 0.518 inch; at 80°, 1:023; and at 100°, pressure falls to the morning minimum. Since the view 1.918 inch, being nearly is the average pressure of the propounded some years ago, that if the elastic force of atmosphere. vapor be subtracted from the whole pressure, what remains In investigating the hygrometry of the atmosphere, the will show only one daily maximum and minimum, has not chief points to be ascertained are-(1), the temperature of been confirmed by observation, it follows that the above the air; (2), the dew-point; (3), the elastic force of vapor, explanation is quite insufficient to account for the phenom- or the amount of barometric pressure due to the vapor ena; indeed, the view can be regarded in no other light present; (4), the quantity of vapor in, say, a cubic foot of chan simply as a tentative hypothesis.

air; (5), the additional vapor required to saturate a cubio Singnlarly enough, Lamont and Broun, a few years ago, I foot of air; (6), the relative humidity; and (7), the weight

Volume.

0.04

b х

of a cubic foot of air at the pressure at the time of obser- descending currents increase in temperature, and conse vation. The vapor of the atmosphere is observed by means quently reduce their relative humidity, it follows that, of the hygrometer (see HYGROMETER), of which it is only over a region from which ascending currents rise, solar and necessary here to refer to Regnault's as the most exact, and terrestrial radiation is very considerably obstructed, but Augusts as the most convenient, and, consequently, the over a region upon which currents descend, radiation is one in most general use. August's hygrometer consists of a much less obstructed. Most of our exceptionally hot dry and a wet bulb, with which are observed the tempera- summer and cold winter weather is to be explained in this ture of the air and the temperature of evaporation. Of way, on which occasions there is generally observed a high these two observed data, the formula of reduction, as barometric pressure overspreading a comparatively limited deduced from Apjohn's investigations, is as follows:-Let region, on which a slow downward movement of the air F be the elastic force of saturated vapor at the dew-point, proceeds. s the elastic force at the temperature of evaporation, d the Of the solar heat which reaches the surface of the difference between the dry and wet bulb, and b the baro- globe, that part which falls on the land may be regarded metric pressure, then

as wholly absorbed by the thin superficial layer exposed d F=f

to the heating rays; and since there is no mobility in the 88 30

particles of the land, the heat can be communicated

downwards only by conduction. On the other hand, the when the reading of the wet bulb is above 32°; and

solar heat which falls on water is not, as in the case of d 6

land, arrested at the surface, but penetrates to a considerх

able depth, the heating effect being in the case of clear

water appreciably felt at a depth of from 500 to 600 feet. when below it. From Regnault's values of the elastic force Since the heat daily received by the ocean from the sun is of vapor, f is found, and d and b being observed, F is diffused downwards through a very considerable depth, calculated. From F the dew-point is found. In calcu- the surface of the ocean on which the atmosphere rests is lating relative humidity, saturation is usually assumed to much less heated during the day than is the surface of the be 100, perfectly dry air 0. The humidity is found by land. Similarly it is also less cooled during the night dividing the elastic force at the dew-point by the elastic by terrestrial radiation. force at the temperature of the air, and multiplying the This points to a chief acting force on which the great quotient by 100.

