[ 77 ] 



became therefore, in all cafes, the meafure of the additional 

 refiftance, and determined with the greateft accuracy the refult of 

 every experiment which I wiflied to try. 



Having found that nearly five pounds and a half was fuffi- 

 cient to give the wheel, when loaded fo as to weigh about four 

 pounds, a velocity of nearly ten feet in a fecond, I placed an 

 obftacle of a quarter of an inch high upon the plane, and it 

 required no lefs than fix pounds and a half to overcome its 

 refiftance. Two fuch obftacles required fourteen pounds and a 

 half. Two obftacles of the fame height, but of a different 

 fhape, each making an inclined plane of three quarters of an 

 inch long and a quarter of an inch high, were fubftituted in 

 the place of the former, and it required but two pounds to 

 overcome their refiftance. The difference, therefore, between 

 two and fourteen muft be attributed to vis-inertias ; for the velo- 

 cities of the carriage and the heights of the obftacles remaining 

 the fame, the only difference that exifts is, that in the one cafe 

 the wheel has much more time to furmount the obftacle than 

 in the other, and confequently had much lefs vis-inertiae to 

 overcome. 



From this confideration it appears that whatever permits 

 the load to rife gradually over an obftacle, without obftruding 

 the velocity of the carriage, will tend to facilitate its draught ; 

 and the application of fprings has this effed to a very confider- 

 able degree. The fame weight of, four pounds being drawn 

 aver the fame obftacles, when fprings were put between the 

 load and the carriage, by four pounds inftead of fourteen. 



This 



