From the facts contained in the foregoing Table, the following results are deduced.
From these experiments we find, that in a well-fitted up carriage, the whole resistance may be reduced to nearly the 500th part of the weight ; and that, taking the resistance of the wheels upon the rails as equal to the 1000th part of the weight, the friction of attrition at the axles amounts to no more than the 60th part of the weight, when the velocity of the surfaces in contact is equal to the progressive motion of the carriage. The resistance from the above experi ments being so much less than that previously found by practice in carriages on railroads, and in the proportion of 60 to 40 less than that found by Southern, Mr. Wood was induced to suppose there might have been some errors either in the experiments or calculations, though, in the prosecution of them, the utmost care was taken ; and the uniformity of the result, in each of the experiments, almost proves that no error could have been committed. The degree of polish given to the axles, though nothing more than what was effected by using the best materials, and causing the carriage to be run up and down the railroad, with the axles lubricated with the best neat's-foot oil, may account for the great reduction compared with that of the former experiments; and, in this case, good neat's-foot oil was used and applied at the commencement of each experiment; whereas, in the former experiments, the grease commonly used on the axles of coal-waggons was used. To put the question, however, beyond all doubt, and, at the same time, to ascertain more particularly all the phenomena of the friction of attrition, Mr. Wood had an axle fitted up, which was placed upon two chairs or bearings, by which the rubbing friction could be ascertained, independent of that of rolling. The axle was placed upon two bearings, at such a height from the ground as would allow a weight to descend 30 feet; a wheel was fixed in the middle of the axles 2 feet diameter, around which a cord was wound, to the end of which a weight was attached, and rings of lead were fastened upon the axle to vary the weight. In each experiment the cord was wound round the wheel, and the weight thus elevated precisely 30 feet from the platform; by withdrawing a in the weight was then let free, and, falling 30 feet, unwound the cord, and put the axle and lead weights into rapid motion ; the cord Then detached itself, and left the axle to turn freely round, until the friction of the axles brought it to rest. By a proper apparatus the
time occupied, during each ten revolutions of the axle, was measured ; as, also, the whole time, until it came to rest: by which means not only the absolute amount of friction was obtained, but also the friction at different velocities ; and by varying the weights from 1331 lbs. to 41401bs. the relative resistance, with different weights, was also ascertained. ' The principal object, however, of instituting this set of experiments, was to ascertain if the friction varied with the surface of bearing; and, if there was any, what size of bearing, subjected to a given pressure, produced the least resistance. With this view, bearings of 3, 4i, and 6 inches, respectively, were used, the diameter of the axle being 3 inches, and, on each of which, the successive weights of 1331, 2465, 3622, and 41401bs. were placed. With these materials the number of experiments made were more than 600, and varied in every possible way, to leave no doubt as to the accuracy of the result; the weight, in each experiment, falling precisely 30 feet.
It is worthy of notice in this place, that a considerable variation in the number of was occasioned by different modes of applying the oil. The axle rested upon the chairs, without any cap or cover, as here shown, where A represents an end view of the axle, and B the chair. At the com mencement of each experiment the axle was oiled in the usual way, with fine neat's-foot oil ; but it was found, that unless the oil was continually feed ing upon the axle as it turned round, the result was never the same, unless the oil was supplied in such quantities, that when the axle turned round, the oil was heaped up against it, as shown above, and thus kept up a continual supply to the axle. When that was not the case, although the axle was well oiled, yet, unless the oil was kept constantly feeding upon the axle as it turned round, a maximum effect did not take place. The following Table, being one of the series of experiments, will show the effect.