movements of the atmosphere depend, viz., simultaneous The elastic force may be regarded as representing local irregularities in the distribution of temperature in approximately the absolute quantity of vapor suspended the atmosphere. The local expansion of the atmosphere in the air. It may be termed the absolute humidity of by heat during the day is greatest over land, where the air the atmosphere. Since the chief disturbing influences at is clear, dry, and comparatively calm, and least over the work in the atmosphere are the forces called into play by ocean, where the sky is clouded, and the air loaded with its aqueous vapor, a knowledge of the geographical distri- moisture. On the other hand, the local contraction by bution of this constituent through the months of the year cold during night is greatest over land, where the air is is of the utmost possible importance. Hence every effort clear, dry, and calm, or nearly so, and least over the ought to be made to place the observation of the hygrometry ocean, where the air is clouded, and loaded with moisture. of the air, and the reduction of the observed data, on a As familiar illustrations of atmospheric movements resultsounder basis than has yet been done. As regards geograph- ing from local expansions by heat and contractions by ical distribution, the elastic force is greatest within the cold, we may refer to the land and sea breezes, and what tropics, and diminishes towards the poles: it is greater over depend upon exactly the same principle, the dry and rainy the ocean, and decreases on advancing inland; greater in monsoons in different parts of the globe. But the illustrasummer than in winter; and greater at midday than in the tion of the principle on the broadest scale is the system of morning. It diminishes with the height generally; but in atmospheric circulation known as the equatorial and polar particular cases, different strata are superimposed on each currents of the atmosphere, which originate in the unother, differing widely as regards dryness and humidity, equal heating by the sun of the equatorial, temperate, and and the transitions from the one to the other are often polar regions. sharp and sudden.

The other principal motive force in atmospheric circuThe relative humidity of the air may be regarded as the lation depends on the aqueous vapor. The many ways degree of approach to saturation. It is greatest near the in which this element acts as a motive force will be seen surface of the earth during night, when the temperature, when it is considered that a large quantity of sensible being at or near the daily minimum, approaches the dew- heat disappears in the process of evaporation, and reappoint; it is also great in the morning, when the sun's rays pears in the process of condensation of the vapor into rain have evaporated the dew, and the vapor is as yet only or cloud; that saturated air is specifically lighter than dry diffused a little way upwards; and it is least during the air; and that the absolute and relative amount of the greatest heat of the day.

vapor powerfully influences both solar and terrestrial Between the humidity, both absolute and relative, of the radiation. The question to be carefully considered here air and the temperature there is a vital and all-important is, how in these ways the vapor produces local irregularconnection. Observation shows that when the quantity of ities in the distribution of atmospheric pressure, thus vapor in the air is great, and also when the relative giving rise to aërial movements which set in to restore the humidity is high, temperature falls little during the night, equilibrium that has thus been disturbed. even though the sky be perfectly clear; but when the It is from these local irregularities-using the word quantity of vapor is small

, or the relative humidity is low, local in a very wide sense in the distribution of atmotemperature rapidly falls. On the other hand, during the spheric pressure, whether the irregularities originate in the day the temperature rises slowly, when the quantity of temperature or aqueous vapor, that all winds, from the vapor is great, or relative humidity high, even though the lightest breeze to the most destructive hurricane, take their sky be clear, but when the quantity of vapor is small, and rise; for, as already stated, wind is merely the flowing humidity low, temperature rapidly rises. These facts are away of the air from where there is a surplus of it to explained by the circumstance that perfectly dry air is where there is a deficiency, diathermanous, that is, it allows radiant heat to pass through In examining weather charts embracing a considerable it without being sensibly warmed thereby: Add vapor portion of the earth's surface, such, for instance, as those to this air, and its diathermancy is diminished. The dia- published in the Journal of the Scottish Meteorological thermancy is also reduced if the temperature approach Society, vol. ii. p. 198, which include a large part of the nearer to the dew-point; in other words, if the relative northern hemisphere, there are seen two different systems humidity be increased. Hence, with an increase of vapor of pressure changing their forms and positions on the globe or with increased humidity, the effects of both solar and from day to day-one set being systems of low pressure terrestrial radiation are much less felt on the surface of the marked off by concentric isobarics enclosing pressures earth-the vapor screen performing, in truth, one of the successively lower as the central space is approached, and most important conservative functions of the atmosphere. the other set being systems of high pressure marked off by

Since ascending currents fall in temperature as they roughly concentric isobarics bounding pressures succesascend, through diminished pressure and consequent dila- sively higher towards their centres. These two systems tation, they increase their relative humidity; and since I are essentially distinct from each other, and without some

