In the asthenic form of dysentery, the tormina, tenesmus, and mucous and bloody stools are attended with great depression of all the organic functions, and extreme prostration of strength. The local dysenteric symptoms, exceedingly urgent from the commencement, are rapidly followed by fever of a low nervous or typhoid type. This form of the disease often prevails as an epidemic; and under circumstances favourable to their accumulation and concentration, exhalations from the stools of the sick seem capable of producing dysentery in persons directly exposed to them, previously in a state of sound health. Dr. Aitken, who saw this disease in the Crimea, says, "It is the mucous membrane of the great intestine, and especially of the rectum and lower portion of the colon, which is the seat of the characteristic lesions in dysentery." These lesions are frequently attended with an exudative process, which involves the whole mucous membrane, although in a number of cases only the solitary and tubular glands are involved. The following are the morbid conditions of the intestine observed by Dr. Aitken. 1. Forms of exudation obvious on the surface of the mucous membrane of the rectum and colon. 2. Forms of exudation not obvious to the unaided eye, but which were seen in all the cases examined by the microscope, to fill the mucous tubular follicles of the large intestine. 3. Forms of exudation obvious to the eye and demonstrable by microscopic examination, as being developed in the solitary vesicular glands of the large intestine. 4. Changes in the exuded material which tend first towards its organisation and subsequently to its destruction and removal by ulceration. 5. Ulcerative changes in the tissues of the mucous membrane itself. 6. Similar dysenteric lesions extending into the small intestines." In chronic cases the exuded matter is either thrown off or it becomes organised, and subject to the ulcerative process in the same manner as the mucous membrane. In all cases the solitary and tubular glands appear to be first attacked. The duration of dysentery is as various as its types. It may prove fatal in a few days or hours, or last for weeks and even months, and ultimately destroy life by inflammation and gangrene of the bowels. In some cases the disease ceases spontaneously, the frequency of the stools, the griping and the tenesmus gradually diminishing, while natural stools return; but in other cases, the disease, with moderate symptoms, continues long, and ends in protracted and exhausting diarrhoea. The causes which predispose to dysentery appear to be long-continued exposure to a high temperature, or alternations of heat and cold; hence the disease is generally most prevalent in summer or autumn, after considerable heats have prevailed for some time, and especially after very warm and at the same time very dry states of the weather. All observation and experience show that a powerful predisposition to the disease is formed by the habitual use of a high and stimulating diet, and especially by indulgence in spirituous liquors, by excessive fatigue, by exposure to night air, by salt diet, and by all causes which enfeeble the constitution in general, at the same time that they over excite the alimentary canal in particular. The exciting causes are long-continued exposure to intense heat, or to sudden and great alternations from heat to cold; exhalations from vegetable and animal matters in a state of decomposition, as from marsh, stagnant, river or sea water, from animalculæ and minute insects, or from the flesh of deceased animals; noxious exhalations from the bodies of persons crowded together in close and confined situations, and more especially, as would appear, from the discharges from the bowels of persons labouring under dysentery; scanty and bad food, consisting more especially of vegetable or animal matter in a state of decay, as tainted meat, stale fish, unwholesome bread, unripe rice, rye, and impure water. DYSLYSINE. [CHOLIC ACID.] DYSLYTE. A nitrogenous product not yet analysed, obtained by the action of nitric acid upon citraconic acid. It is produced simultaneously with eulyte. DYSPEPSIA (Avoπevía, dyspepsia), Indigestion, the difficult and imperfect conversion of the food into nutriment. Digestion is a part of the great function of nutrition; its ultimate object is to convert the aliment into blood. Between the articles taken as food and the nutrient fluid of the body-the blood, there is no obvious analogy, and there is a wide difference in nature. Hence the function of digestion consists of a succession of stages, at each of which the food undergoes a specific change. Each change is effected by a peculiar process, for the accomplishment of which a special apparatus is provided. Of these processes the chief are mastication, deglutition, chymification, chylification, and fæcation. [DIGESTION, in NAT. HIST. DIV.] The healthy condition and the natural action of every individual organ belonging to the portion of the apparatus proper to each of these processes is necessary to the sound state of the function of digestion. It