182 ADVANTAGE OF LARGE WHEELS IN RAILWAY CARRIAGES. A locomotive engine mounted on wheels of 3 feet diameter, drew after it 9 loaded waggons weighing 731 cwt. 36 miles in 9 hours and 35 minutes: the wheels were then removed and replaced by others of 4 feet diameter; and with this single alteration the engine drew the same load 48.8 miles in 9 hours and 27 minutes, the consumption of fuel being the same in both cases, viz. 2534lbs.: hence, as Mr. Wood observes, if the load and the consumption of fuel be the same, the space passed over in a given time will be as the diameter of the wheels for 3 is to 4, as 36 to 48, and also by enlarging the diameter of the wheels of the engine the velocity is increased, and the consumption of fuel diminished at the same time: for if we suppose 36 bushels to have been consumed in each experiment, the expense per mile will be one bushel in the first experiment, and only three-quarters of a bushel in the second; whilst the velocity in the first experiment was 3.7 miles per hour, but in the second 5.15 miles per hour. Although from the great scale on which the experiment was made, and the time it occupied, it is decisive upon the subject the result has certainly an air of paradox, for there appears to have been a gain of power and velocity at the same time; and as the wheels may be supposed of any diameter, it may seem to prove that the mechanical effect of a given power may be indefinitely increased, and consequently exceed the power expended; but if we examine what is the real mechanical effect in moving a carriage along a horizontal plane, the air of paradox will disappear, and it will be seen that the result of the experiment is in accordance with the laws of motion. The resistance to the motion of wheel-carriages (independent of that caused by the air) may be divided into two parts, that at the rim and that at the axles; the resistance at the rim is occasioned by the inequalities of the surface on which the carriage travels, the whole weight of the carriage requiring to be lifted through the perpendicular height of each obstacle, and of course requiring a force to be exerted equal to its own weight, moving through the same space. But in a well-laid horizontal railway of good construction, the resistance at the rim of a perfectly circular wheel is so very small, that we may in practice consider the resistance at the axles as the only obstacle to the motion of the carriage. The resistance at the axles is the friction of the wheel during their revolution on the axle, and the amount of this friction in any given time will depend upon the weight with which the carriage presses the axles; and the space through which this pressure acts, and the amount of friction overcome in any given time will be the real measure of mechanical effect in that time; therefore if the weight of the carriage be always the same, and the wheels perform the same number of revolutions in any given time, the friction or mechanical effect will always be the same, and will only require the same expense of power; but the space through which the wheels roll at each revolution on their axles will be as their diameter, and if the revolutions be made in equal times in all cases, the horizontal velocity of the carriage must also be as the diameter. Thus it will be seen that the real measure of effect in transporting carriages along a horizontal plane, is as the pressure at the axles multiplied by the number of revolutions, and not as the weight of the carriage multiplied by the space through which it passes, as would be the case if the carriage moved in a vertical instead of a horizontal path. If the plane be inclined, then in addition to the friction of the axles there is the gravitating power of the load to be considered, which will be as the weight of the carriage and the perpendicular height of the plane; and as it is only the friction at the axles which is reduced by increasing the diameter of the wheels, the increase of velocity will not be as the increase of diameter, but as the increase of diameter divided by the proportion which the friction bears to the gravitating power of the load. Thus, if it require a force of 16 cwt. to overcome the friction of a carriage on a horizontal plane, the carriage weighing 10 tons, it will require to move the same carriage up an in clined plane, 100 feet long, with a rise of 2 feet, an additional force of 10 tons ADVANTAGE OF LARGE WHEELS IN RAILWAY CARRIAGES. of moving through 2 feet, or 2 cwt. through 100 feet; and the whole force required will be 3 cwt. through 100 feet. Moreover, as the friction is only the whole resistance, the increased velocity obtained from increasing the diameter of the wheels is only of what it would be on a horizontal plane. Another advantage which large wheels possess over small ones is, that they are less liable to slip or turn round on the rails without advancing the carriage; and this advantage Mr. Wood has wholly overlooked, for in his experiments to ascertain the amount of adhesion of the wheels to the rails, he states it to be of the weight of the engine, without adverting to the size of the wheels. Now the adhesion evidently depends upon the weight of the carriage and the diameter of the wheels jointly, for the force with which the circumference of the wheel is pressed by the carriage against the rail, may be considered as a weight acting at the end of a lever, whose length is the radius of the wheel, and the resistance will be as the radius is to the length of the crank by which the wheel is turned: or, as the amount of friction is as the pressure and as the space through which it acts, the friction of the wheel in slipping during any portion of a revolution will be as the diameter of the wheel; therefore the weight of the engine may be diminished or the weight of the load to be drawn may be augmented, if the diameter of the wheels be increased, provided the power of the engine bear always the same proportion to the load. The superiority of large wheels over small ones being so evident, I have no doubt that in future attempts at improvements on locomotive engines greater attention