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knowledge of them the circulation of the atmosphere can- | in the space covered for the time by it being, on account not be understood.

of its dryness and clearness, more fully under the influence 1. Areas of Low Pressure, or Cyclones. The annexed of both solar and terrestrial radiation; and consequently woodcut, fig. 1, is a good representation of a cyclone in winter it is accompanied with great cold, and in summer which passed over north-western Europe on the morning with great heat. As shown by Buchan, in reviewing the of 2d November, 1863. The pressure in the central space weather of north-western Europe for 1868, the intense is 28-9 inches, from which it rises successively, as shown heat which prevailed in Great Britain during 2–4th August by the isobarics, to 29·1, 29-3, 29-5, 29-7, and 29-9 inches. of that year was due to the high barometric pressure accom

panying this anticyclone, the comparative calmness of the atmosphere, the clearness of the sky, the dryness of the air, and the strong insolation which took place under these circumstances.

Thus, then, the tendency of the winds on the surface of the earth is to blow round and in upon the space where pressures are low and out of the space where pressures are high. Now, since vast volumes of air are in this way poured into the space where pressure is low, without increasing that pressure, and, on the other hand, vast volumes flow out of the space where pressure is high, without diminishing that pressure, it necessarily follows that the air poured in is not allowed to accumulate over this space, but must escape into other regions; and also that the air which flows out from the anticyclonic region must have its place supplied by fresh accessions from above. In other words, the central space of the cyclone is occupied by a vast ascending current, which after rising to a considerable height flows away as upper currents into surrounding regions; and the central space of the anticyclone is filled by a slowly descending current, which is fed from upper currents, blowing towards it from neighboring regions.

When the area of observation is made sufficiently wide, Fig. 1.–Weather chart, showing cyclone.

cyclones are seen to have one, or sometimes more, anti

cyclones in proximity to them, the better marked antiThe arrows show the direction and force of the wind, the cyclones having two, and sometimes more, cyclones in their force rising with the number of feathers on the arrows. vicinity. In fig; 2, a part of a cyclone in Iceland is seen, The two chief points to be noted are the following :-(1.) and another cyclone in the Crimea accompanied the antiThe direction of the arrows shows a vorticose motion of cyclone there figured. Hence the cyclone and the anticythe air inwards upon the space of lowest pressure, the clone are properly to be regarded as counterparts

, belonging motion being contrary to that of the hands of a watch. It I to one and the same great atmospheric disturbance. will be observed that the winds blow in conformity with what is known as Buys-Ballot's “Law of the Winds," already referred to, but which may be otherwise thus put:-Stand with your back to the wind, and the lowest barometer, or centre of depression, will be to your left in the northern hemisphere (in the southern hemisphere to the right); this rule holds universally. (2.) The force of the wind is proportional to the barometric gra

29.8 dient, or the quotient of the distance between two places stated in miles by the difference of pressure stated in inches of mercury as observed at the two places. Hence in the Channel, where the isobarics are close together, winds are high, but in the north of Scotland, where the isobarics are far apart, winds are light. This rule also 299 holds universally, though the exact relation requires still to be worked out by observation. As regards the important climatic elements of temperature and moisture, the air in the S.S.E. half of the cyclone is mild and humid, and much rain falls; but in the other half it is cold and dry, and little rain falls. A succession of low pressures passing eastward, in a course lying to northwards of Great Britain, is the characteristic of an open winter in Great Britain; on the other hand, if the cyclones follow a course lying to the southward, the winters are severe. This is a chief point of climatic importance connected with the propagation eastward of these cyclonic areas.