is easy therefore to see by how many causes it may be disturbed; in how many different organs the source of the disturbance may have its seat, of how varied a nature the disturbance may be, and how greatly the disturbance of the digestive function may derange the other functions of the body. In the history of the human family there is no known community of human beings in any country, and no age of human life, in which the first necessity of existence, that of taking food for the nourishment of the body, is not the cause of disease and death to great numbers, and of uneasiness, nay, sometimes even of intense pain, to far greater numbers. Why is this? Why is the digestive process more productive of suffering, disease, and death in man than in the lower animals of a similar structure, in which the function, considered in a physiological point of view, is scarcely at all less complex? The correct answer to this question would include a clear account of the causes of dyspepsia, and would suggest the appropriate remedies for the disease. Digestion being an organic function, when this function is healthfully performed, for reasons which have been fully developed, it is unattended with consciousness. The first effect of the disturbance of this function is to render the patient not only conscious, but painfully conscious, that he has a stomach. A sense of nausea, sometimes, when the affection is severe, even vomiting, an obscure feeling of uneasiness, fulness, distension, weight in the region of the stomach, occasionally amounting to pain, and even severe pain, flatulence, eructation, a sensation of sinking, and lastly, a loss of appetite, constitute the train of uneasy sensations which, coming on after the reception of food, indicate disordered digestion, and which take the place of the feelings of refreshment and exhilaration which result from healthy digestion. When these uneasy sensations are occasioned by a disordered state of the stomach, the disorder may consist in a derangement either of its secreting arteries, or its mucous glands, or its organic nerves, or its muscular fibres, inducing a deficient secretion of the gastric juice, a deficient secretion of mucus, a diminished or increased irritability of the muscular fibres, by which the motions of the stomach are disturbed. If the gastric juice be deficient, the first step in the digestive process cannot take place, the food cannot be dissolved; if the mucus be excessive, the contact of the gastric juice with the food may be prevented: if the muscular fibres of the stomach are torpid or too irritable, the food may be detained too long or too short a time in the stomach. The causes of dyspepsia are either those which act directly and immediately upon the stomach itself, or those which act upon the whole body or upon particular parts of it, but which still affect the stomach principally and almost solely. In the acute form of dysentery, when the fever is high, the pain Of the first kind are noxious, irritating, and indigestible substances intense, and the inflammation active, blood-letting from the arm has taken into the stomach as articles of food or drink, such as tainted been recommended, and also the local abstraction of blood by leeching meat, decayed vegetables, unripe fruit, very acid matters, ardent or cupping. The employment of purgative remedies in dysentery spirits, &c.; and even wholesome food taken too frequently or in too requires the greatest discrimination and caution. If the colon be dis- large a quantity, especially when its nature is very nutritious, as when tended with feculent matter which it cannot discharge, no remedies it consists principally of animal matter, or when a large quantity of will succeed until this accumulation is removed; if, on the contrary, nutriment is presented to the stomach in a very concentrated form, or there have been already frequent and copious discharges of feculent is rendered too stimulating by being highly seasoned; the abuse of matter, the administration of purgatives should be avoided. The fermented and spirituous liquors, which is one of the most frequent practitioner should therefore carefully examine the state of the bowels causes of dyspepsia in its severest and most fatal forms; and large with regard to their fulness or emptiness of fecal matter, and their quantities of fluids, habitually taken at too high a temperature, as very actual state in this respect can almost always be ascertained with a great hot tea, coffee, or soup. degree of certainty if due pains be taken to discover it. If there be reason to suppose that there is any accumulation of fæces, the mildest purgatives should be given, of which the best is castor oil, and this should be cautiously repeated until the irritating matter is wholly removed. Great relief is at the same time afforded to the distressing tormina and tenesmus by emollient and opiate enemas injected in very small quantities. After the subdual of the inflammatory state by blood-letting, and the evacuation of the accumulated fæces by mild purgatives, the great object is to soothe the