In conducting these experiments, the first four were made with the axle oiled, so as to keep it constantly feeding on, as shown in the figure. The weight being drawn up was liberated, and falling 30 feet, the respective number of revolutions were made before the axle came to a state of rest ; the second column being the time in oscillations of a pendulum vibrating 300 times in 157 seconds. At the end of experiment 257, the oil which was resting upon the bearing, heaped up, as shown in the figure, was merely removed, as cautiously as possible, so as to allow that which surrounded the axle to remain; the weight was drawn up as before, and falling precisely the same distance, the number of revolutions was, in that experiment, 189. No additional oil being applied, the weight was successively drawn up and liberated as before, and the number of revolutions were found, as shown in the table, until the end of the 300th experiment, when the number of revolutions, by the same moving force, was only 37; during the whole of which period the axle was never touched, no oil was applied, and none removed. At the end of the 300th experiment, the axle was again copiously oiled, so as to feed on during the whole of the 301st experi ment, when the number of revolutions were 265. The oil was then removed as before, when the number regularly diminished until the 323d experiment, when it was again reduced to 36; and when, in the next experiment, the oil was applied as before, the number was increased to 278, by the game weight falling precisely the same distance, which, in the previous experiment, only pro duced 36 revolutions. The oil used should be very fluid, so as to present the least resistance to the bodies sliding over one another, yet of sufficient viscidity to prevent them coming actually in contact. The fine purified plumbago, pnepared as described under our article PLUMINA00, seems to us well deserving the attention of the experimentalist on the friction of running axles. It has, here tofore, been used only in a very impure, and, coesequently, ineffective state.
The following Table being the result of part of the experiments previously alluded to, having been made upon the friction of axles alone, the bearing sur faces and insistent weights of which being also more varied, will show the comparative effect of different sized bearings, with the finest neat's-foot From the various experiments made by Mr. Wood on the friction of carriages, he arrived at the following conclusions ; viz.
" That in practice we may consider the friction of carriages moved along railways as a uniform and constantly retarding force.
" That there is a certain area of bearing surface compared with the insistent weight, when the resistance is at a minimum.
" That, when the area of bearing surface is apportioned to the insistent weight, the friction is in strict ratio with that weight." The area of bearing surface in the axles of carriages, calculated to give the minimum of friction, he found to be one inch to every 98 lbs. of the insistent weight.
We shall now proceed to the consideration of the retarding effects of the air to the motion of carriages, which, although inconsiderable at low velocities, presents a great resistance at high velocities, and becomes, at length, so con siderable by a farther increase of speed, as to constitute, comparatively speaking, the only cause of resistance worth mentioning. The author of a series of papers that were published in the Scotsman some years ago, has elucidated this part of our subject with admirable simplicity. "During high winds (he observes) this resistance is so considerable, that means should be taken to lessen its amount, first, by making the vehicle long and narrow rather than broad and short; and, secondly, by giving the front a round or hemispherical form. Let us suppose, then, that there are two steam vehicles, each weighing with its engine, fuel, and lead, 15 tons (or 30,000 lbs.) The one a steam-waggon, for
conveying goods, 6 feet high and 5 feet wide, and having, of course, a front of 30 square feet, which, in reference to the pressure of the sir, is reduced to 15 feet by giving it a rounded form. The other, a steam-coach, for carrying passengers, is 8 feet high, and 8 feet wide ; or 7 feet high, and 9 wide, presenting a front of 60 square feet, but reduced to 30 by its rounded form. Now, still it is found, by experiment, to press with a force of 16 grains upon a body presenting a front of 1 foot square, and moving at the rate of 1 foot in a second, and the pressure increases as the square of the velocity. Hence, our steam-coach, when moving at 4 miles an hour, in a still atmosphere, would encounter a resistance from the pressure of the air of 2* pounds ; at 8 miles an hour the resistance would be 9lbs. ; at 12 miles an hour, 20 lbs. ; at 16 miles an hour, 36 lbs. ; at 20 miles, 57 lbs. The steam-waggon, presenting only half the surface in front, would experience only half the resistance.
Note.—To affect minute accuracy in calculations of this kind, is a mere deception. Fractional quantities are therefore rejected. In point of fact, the resistance increases rather faster than in the simple ratio of the surface, and the resistance of a sphere is less than the half of that of its diametrical section. On the other hand, the resistance increases in a ratio rather less than that of the square of the velocity.
" Let us assume, according to what we have already stated, that a power of 150 lbs. would just put the steam-coach in motion ; then, if we allow an addi tional power of 33 lbs. for acceleration, making 183 lbs. altogether, we find that if the air did not oppose its progress, it would move over 43 miles in one hour. Now, since it is propelled only by a force of 33 lbs., as soon as the resistance of the air pressed it back with a force of 33 lbs., the acceleration would cease, and the motion become uniform. This would take place within 12 or 15 minutes, and, when the velocity had risen, to 14 or 15 miles an hour. With the steam-waggon, presenting only half the front, the velocity would become uniform at 22 miles an hour. Hence we see, that if we had a perfect calm in the atmo sphere, we could impel 15 tons along a railway with a velocity of 15 or 22 miles an hour (according to the extent of surface the vehicle presented) by a force of 183 lbs." The intelligent author next proceeds to compare the resistance on a railway with that in a canal or arm of the sea, in a calm atmosphere. Although this mode of treating the subject is somewhat irregular, yet it places the matter in such a striking and interesting point of view, that we think the digression will be excused. The force required to impel a vessel weighing, with her load, 15 tons, through water at different velocities, would be as follows:— ' We may now combine the two tables into one, and exhibit the results in horse power, as well as pounds, reckoning one horse power equal to 180 lb& We see from this Table the astonishing superiority of the railway over the canal for all velocities above 4 miles an hour. Nearly three times as much power would be required to move an equal mass at 6 miles per hour on a canal as on a railway ; 5 times as much power would be required at 8 miles an hour; 10 times as much at 12 miles ; 15 times as much at 16 miles ; and 24 times as much at 20 miles an hour. It is evident, also, that an addition of power, too trifling to add any thing material to the weight of the vehicle, would raise the terminal or uniform velocity from 4 miles an hour to 20.; and that, speaking practically, it would cost no more to command a velocity of 20 miles an hour on a railway than a velocity of one. Except for the chances of Injury to the railway or the vehicles, there would not be the smallest reason for conveying goods, even of the coarsest kinds, at 4 miles rather than, at 20 miles the hour.