2. Areas of High Pressures, or Anticyclones.—The accompanying weather chart, fig. 2, for 2-4th August, 1868, represents an anticyclone or region of high pressure, which overspread the greater part of Europe at that

30.1 time. Here the highest pressure is in the centre of the system, and, as usually happens, the isobarics are less symmetrical than those near the centre of a cyclone. The winds, as usual in anticyclones, are light; this, however, is the essential point of difference the winds do not flow inwards upon the centre, but outwards from the re

Fig. 2.-Weather chart, showing anticyclope. gion of high pressure; and it will be observed that in many cases they cut the isobarics at nearly right angles. From this it follows that observations of the winds Another important point of difference is in the air over the cannot be conducted, and the results discussed, on the region covered by the anticyclone being, particularly in its supposition that the general movement of the winds felt on centrai portion, very dry, and either clear or nearly free the earth's surface is horizontal, it being evident that the from clouds.

circulation of the atmosphere is effected largely through Climatically, the significance of the anticyclone consists 1 Allas Meléorologique de l'Observatoire Impérial, Année 1868. D. 39.

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systems of ascending and descending currents. The only of unwholesome places, as first shown by Regnault. The satisfactory way of discussing the winds, viewed especially following figures, showing the volume per cent. of oxygen, in their climatic relations, is that recently proposed by rest on the authority of Dr. Angus Smith, who has given Köppeu of St. Petersburg, and applied by him with very much attention to this subject :-Sea-shore of Scotland and fruitful results in investigating the weather of that place Atlantic (lat. 43° 5' N., long. 17° 12' W.), 20-99; tops of during 1872 and 1873. In attempting an explanation of Scottish hills, 20-98; in sitting-room feeling close but not these phenomena, we are met with several as yet insuper- excessively so, 20:89; backs of houses and closets, 20-70; able obstacles :-(1.) An imperfect knowledge of the mode under shafts in metalliferous mines, 20:424; when candles of formation and propagation of low-pressure systems; (2.) go out, 18.50; when it is very difficult to remain in the air Imperfect knowledge of the relations of the formation of many minutes, 17.20. The variations in the amounts of cloud and aqueous precipitation to barometric fluctuations; carbonic acid in different situations are great; thus—in the (3.) A want of information with reference to the merely London parks it is .0301; on the Thames, 0343; where mechanical effects of ascending, descending, and horizontal fields begin, 0369; in London streets in summer, 0380, currents of air on the barometric pressure; in other words, during fogs in Manchester, .0679; in workshops it rises to we do not know how far the barometric pressure is an in- :3000, and in the worst parts of theatres to :3200; and the dication of the mass of air in the column vertically over largest amount, found in Cornwall mines, is 2-5000. it, when that column is traversed by air-currents; (4.) An Great differences have been observed by Dr. A. Smith almost total absence of really good wind observations; and between country rain and town rain: country rain is neu(5.) Deficient information in nearly everything that re- tral; town rain, on the other hand, is acid, and corrodes spects aqueous vapor--its relation to radiant heat, both metals and even stones and bricks, destroying mortar rapsolar and terrestrial; its mode of diffusion vertically and idly, and readily spoiling many colors. Much information horizontally in the free atmosphere, especially from an has been obtained regarding impurities in the air of towns evaporating surface; the influence which its condensation and other places by examining the rain collected in differinto cloud and rain exerts on aërial currents,-in regard to ent places. The air freest from impurities is that collected all which more satisfactory methods of observing this vital at the sea-coast and at considerable heights. Again, amelement, and discussing the results of observation, are monia is found to diminish, while nitric acid increases, in greatly to be desired. There are here large important ascending to, at least, habitable heights. As regards orfields of inquiry awaiting experimental and observational ganic matter in the air, it corresponds to a considerable physicists.