irritated membrane by opiates, on the judicious employment of which, and the skilful combination and alternation of this class of remedies with mild purgatives and astringents, the successful treatment of ordinary dysentery mainly depends. The asthenic forms with typhoid symptoms need a guarded yet active treatment, nearly the same as that which is proper to typhus fever with abdominal affection. [FEVER.] Of the second kind, or the causes which act upon the whole body or upon particular parts and functions of it, are—want of pure air; hence the frequency of dyspepsia in large and crowded cities, and more espe cially in narrow and confined lanes and alleys, in the dirty and illventilated houses of the poor. Want of exercise: from physical inactivity all the organs of the body languish, but the stomach first and most. Intense study or close application to business too long continued, implying both want of air and want of exercise. Mental emotion, more especially the depressing passions, fear, grief, vexation, disappointment, anxiety, and hope deferred. Exposure to the influence of cold and moisture. In persons with weak stomachs and delicate skins, a cold damp day, more especially suddenly succeeding a hot day, often produces a severe attack of dyspepsia. Hence it is that dyspeptic complaints are so prevalent when cold and damp weather first sets in. Cold is a sedative to the nervous system, as heat is an excitant; and 715 DYSPEPSIA. the depressing effects of cold seem to be peculiarly manifested in the The state of dyspepsia is most frequently a state merely of dis- But dyspepsia is often the result of disease situated not in the DYSPEPSIA. stomach, but in some other organ. The stomach has been justly are to E occupies the fifth place in the Hebrew alphabet and those derived from it. The vowels, when arranged according to their physical affinity, would lie in the series i, e, a, o, u [ALPHABET], and accordingly the vowel e is frequently interchanged with its neighbours and a. It is also under special circumstancs occasionally convertible with o and u. 1. E is interchanged with i. Thus in Latin the old datives heri, mani, ruri, musai, afterwards took the forms here, mane, rure, musae; and the words magis, videris, tristis, when they appeared without an 8, were written mage, videre, triste. The same interchange appears in the declension of the adjective is, ca, id, and the conjugation of the verbs eo and queo. 2. E in Latin often corresponds to oi in French. Thus many Latin infinitives in ere reappear in French with the termination oir, as habere, debere, avoir, devoir. The Latin past imperfect has the suffix eba, which passed through the forms eva and ea to oie and oi. Thus from habebam were deduced avera, area, avoie, and lastly avois or avais. This final 8, so far as regards the first person, does not appear in the oldest forms of the French language. Other instances of the change of o into oi may be seen in the Latin adjectives and other words in ensis or esis, which in French have the suffix ois, Viennensis, Viennois; mensis, mois. 3. E Latin into ie French, as mel, bene, ped-; miel, bien, pied. 4. E into a. This is well marked in the dialects of the Greek oopin, Ionic; σopia, Doric, &c. Hence the Latins have often an a where the common dialect of the Greek had e, as unxavn, wλnyn; Lat. machina, plaga. Both forms often coexist in Latin, as tristitia- and tristitie. The a is often changed into e in Latin, if a prefix is added, particularly if two consonants follow the vowel, as factus, confectus; pars, expers; castus, incestus; ars, iners. 5. E into o. Especially in Greek, when a strong vowel follows, as Xeyw, λoyos; veμw, voμos. The Latin language prefers an o, as que, romo; NETTW, coquo; veos, novus; e, ob. This change is particularly common in words beginning with a w, or with what was pronounced as a w, the Latin v. Thus vester, verto, velo, were once written roster, vorto, voto. Even in our own language worm (vermis, Lat), and work (pyov, Gr.), are now pronounced as if written with an e. The Greek even interchanges a long o with a long e as wатηρ, àτаτWР, EÙTатwр; and so too in Latin we have Anio Anienis. 6. A short e often gives place in Latin to a short u when followed by one of the liquids 1, n, m, as in Siculus, (Gk. Zikeλos) perculsus (perce), tabula (Germ. tafel), decumus (decem), contumax (temno), funda (opevồóvn), faciundus (faciendus) euntem. EAGLE, CONSTELLATION. [AQUILA.] EAGLE, Roman Standard. The eagle, as a symbol of empire, is often seen on ancient coins and medals, and on none more frequently than on those of the Ptolemies of Egypt and the Seleucidæ of Syria. As an ensign or standard, borne upon a spear, it was used by the Persians in the time of the younger Cyrus. (Xenoph., Anab.' i. 10.) Pliny (Hist. Nat.' li. x., c. 4, edit. Hardouin, tom. i., p. 549) says that, till the time of C. Marius, the Romans used five different animals for standards, the wolf, the minotaur, the horse, the boar, and the eagle,--but that in Marius's second consulate they adopted the eagle as the sole ensign for their legions. The