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The load upon a carriage in paffing over an obftacle refifts 

 the power which draws it, not only by its weight, but by its 

 vis-inertias ; after a carriage has been once fet in motion upon 

 a fmooth road with any given velocity, its motion, fo long as 

 that velocity is continued, is neither retarded nor promoted by its 

 vis-inertiae ; but whenever it pafTes over any height, not only 

 the weight of the carriage muft be lifted up, but the vis-inertiap 

 of that weight muft be overcome in a new diredion, and as 

 much velocity muft be communicated to it in that new diredtion 

 as will enable it to rife to the height of the obftacle whilft it 

 paffes over its bafe. When an obftacle is of fuch a fize and 

 fhape that a wheel of fix feet diameter muft ftrike the top of 

 it at once, and not roll from the bottom upwards, and when its 

 fliape will permit a fmaller wheel to touch it during its whole 

 afcent, as there is more time allowed for overcoming the vis- 

 inertiae of its weight in the latter cafe than in the former, 

 the fmaller wheel may be drawn forward by a lefs power than 

 the larger, notwithftanding the advantage of lever, which is in 

 favour of the larger wheel. 



To determine thefe circumftances by experiment, it was 

 neceffary to conftrudt an apparatus different from that which I 

 have defcribed. I at firft made ufe of an inclined plane of five 

 or fix feet long, and one foot high, placed upon a fmooth hori- 

 zontal floor. The diftance to which the carriage was driven 

 upon the floor by the velocity which it acquired in its defcent 

 down this inclined plane, I aifumed as the meafure of its ac- 

 quired force, and the refiftance of any obftacle which I placed 

 in its way I determined by the diminution of this diftance. But 



L 2 though 