extent with the density of the population. As might have The law of the dilatation of gases, known as the “ Law been supposed from the higher temperature, more nitric of Boyle" or "Law of Mariotte," is this: The volume oc- acid is contained in rain collected on the Continent than in cupied by a gas is in inverse ratio to the pressure under the British Islands. This inquiry, which is only yet in its which it exists, if the temperature remains the same; or infancy, will doubtless continue to be vigorously prose the density of a gas is proportioned to its pressure. Con- cuted, particularly since we may hope thereby to arrive at sequently, air under a pressure equal to that of two atmo- the means of authoritatively defining the safe limits of the spheres will occupy only half the volume it occupied under density of population, and the extent to which manufacthe pressure of one atmosphere; under the pressure of tures may be carried on within a given area. The influthree atmospheres, one-third of that volume, &c. By doub- ence of atmospheric impurities on the public health has ling the pressure we double the elasticity. If, however, the received a good deal of attention. temperature be increased, and the air occupy the same The relation of weather to mortality is a very important space, the pressure will be increased; but if the pressure inquiry, and though a good deal has been known regarding is to remain the same, the air must occupy a larger the question for some time, yet it has only recently been space. From Regnault's experiments, it is concluded that systematically inquired into by Dr. Arthur Mitchell and the co-efficient which denotes increase of elasticity for 1° Mr. Buchan, the results of the investigation which deals Fahr. of air whose volume is constant equals .002036; with the mortality of London being published in the Jourand that the co-efficient which denotes increase of vol- nal of the Scottish Meteorological Society (New Series, Nos. ume for 1° Fahr. of air whose elasticity is constant equals 43 to 46). Considering the weather of the year as made 002039.

up of several distinct climates differing from each other Those portions of the atmosphere in contact with the according to temperature and moisture and their relations earth are pressed upon by all the air above them. The air to each other, it may be divided into six distinct climates, at the top of a mountain is pressed upon by all the air characterized respectively by cold, cold with dryness, dry. above it, while all the portion below it, or lying between ness with heat, heat, heat with moisture, and cold with the top of the mountain and the surface of the sea, exerts moisture. Each of these six periods has a peculiar influno pressure whatever upon it. Thus the pressure of the ence in increasing or diminishing the mortality, and each atmosphere constantly diminishes with the height. If, has its own group of diseases which rise to the maximum, then, the pressure of the atmosphere at two heights be or fall to the minimum mortality, or are subject to a rapid observed, and if at the same time the mean temperature increase or a rapid decrease. The mortality from all and humidity of the whole stratum of air lying between causes and at all ages shows a large excess above the the two levels were known, the difference in height be average from the middle of November to the middle of tween the two places could be calculated. For the devel. April, from which it falls to the minimum in the end of opment of this principle, see BAROMETRIC MEASURE- May; it then slowly rises, and on the third week of July MENTS OF HEIGHTS, p. 332.

suddenly shoots up almost as high as the winter maximum The air thus diminishing in density as we ascend, if it of the year, at which it remains till the second week of consists of ultimate atoms, as is no doubt the case, it follows August, falling thence as rapidly as it rose to a secondary that the limit of the atmosphere will be reached at the minimum in October. Regarding the summer excess, height where the force of gravity downwards upon a single which is so abrupt in its rise and falì

, it is almost altogether particle is equal to the resisting force arising from the re- due to the enormous increase of the mortality among mere pulsive force of the particles. It was long supposed, from infants under one year of age; and this increase is due the results of observations on the refraction of light, that not only to deaths at one age, but to deaths from one the height of the atmosphere did not exceed 45 miles; but class of diseases, viz., bowel complaints. If the deaths from the observations of luminous meteors, whose true from bowel complaints be deducted from the deaths from character as cosmical bodies was established a few years all causes, there remains an excess of deaths in the cold ago, it is inferred that the height of the atmosphere is at months, and a deficiency in the warm months. In other least 120 miles, and that, in an extremely attenuated form, words, the curve of mortality is regulated by the large it may even reach 200 miles.