eagle used by the Romans as a standard was of gold or silver: the latter metal, we are told by Pliny, was most frequently used, as the more glittering, and of course more readily seen. It was borne, like the Persian eagle, on the summit of a spear, and was of the size of a pigeon, with its wings displayed. It sometimes rested upon a cross-bar on the top of the spear, and sometimes upon shields piled up. On the reverses of some of the coins of Augustus and Galba, in second brass, the legionary eagle is represented holding the thunderbolt in its talons. The small size of the eagle often contributed to its concealment, when the legion to which it belonged was defeated. The name of the legion was usually engraved upon it. Tacitus, in his 'Annals,' 1. i. 60, relates the finding of the eagle of the nineteenth legion by Germanicus, which had been lost in the massacre of Varus. Cicero (Catilin.' i., c. 24) says that Catiline had a silver eagle in his house as his titular divinity, which was also his standard in war. A Roman eagle in steel, found at Silchester, presumed to have been a legionary eagle, was exhibited to the Society of Antiquaries in 1788 by the then bishop of Carlisle. The reader will see a great deal of learning displayed upon this and the standard of the cohorts in M. Le Beau's Quatorzième Mémoire sur la Legion Romaine; Des Enseignes.' Mem. de l'Académie des Inscript. tom. xxxv., 4to, Par. 1770, pp. 277-308. The eagle has also been adopted as the standard in the modern French army. It is borne perched at rest on a small base at the top of the banner-staff. Austria bears a double-headed eagle on her banners; and Prussia and Russia have taken the eagle as their national E symbol. The United States of America use also the eagle as the national emblem, as well as for the designation of a gold coin of the value of 43s. 8d., or 10 dollars. EAGRE. [BORE.] EAR-RING; a ring hung from a hole, perforated for that purpose through the ear, sometimes set with pendant jewels, pearls, or other precious stones. The word is Anglo-Saxon, ear-hring. Ornaments of this sort, large or small, have been worn in almost all countries by women, from the earliest ages; but more rarely by the men. In ancient Egyptian paintings females are represented with large circular ear-rings; but no male figures, except of captives, wear them. In Assyrian sculpture, on the other hand, in which few native female figures occur, all the males of rank from the great king downwards wear large ear-pendants. Botta (plate 161) has figured several of them. Among Oriental nations generally the ear-ring appears always to have been a customary ornament with both sexes. Among the Greeks and Romans it was confined exclusively to the men. Homer makes Hera, when adorning herself so as to appear most fascinating in the eyes of Zeus, place in her ear-tips three-gemmed ear-rings. (Iliad,' xiv. 182.) Several of the most beautiful Greek female statues have the ears pierced for the reception of ear-rings, which were no doubt of gold or gems. In the Latin of the middle age ear-rings are termed pendentes, from the more common form of the ornaments usually attached to the ring itself. Ear-pendants have been occasionally found in British and Saxon barrows in various parts of England. EAR-TRUMPET, a curved tube employed to aid defective hearing. The rays of sound, proceeding from any source, enter the tube nearly parallel, and its inner surface is so curved that, after one or more reflections, they converge upon the membrane of the tympanum, and thus act with increased force. In early notices of acoustic instruments there is some difficulty in distinguishing such as were intended to be applied to the ear, to assist in collecting sound, from such as were employed in aid of the voice, to enable a speaker to produce articulate sounds in such a manner as to insure their transmission to a considerable distance. [SPEAKING TRUMPET.] To a certain extent, indeed, such instruments may be employed for either purpose, some speaking-trumpets being so formed that, if applied to the ear, they would act as hearing- or car-trumpets. The common ear-trumpet is a conical tube of metal, the larger end of which expands like the mouth of a trumpet, while the smaller is so shaped as to enter the ear, and conduct the vibrations of sound collected at the wide end direct to the membrane of the tympanum. The smaller end is frequently curved, in order that it may be applied properly to the ear while the mouth is directed forwards to receive sounds from a speaker in front of the person using it. For the sake of portability, ear-trumpets are frequently made in two, three, or more portions, sliding within one another, somewhat in the same manner as the tubes of an opera-glass. Various other instruments, of doubtful value, are more or less employed in aid of defective hearing: among these may be mentioned, the auricle, a small shell-like instrument, formed of gold, and worn in the ear, so that nothing but the expanded mouth is visible; car-cornets, which are small instruments, made of various shapes and sizes, somewhat