Upon a reference to some of Mr. Wood's experiments, as detailed in his valuable treatise, we find that the results confirm our views as to the resistance upon plate rails. The rails he used were 4 feet long, 31 inches broad where the wheel wheel runs upon them, and the height of the upright ledge 3 inches. In an experimentmade with two loaded carriages, each weighing 8512 lbs., cast-iron wheels inches diameter, 11 inch broad upon the rim which runs upon the rails, brass bearings 11 inch broad, and diameter of axle 21 inches. the resist ance tsjs) a certain inclined plane was found to be 168, and down the same 126, making the mean resistance 147, which is equal to the 116th part of the weight moved.
With respect to edge-rails, it was usual, until recently, to estimate the amount of resistance at the two hundredth part of the insistent weight; but the improve ments which have of late years been made, both in the rails and the carriages, have reduced this resistance to about the 240th part of the weight: according to which, the following table has been calculated by Mr. Wood.
It becomes now an interesting point of inquiry to ascertain the extent of those several resistances to motion of which the foregoing table has given the total. For this purpose we are obliged again to resort to the ably conducted experiments of Mr. Wood; but as it would be impossible for us to give a detail of those experiments, or of the useful tables calculated therefrom, within the compass of our article upon this subject, we must content ourselves with a notice of the results derived therefrom, and to refer the reader who may be desirous of more precise information to the author's valuable work.
Mr. Wood found that the ratio of resistance to the rolling of the wheels upon a railway, was not increased by an increase of the weight in the carriage; and they were very nearly the same in velocity, varying from 5.50 to 14.45 feet per second ; so that Me resistance by the roUing of the wheels is an uniformly retarding force, both with respect to velocity and weight. Taking the resistance of the wheels as equal to the 1000th part of the weight, and knowing the whole amount of resistance, we obtain that of the friction of the axles; applying this to the experiments detailed by Mr. Wood, the following results are given in that gentleman's work :— We thus find that, in the above experiments, the resistance by the attrition of the axles amounts, in the most favourable case, to the 23d part of the insistent weight; or, taking the numbers 1 to 6, and the following experiments, equal to the 20th part of the weight ; while, in some of the experiments on the empty carriages, the friction appears much greater ; from whence we would be inclined to conclude, that the resistance is diminishedby an increase of pressure. There
is no subject in science, perhaps, on which there is a greater diversity of opinion than in the laws which govern friction; and the previous experiments, though sufficient, in many cases, for practical purposes, yet by no means tend to bring the inquiry into any more settled state. In Nos. 1 and 6, and the following experiments, the ratio only varies (except in one instance) from the 19th to the 21st part of the weight : and as, perhaps, in the other experiments, the resist-. ance of the wheels—the state of the axles—the construction of the carriages— or some other adventitious cause, might have operated to increase the friction, so as to induce us to leave these experiments out of the question, and take the former as the more correct amount; yet still this ratio is greater than shown by former experimentalists.
In some experiments by Mr. Southern, in 1801, communicated to the Royal Society, and printed in the sixty-fifth volume of their Transactions, the friction of the axles of a grindstone weighing 3700 lbs. amounted to less than the for tieth part of its weight. Now there does not appear any reason why, in well constructed carriages, the resistance on the axles should be greater than in other machinery; and, therefore, we are obliged to conclude, either that the resist ance of the wheels must be greater than we have assigned, or that there were some defects in the construction, either of the carriage or axles. Under these circumstances, and considering the importance of obtaining the most correct information on the subject, Mr. Wood had an experimental carriage made, and fitted up with the utmost care; the axles and bearings of which were of the best material, and were kept in use a considerable time before the experiments were made, to render them as smooth as possible. The same wheels were used as in experiment 12, and the experiments were also made upon the same piece of railroad. Bearings of brass and cast-iron were both used to ascertain which gave the least friction; and the carriage was loaded with different weights to ascertain the relative resistance. The experiments, which were conducted with the utmost care, and repeated several times to obtain correct results, are given by Mr. Wood in a series of tables. We annex those which relate to the cast-iron bearings, as that metal evidenced in every experiment less friction than the above-mentioned alloy, to the amount of about one-thirteenth part.