will be paid to this point, and that the diameters will be increased as far as is consistent with strength, and without adding too much to the weight; and the limits in this case will I think be best attained by wheels on Mr. T. Jones's construction, which may now be had formed entirely of wrought iron, and which far exceed in lightness and strength any wheels hitherto employed on railways. What these limits may be is hard to say, , but 183 it seems not too much to expect that wheels of 8 or 9 feet diameter may be employed; for the wheels of timber waggons and also of the French diligences are frequently 6 feet in diameter, although made of wood, and exposed to great transverse strains from the inequalities of the road. Eight feet therefore does not seem to me too great a diameter for iron wheels upon a railway, and exposed to no transverse strain; and the stability might be rendered even greater than at present, as the diameter would admit of the boiler and the rest of the apparatus being placed below instead of above the axles. There is another point to which I would advert, viz. the means of ascending a plane, the acclivity of which is too great to be overcome by the adhesion of the wheels: in this case the carriages are in general hauled up by a rope wound round a barrel turned by a fixed steam-engine; but the friction of the rope along the plane, although supported by friction-rollers, amounts in many cases to nearly one-third of the whole resistance, whilst in most cases the ascent might be surmounted by either of the following methods:1st. By means of a toothed-rail along the whole length of the inclined plane, into which a toothed-wheel attached to the locomotive engine works, as on the Middleton Colliery Railway at Leeds. Or, 2nd, by means of a chain extending the whole length of the plane, and made fast at each extremity, and a spike-wheel attached to the locomotive working into the links of the chains: for this method Mr. Chapman has a patent, if it be not expired. By either of these methods the friction beforementioned wouldbe avoided, and fixed engines be unnecessary, which would be a great advantage, especially if rail. ways should ever be laid by the side of turnpike-roads. If you can spare a space in the "Mechanics' Magazine" for the insertion of these suggestions, You will much oblige Your most obedient servant, J. MURDOCH, Mech. Draftsman. 4, Vittoria-place, Mile-End-roa!, October 27, 1829. SAFETY RAILWAY CARRIAGE. Sir, Since the result of the late decisive experiments on the Manchester and Liverpool Railway have been made known to the public, there appears to be a very general feeling that locomotive engines on railways are likely to be very speedily brought into use for the general purposes of travelling, as well as the conveyance of merchandise; and it must be confessed, that the late surprising display give us every reason to think that opinion well-founded. But it ought not to be forgotten, that there are prejudices to be removed and fears to be allayed, which, if not fairly met, may retard the attainment of what appears to be a desideratum with all men, to wit," cheap and expeditious travelling;" and therefore it may perhaps be well if any thing can be suggested that will add to the security of the railway passenger: for we ought to bear in mind that the argu ment of the infrequency of accidents on railways, does not apply to the velocities proved to be attainable, and purposed to be used. With this view, I beg to suggest to your readers a few observations on the subject, and in explanation of the preceding figures. I presume, in the first place, that it will be indispensably necessary to have railways securely fenced, whether they be laid on new lines or on the sides of public roads. If that be so, might not such fence or railing be so constructed as that it might check any tendency great velocities may give the carriage to fly from the rail, as well as prevent straying cattle or thoughtless pedestrians from getting into danger? Fig. 1 (a section) is intended to show this double use of a fence or railing: a a are the sleepers or foundation of the railway; bb the iron rails resting on the sleepers; ee the wheels of the carriage in the common form with the inner edge ON THE MODES OF KEEPING METEOROLOGICAL DIARIES. projecting; dd upright posts or rods fixed in the sleepers, and supporting ee, the horizontal rails of the fence, which overhang and run parallel with the railway, and being distant from it the breadth of the wheel something less than the depth of its projecting rim edge. It will be seen that the wheel being thus, as it were, boxed in, cannot leave the rail so long as the parallelism of the rail and the horizontal fence-rail is preserved; any tendency to do so, being checked by the overhanging or horizontal fence-rail on this construction. If it be objected that the smallness of the space between the upper part of the wheel and the overhanging fencerail will cause friction, by the accumulation of mud or snow, it may be answered, that an easy remedy may be had by increasing the depth of the projecting edge of the wheel, and the space between the upper surface of the wheel and the horizontal railing; but if that he inconvenient, let the horizontal railing e be placed in a position equidistant from the top of the wheel and the side of the carriage, and the chances are that an overturn will be prevented; but by that arrangement the wheels will not be prevented leaving the rail, as they would if on the former construction. Perhaps it might be worth while to inquire how far this overhanging railing might also protect the railway from wet, and thereby diminish the waste of iron. I believe some minute inquiries were made in the neighbourhood of Newcastle some years ago, as to the destruction of iron by the scaling or rusting of the exposed rails used in that district; but I am not acquainted with the results: probably in this damp climate it will be found that the exposure to rain does not much add to the unavoidable destruction of iron.