resembling a French horn or a musical trumpet in appearance, applied to the ears and held in their place by slender springs; speaking or conversation tubes, which are flexible elastic tubes of India-rubber and silk, kept open by spiral wire springs, and terminating at one end in an ear-piece, and at the other in an open bell-shaped vessel, which is held before the mouth of the speaker; and table sonifers, which consist of a revolving trumpet-shaped cowl mounted on a pedestal, which may be placed upon a table, so as to be turned towards any part of the room where conversation may be going on, and of communicating the sound through a flexible tube to the ear of the deaf person. When flexible tubes are employed for such a purpose, their effect is increased by making them of a tapered or conical shape. An ingenious instrument of the ear-trumpet kind is made in the form of a walking-stick. Another, which is held so as to reflect sound into the ear, is styled the ear-conch, and may be termed an auxiliary ear: it is formed of plated metal. The commoner kinds of acoustic instruments are made of tinplate, japanned; but the better sorts are sometimes formed of silver, or of gong-metal, which is supposed by some to be the best metal for the purpose. Hebert (Engineer's and Mechanic's Encyclopædia,' vol. i., p. 463) quotes an opinion from Dr. Morrison, of Aberdeen, that that end of an ear-trumpet which is applied to the ear should not be made so small as to enter the ear, but should be large enough to include the whole of the external ear; for that gentleman, having been deaf for many years, experienced no relief from ordinary ear-trumpets, but found one which he had made of block-tin, on the proposed plan, to succeed. EARL. The title of count or earl, in Latin comes, is the most ancient and widely spread of the subordinate or subject titles. This dignity exists under various names in almost every country in Europe. By the English it is called earl, a name derived to us from the ealderman of the Anglo-Saxons and the eorle of the Danes. By the French it is called comte; by the Spaniards conde; and by the Germans graf, under which generic title are included several distinct degrees of rank, -landgraves, or counts of provinces; palsgraves, or counts palatine, of which there are two sorts; markgraves, or counts of marches, or frontiers (whence marchio, or marquess); burghgraves, or counts of cities; counts of the empire; counts of territories; and several others. [COUNT; BARON.] After the battle of Hastings, William the Conqueror recompensed his followers with grants of the lands of the Saxon nobles who had fallen in the battle, to be held of himself as strict feuds; and having annexed the feudal title of earl to the counties of the Saxon earls (with whom the title was only official), he granted them to his principal captains. These earldoms were of three kinds, all of which were by tenure. The first and highest was where the dignity was annexed to the seisin or possession of a whole county, with jura regalia. In this case the county became a county palatine, or principality, and the person created earl of it acquired royal jurisdiction and seigniory. In short, a county palatine was a perfect feudal kingdom in itself, but held of a superior lord. The counties of Chester, Pembroke, Hexham, and Lancaster, and the bishopric of Durham, have, at different times, been made counties palatine; but it does not appear that the title of earl palatine was given to the most ancient and distinguished of them namely, the earl of Chester-before the time of Henry II., surnamed Fitz-Empress, when the title of palatine was probably introduced from the empire. The earls of Chester created barons and held parliaments, and had their justiciaries, chancellors, and barons of their exchequer. This county palatine reverted to the crown in the reign of Henry III. The second kind of earls were those whom the king created earls of a county, with civil and criminal jurisdiction, with a grant of the third part of the profits of the county court, but without giving them actual seisin of the county. The third kind was where the king erected a large tract of land into a county, and granted it with civil and criminal jurisdiction to be held per servitium unius comitatûs. Under the early Norman kings, all earls, as well as barons, held their titles by the tenure of their counties and baronies; and the grant, or even purchase, with the licence of the sovereign of an earldom or a barony, would confer the title on the grantee or purchaser; but with the solitary exception of the earldom of Arundel, earldoms by tenure have long since disappeared, and in late times the title has been conferred by letters patent under the great seal. Earls have now no local jurisdiction, power, or revenue, as a consequence of their title, which is no longer confined to the names of counties or even of places; for several earls, as Earl Spencer, Earl Grey, and others, have chosen their own names instead of local titles. The coronet of an English earl is of gold surmounted with pearls, which are