number of deaths from diseases of the respiratory organs. Though there are considerable differences in the specific The curve of mortality for London, if mere infants be gravities of the four constituent gases of the atmosphere, excepted, has thus an inverse relation to the temperature, viz., oxygen, nitrogen, carbonic acid gas, and aqueous va- rising as the temperature falls, and falling as the temperapor, there is yet no tendency to separation among them, ture rises. On the other hand, in Victoria, Australia, owing to the law of diffusion obtaining among elastic fluids where the summers are hotter and the winters milder, the mixed together. While the proportion of these gases is in curves of mortality and temperature are directly related to a general sense constant, there are, however, consistent dif- each other-mortality and temperature rising and falling ferences in the amounts of oxygen and nitrogen in the air together; the reason being that in Victoria deaths from

bowel complaints are much greater, and those from diseases that these smaller drops can be divided again, and the of the respiratory organs much less than in London.

theory goes on to assert that there is nothing in the nature The curves show that the maximum annual mortality of things to hinder this process of division from being from the different diseases groups around certain specific repeated over and over again, times without end. This is conditions of temperature and moisture combined. Thus, the doctrine of the infinite divisibility of bodies, and it is cold and moist weather is accompanied with a high death- in direct contradiction with the theory of atoms. rate from rheumatism, heart diseases, diphtheria, and The atomists assert that after a certain number of such measles; cold weather, with a high death-rate from bron- divisions the parts would be no longer divisible, because chitis, pneumonia, etc.; cold and dry weather, with a high each of them would be an atom. The advocates of the death-rate from brain diseases, whooping-cough, convul- continuity of matter assert that the smallest conceivable sions; warm and dry weather, with a high death-rate from body has parts, and that whatever has parts may be disuicide and small-pox; hot weather, with a high death-rate vided. from bowel complaints; and warm moist weather, with a In ancient times Democritus was the founder of the high death-rate from scarlet and typhoid fevers. (See atomic theory, while Anaxagoras propounded that of conCLIMATE.)