-Sed query. I There remains to be considered the means of preventing collision. should not have troubled you on this head, had not a little contrivance occurred to me in answering the objec tions of an old gentleman a few days ago. My friend is one of those who derive their information, scientific and political, from the newspapers; and through the medium of that well 185 known journal, The Times, he has become acquainted with the Liverpool experiments, and has set his mind upon going into the country next spring at the rate of 30 miles an hour, and nothing less. But notwithstanding his desire to fly with the dove, he has serious misgivings as to the effects to be apprehended from meeting in collision the " up coach." I found it in vain to explain that the having two lines of railway would prevent such an occurrence; for while the possibility remains, so, he says, will his apprehensions. I then suggested the contrivance figured in No. 3, being simply a flat piece of iron c fixed on the horizontal railing b, and easily yielding to the passage of a carriage in the direction from d to e, but offering resistance to any passage in the opposite direction. If such springs were placed on the horizontal railing at equal distances, say a quarter or half a mile apart, they might be made to move a small clock-work attached to the carriage, which should show the distance passed over: the reading of mile-stones being, as I take it, out of the question with the new veloci ties. Perhaps some of your ingenious readers will suggest the plan of a railway meter. I write these things for the consideration of those practically acquainted with the subject, which, as you may have perceived, is not the case with, Your obedient servant, REPLY BY MR. SQUIRE TO DR. BUR- Sir, I should wish to say a few words in reply to Dr. Burney's remarks, in No. 311, on my meteorological summary in your 301st Number. Without recapitulating, I will just observe, that as the height of the Horticultural Society's barometer above high-water in the Thames at Chiswick is known, and that the goodness of the instruments and accuracy of the observations cannot be disputed; for ON THE MODES OF KEEPING METEOROLOGICAL DIARIES. 186 these reasons, I shall take the liberty of making a comparison between the barometrical observations made at Gosport and those at Chiswick. = Now, the mean altitude of the barometer at Chiswick, in 1827, was 29.950, and at Gosport 29.900, making a difference 05; in 1828, it was at Chiswick 29.921, and at Gosport 29-868, making a difference of '053. Hence the observations of the former year make the barometer at Gosport to be 45.422 feet above that at Chiswick, and those of the latter year 48.427 feet, giving a mean of 46.9245, or nearly 47 feet. But the Horticultural So ciety's barometer is stated to be 14 feet above high-water in the Thames at Chiswick, and Dr. Burney's 50 feet above low-water mark in Portsmouth harbour; therefore 46.9245+14-50=10.9245 feet. So that it appears, from these obvervations, that the low-water mark at Gosport is nearly 11 feet above highwater in the Thames at Chiswick!! Now, as we can safely depend upon every thing connected with the Chiswick observations, and that it is impossible for low-water mark at Portsmouth to be above that of high-water in the Thames at Chiswick; ergo, Dr. Burney's barometrical numbers must, in some way or other, be in correct. The Doctor says my estimate of the mean temperature in the shade at Epping is too high by about of a degree; from which I judge he has taken Mr. Howard's or Mr. Daniell's result for his standard of comparison. That these gentlemen make the mean for the neighbourhood of London to be 49.5° is certain: yet as this was obtained from the daily maxima and minima, it must for that reason be too low; as it is well known that the real annual mean always exceeds this by about a degree, varying a trifle more or less in extremes. Instead of considering 50.32° in excess for the mean temperature of the external air in the shade at this place, I have many reasons for believ ing that it is, on the contrary, rather in defect. According to theory, by the formula 31+53c2, where c denotes the cosine of the latitude, the mean temperature of Epping comes out 51.36°: spring water, and a shady part of the house which opens to the north, where the annual mean oscillates within the limits of about two degrees, give mean results but little short of that obtained by theory. But a series of careful observations made on the water in a well that is about 20 or 30 feet deep, and perfectly shaded, is the best criterion for determining the mean temperature of any place. On this account, Dr. Burney's observations on the temperature of spring water at Gosport must be considered valuable; the mean of which for 8 years 51.884°, that of the external air in the shade for the same period =52-855°, which latter mean is certainly too high. Let us see what the above formula will give: now the latitude of Dr. Burney's observatory is 50° 47' 20"; hence 31 +5302 = 52.182°, which is only about of a degree greater than the mean of spring water, but is nearly less than that assigned to the air. I should consider the mean temperature for the parallel of Gosport to be extremely near 52°. Dr. Burney is doubtless aware that good barometers, such as ought always to be used for philosophical purposes, do not require to be compared with any others for the verification of their accuracy, as they are always standard instruments of themselves. It cannot, however, be denied that many are very imperfect such may do for the purpose of common weather-glasses, but not so if intended for scientific pursuits; these require, in every respect, the greatest care and circumspection in their construction. The best barometers are those made by Mr. Newman, Philosophical Instrument Maker, Regent-street. This gentleman has long been convinced of, the necessity of giving to this instrument every improvement that it seems capable of, both in its construction as well as adjustment; and he is well known to spare no pains for the accomplishment of these desirable ends, in which he has so happily succeeded. I am, Sir, Yours, &c. THOMAS SQUIRE. Epping, Oct. 28, 1829. |