placed at the extremity of raised points or rays, placed alternately with foliage. The form of their creation, which has latterly been superseded by the creation by letters patent, was by the king's girding on the sword of the intended earl, and placing his cap and coronet on his head and his mantle on his shoulders. The king styles all earls, as well as the other ranks of the higher nobility of peerage, his cousins. An earl is entitled right honourable, and takes precedence next after marquesses, and before all viscounts and barons. When a marquess has an earldom, his eldest son is called earl by courtesy ; but notwithstanding this titular rank, he is only a commoner, unless he be summoned to the House of Lords by such title. So the eldest sons of dukes are called earls where their fathers have an earldom but no marquisate, as the Duke of Norfolk, &c. The number of English earls in the House of Lords is at present (1859) 110. Of Scotch earls there are 42, and of Irish earls 65, of whom many have seats in the House of Lords in consequence of possessing a British peerage also. EARL MARSHAL OF ENGLAND, one of the great officers of state, who marshals and orders all great ceremonials, takes cognisance of all matters relating to honour, arms, and pedigree, and directs the proclamation of peace and war. The curia militaris, or court of chivalry, was formerly under his jurisdiction, and he is still the head of the heralds' office, or college of arms. Till the reign of Richard II., the possessors of this office were styled simply marshals of England; the title of earl marshal was bestowed by that king, in 1386, on Thomas Lord Mowbray, Earl of Nottingham. The office is now hereditary in the family of Howard, and is enjoyed by the Duke of Norfolk. EARTH (Astronomy). In the language of astronomers, the earth is rarely treated as a planet. All the phenomena connected with its motion are seen in the apparent motion of the SUN, to which article we therefore refer. EARTH, CONTROVERSY ON THE MOTION OF THE. [MoTION OF THE EARTH.] EARTH, FIGURE OF THE. [GEODESY.] EARTH, MEAN DENSITY OF THE. The quantity of matter which the earth contains must ultimately be our only guide to that of any other planet. The relative masses of two planets can be found by calculation of the effects which they produce upon any third body; but the mass of a planet with reference to any given substance, as water, cannot be directly determined upon any instance except our own earth. Perhaps a problem could hardly be proposed which would seem more impracticable to the ordinary reader than that of determining the mean density of the earth. It amounts to asking this:If it were required to substitute for the earth a solid globe of the same size, but of uniform material, in such a manner that the absolute weight of bodies on its surface should remain the same, and the attraction of the whole on other planets remain the same-what must the material be? Of necessity this question was started by Newton, whose system was the first in which it became of much interest. Having no means of submitting it to experiment, he made one of those sagacious guesses which, had they been collected and preserved, would alone have kept his memory alive. "Unde cum terra communis suprema quasi duplo gravior sit quam aqua, et paulo inferius in fodinis quasi triplo vel quadruplo aut etiam quintuplo gravior reperiatur: verisimile est quod copia materiæ totius in terrâ quasi quintuplo vel sextuplo major sit quam si tota ex aquâ constaret." (Principia,' iii. 10.) That is, he judged the earth to be between five and six times as massive as the same bulk of water; which is the truth. The relative masses of two planets are determined by the observation of their effects upon a third. Two preliminaries are required: first, the great assumption of the theory of gravitation, that any two particles of matter must attract one another with forces which at different distances are directly as their masses, and inversely as the squares of those distances; secondly, the mechanical consequence of this law of action, namely, that two spheres, having their centres at any given distance, attract one another in the same manner as if each were collected in its centre. Without describing the mode of arriving at such a result from observation, suppose it is ascertained that two planets, A and B, whose distances from a third are as 4 to 3, attract that third with forces which are as 7 to 2. If both planets be brought to the distance 1 from the third, the attraction of the first will be made (4 x 4, or) 16 times as great as before, and that of the second (3 x 3, or) 9 times. Consequently, the new attractions will be as 7 x 16 to 2 x 9, or as 112 to 18. But at equal distances the attractions are in the proportion of the masses; therefore these masses are as 112 to 18. Now suppose the radii of the planets to be as 3 to 2; then their solidities are as 27 to 8, and if the densities (mean) are 8 and 8', the masses are as 278 to 88'. Therefore 278: 88':: 112: 18, or 8 : 5 :: 112 x 8: 18 x 27: 896: 486. If then the mean density of