(A. B.) tinuity, under the name of the doctrine of homæomeria ATMOSPHERIC RAILWAY, a railway in which the ('Opoiouépia), or of the similarity of the parts of a body to pressure of air is used directly or indirectly to propel car- the whole. The arguments of the atomists, and their riages, as a substitute for steam. It was devised at a time replies to the objections of Anaxagoras, are to be found in when the principles of propulsion were not so well under- Lucretius. stood as they are now, and when the dangers and inconve- In modern times the study of nature has brought to niences attendant on the use of locomotives were very much light many properties of bodies which appear to depend on exaggerated. It had been long known that small objects the magnitude and motions of their ultimate constituents, could be propelled for great distances through tubes by air and the question of the existence of atoms has once more pressure, but a Mr. Vallance, of Brighton, seems to have become conspicuous among scientific inquiries. been the first to propose the application of this system to We shall begin by stating the opposing doctrines of atoms passenger traffic. He projected (about 1825) an atmo- and of continuity before giving an outline of the state of spheric railway, consisting of a wooden tube about 6 feet molecular science as it now exists. In the earliest times 6 inches in diameter, with a carriage running inside it. A the most ancient philosophers whose speculations are diaphragm fitting the tube, approximately air-tight, was known to us seem to have discussed the ideas of number attached to the carriage, and the air. exhausted from the and of continuous magnitude, of space and time, of matter front of it by a stationary engine, so that the atmospheric and motion, with a native power of thought which has pressure behind drove the carriage forward. Later invent- probably never been surpassed. Their actual knowledge, ors, commencing with Henry Pinkus (1835), for the most however, and their scientific experience were necessarily part kept the carriages altogether outside the tube, and limited, because in their days the records of human thought connected them by a bar with a piston working inside it, were only beginning to accumulate. It is probable that this piston being moved by atmospheric pressure in the the first exact notions of quantity were founded on the conway just mentioned. The tube was generally provided sideration of number. It is by the help of numbers that with a slot upon its upper side, closed by a continuous concrete quantities are practically measured and calculated. valve or its equivalent, and arrangements were made by Now, number is discontinuous. We pass from one number which this valve should be opened to allow the passage of to the next per saltum. The magnitudes, on the other the driving bar without permitting great leakage of air. hand, which we meet with in geometry, are essentially About 1840, Messrs. Clegg & Samuda made various experi- continuous. The attempt to apply numerical methods to the ments with an atmospheric tube constructed on this prin- comparison of geometrical quantities led to the doctrine ciple upon a portion of the West London Railway, near of incommensurables, and to that of the infinite divisibility Wormwood Scrubs. The apparent success of these in- of space. Meanwhile, the same considerations had not duced the Dublin and Kingstown Railway to adopt Clegg been applied to time, so that in the days of Zeno of Elea & Samuda's scheme upon an extension of their line from time was still regarded as made up of a finite number of Kingstown to Dalkey, where it was in operation in 1844. “moments,” while space was confessed to be divisible withLater on, the same system was adopted on a part of the out limit. This was the state of opinion when the celeSouth Devon line and in several other places, and during brated arguments against the possibility of motion, of the years 1844-1846 the English and French patent rec- which that of Achilles and the tortoise is a specimen, ords show a very large number of more or less practicable were propounded by Zeno, and such, apparently, continued and ingenious schemes for the tubes, valves, and driving to be the state of opinion till Aristotle pointed out that gear of atmospheric railways. The atmospheric system time is divisible without limit, in precisely the same sense was nowhere permanently successful, but in all cases was that space is. And the slowness of the development of eventually superseded by locomotives, the last atmospheric scientific ideas may be estimated from the fact that Bayle line being probably that at St. Germains, which was worked does not see any force in this statement of Aristotle, but until 1862. Apart from difficulties in connection with the continues to admire the paradox of Zeno. (Bayle's Dicworking of the valve, the maintenance of the vacuum, &c., tionary, art. “ Zeno.") Thus the direction of true scientific other great practical difficulties, which had not been indi- progress was for many ages towards the recognition of the cated by the experiments, soon made themselves known infinite divisibility of space and time. in the working of the lines. Above all, it was found that It was easy to attempt to apply similar arguments to stationary engines, whether hauling a rope or exhausting a matter. If matter is extended and fills space, the same tube, could never work a railway with anything like the mental operation by which we recognize the divisibility of economy or the convenience of locomotives, a point which space may be applied, in imagination at least, to the matter is now regarded as settled by engineers, but which was not which occupies space. From this point of view the atomic 80 thoroughly understood thirty years ago. Lately, the doctrine might be regarded as a relic of the old numerical principle of the atmospheric railway has been applied on a way of conceiving magnitude, and the opposite doctrine of very large scale in London and elsewhere, under the name the infinite divisibility of matter might appear for a time of " PNEUMATIC DISPATCH” (q.v.), to the transmission of the most scientific. The atomists, on the other hand, small parcels in connection with postal and telegraph work, asserted very strongly the distinction between matter and for which purpose it has proved admirably adapted. (See space. The atoms, they said, do not fill up the universe; paper by Prof. Sternberg of Carlsruhe in Hensinger von there are void spaces between them. If it were not so, Waldegg's Handbuch für specielle Eisenbahntechnik, vol. i. Lucretius tells us, there could be no motion, for the atom pt. 2, cap. xvii.)

which gives way first must have some empty place to ATOM (årouoc) is a body which cannot be cut in two. move into. The atomic theory is a theory of the constitution of bodies, which asserts that they are made up of atoms. The op

Quapropter locus est intactus, inane, vacansque

Quod si non esset, nulla ratione moveri posite theory is that of the homogeneity and continuity of

Res possent; namque, officium quod corporis exstat, bodies, and asserts, at least in the case of bodies having no

Officere atque obstare, id in omni tempore adesset apparent organization, such, for instance, as water, that as

Omnibus : haud igitur quicquam procedere posset, we can divide a drop of water into two parts which are Principium quoniam cedendi nulla daret res.” each of them drops of water, so we have reason to believe

-De Rerum Natura, i. 335. Vol. III.-99

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