On Friction in Machinery. 215 



that of the trough ; hollow parallelopipeds of glass are substi- 

 tuted for tumblers or cells. The plates are suspended to bars 

 counterpoised like window sashes. 



The advantages are as follow : The material is one of the best 

 non-conductors, is easily cleansed, and is tlie most impervious to 

 solvents. The fracture of one of the cups is easily remedied by a 

 supernumerary. They maybe procured (as in the United States) 

 where porcelain cannot be had. The shock from 300 pairs is such 

 as few will take a second time : some of the effects have already 

 been stated *. 



XXXVI. On Friction in Machinery. By Mr. Henry Meikle. 



To Mr. Tilloch. 



A 35, Berners Street, Aug. 2/, 1819. 



s hinted at the conclusion of my miscellaneous remarks 

 on friction, carriages, &c. in vour Number for Marcli, 1 again 

 venture to resume that inexhaustible subject. 



From the very accurate experiments of Professor Vince and 

 M. Coulomb, it appears that, on hard bodies, friction is an uni- 

 formly retarding force ; or, that it is constant during all the 

 changes of an uniformly varied motion. This is, perhaps, the 

 most valuable and general mode of experimenting, in order to 

 discover the laws of friction, so far as the velocity is concerned ; 

 because, from it, we learn that friction is, cceleris paribus, the 

 same for all the different uniform velocities which do not exceed 

 the greatest attained in the experiment. We must not, however, 

 like Chemists with their laws of expansion and temperature, sup- 

 pose that friction is the same for velocities greater than those 

 experimented on. Though tiiis seems probable, it is not certain; 

 and to presume on it, we might meet with the same downfall as 

 the fanciful laws of temperature did when they attempted to soar 

 above the ken of observation. 



I formerly noticed that, on account of heat, friction sometimes 

 increases a little with the velocity. This, the experiments re- 

 ferred to, were hardly calculated to detect, as it is not likely they 

 were continued long enough to accumulate heat sufficient to af- 

 fect the result. Rut there is, perhaps, some ground for suspect- 

 ing that the friction of bodies which have long rubbed on each 

 other, is somewhat different from the friction of those that only 

 pass a few trials for the sake of experiment. I have often thought; 

 that the difference between the results obtained ijy Professor Vince 

 and M. Coulomb, relating to the j)ressuic, was probably owing 

 to causes of this nature. 



• The glasses may be had by applying to Hdw. A. Pearson, No. 71» Corn* 

 hill, Boston. 



4 In 