either planet be known, that of the other can be found. The principle of the preceding process exists in every attempt which has been made to find the mean density of the earth. The earth itself is made one of the planets; some known substance, a mountain or a ball of lead, is made the other planet. The attracted body is not a planet, but a pendulum or a plumb-line, and the effect of the mountain or ball of lead upon the plumb-line is measured, that of the earth being either measured or previously known. The actual attraction of the mountain or ball of lead being thus determined, its effect as it would be if placed at the centre of the earth can be calculated; which effect is to the effect of the earth as the mass of the mountain or ball of lead to that of the whole earth. The result of this process, as usual, is condensed into a formula, in which the mode of making the steps is lost sight of: but the above is not the less the manner in which the experiment must be explained. The hint given by Bouguer, the experiment of Maskeleyne, and those of Cavendish and Zach, have been briefly described in ATTRACTION. Since their time two repetitions of Cavendish's experiment have been made the first, by Dr. Reich, of Freyberg, of which an account was published in 1838; the second, by Mr. Baily, at the desire of the Astronomical Society, and at the expense of the government. (Mem. Ast. Soc.,' vol. xiv.) The former obtained the same result as Cavendish, but the experiments were few in number; the latter obtained a result slightly differing from that of Cavendish, but in so many different ways and by so large a number of experiments, that it is impossible to doubt the superior correctness of the conclusion. We shall give such a slight general account of this process (which is substantially that of Cavendish) as our limits will admit, referring to the volume already cited for more detail: very few experiments have been either so well performed or so satisfactorily described. A TORSION pendulum (76 inches long) was provided, moving on a single or double metal wire, or on a double silk line, the mode of suspension being varied from time to time. At each end was suspended a metal or other ball; and these balls (a and b) were the principal attracted substances. The whole torsion-rod with the suspension was inclosed in a case, with a glass at one end. Large leaden balls (a and B) of about twelve inches diameter (the attraction of which on the torsion pendulum is the quantity to be measured in the experiment) were made to travel on a frame in such manner that they could quickly be brought up laterally on opposite sides of the balls, as in the diagram. We must leave out the whole account of the precautions against electricity or radiation, the manner of noting the actual position of the pendulum, &c., and confine ourselves to the principle of the experiment. When a torsion pendulum, such as that described, is left to itself, one side and the other. Even this mean position is continually shifting its direction, so that it cannot be permitted to take a series of observations and make use of them all in determining one mean place. The mode of finding the point of rest, that is of deducing it from observing the extremes of the vibration, is described in the work cited. As soon as the line of rest of the undisturbed pendulum is ascertained, and the large balls are brought into the attracting positions at A and B, on continuing the observation an immediate alteration of the line of rest is seen towards the large balls; say that it becomes nn'. Then the position of equilibrium of the pendulum is altered by the angle nom, in consequence of the approach of the balls. The precautions taken are abundantly sufficient to assure us that the alteration is no consequence of heat, electricity, magnetism, or any of the variable accidents of matter; there is nothing to which it can be referred except that attraction which, when the earth is the agent, we know under the name of weight, and the assumption of which, as a universal property of matter, led Newton to his explanation of the planetary motions. Many of those who were content to receive Newton's hypothesis to this extent, that the planets attract each other, were staggered by the idea that every particle in the universe attracts every other. Such objectors might have here received conviction from the evidence of their own senses, which would have rendered obvious not only the attraction of the balls upon each other, but its transmission through the wood, flannel, and gilding, which it was found necessary ultimately to interpose between the attracting substances and the torsion-rod in order to destroy the effects of radiant heat. Two observations are necessary, that of the time of oscillation of the pendulum, and that of the displacement of the line of rest which the approach of the larger balls produces. The first observation, the time, enables the observer to deduce the force of torsion, or the quantity of pressure required to produce any given displacement. And in this particular it was found that the pendulum altered its character from one quarter of an hour to another; showing that the instrument was so delicate, that circumstances of which no explanation can be given were continually altering its character. The consequence was, that at every new trial, both the time and displacement had to be scrupulously observed together, in order that to each displacement produced the proper producing attraction might be applied. The complete formula for calculating the mean density of the earth, implies,-1. The calcu lation of the character of the pendulum, or the amount of attraction 'necessary to alter its line of rest by a given quantity; 2. The determination of the attraction actually employed, namely, that of the larger balls, by means of the displacement actually observed; 3. The determination of the attraction which the larger ball would exert, if it had been at the centre of the earth, instead of at the distance employed; 4. The number of times the whole earth would contain such a leaden ball, and its easy consequence, namely, the number of times the whole earth would contain a similar bulk of water; 5. All the necessary corrections for the attraction of the other parts of the apparatus upon the torsion pendulum. The larger masses were leaden balls, but the smaller balls attached to the torsion pendulum were changed from time to time, and different substances were used. The following table of results will be more interesting than any description we could give in the same space. It shows the result of the experiments made after the effects of radiation were removed ‡ by additional precautions. The first column In making the experiment the effect was usually doubled by placing the large balls first on one side of the smaller ones, and then on the other, and noting the whole of the double displacement. This distance of course was accurately measured. An enormous mass of experiments was made and rejected in the course of the attempts to remove singular discorda ces, of which no explanation could be given. The mean result of these would not have differed much from those of the more correct sets, but would, of course, have been less satisfactory. The removal of the discordances was due to the suggestion of Professor Forbes, of Edinburgh, who continued to believe they might arise from radiation long after others thought the precautions then already taken must have been sufficient to remove that source of disturbance. ARTS AND SCI. DIV. VOL. III. The results of individual experiments vary considerably, but it is important to observe that there is nothing which indicates that different kinds of matter attract each other according to different laws. If the large ball of lead exerted different attractions upon particles of brass and ivory of the same weight, the effect would be to give the whole earth one mean density or another, according as the smaller ball is of brass or ivory. Now it is true that the experiments give all manner of results from 5.500 to 6:154, but on examining the results, there appears no evidence whatever of the larger balls attracting the different smaller ones differently. If such were the case, undoubtedly the mean densities obtained from different substances would be different; but though such is the case in the preceding list, and even though it would seem that the lighter balls give the larger densities, yet there is every reason to suppose that the effect is to be attributed to the alteration of the pendulum. Thus it will be seen that there is not so much difference between the results of 2-inch ivory and lead balls suspended in the same way by a single copper wire, as there is between the results of 2-inch lead balls suspended by bifilar iron wires, and the same suspended by bifilar brass wires, and also that ivory balls differently suspended give results which have differmerely increasing the weight of the smaller balls, appear much to ences as great as any. The mode of suspension and the effect of exceed that of applying different substances; but not according to any law. In fact, the differences are altogether of that character to which the term discordance is applied; following no settled rule, and exhibiting every appearance of as often affecting the truth by a positive as by a negative error. The first three sets, in-which a brass rod alone is used, were rather a defiance to the apparatus to fail if it could, than seriously intended to help the result. Almost all the experiments were made with a light wooden torsion-rod, and comparatively heavy balls appended. The trying a brass rod by itself, that is, the attempt at obtaining a mean density by noting the attraction of the larger balls upon the torsion-rod only, was the introduction of an extreme case, to increase confidence in the more ordinary experiments. The mean of the whole is 5-6747, and, rejecting the experiments of which the character would be à priori most doubtful (though it is not certain they ought to be rejected) it is reduced to 5-6604. From the That it is not the case, is also established without a doubt by pendulum 3 A experiments. |