Chap. 7.] Carriages. 73 



fo by the weight of the carriages which pafs over 

 them. Thefe inequalities, whether of the wheels or 

 of the earth, therefore caufe the wheel to be fupporred 

 by a radius C Q_or C N, oblique to the direction of 

 the power C P, or to the direction of the refiftance 

 C M. Then rhe weight which is fuppofed to prefs at 

 C refills the power which cannot make it advance but 

 by caufing it to rife as much as the point Q^or N is 

 above the point M. The power is then obliged to 

 fuftain part of the weight of the carriage, as if it was 

 placed upon an inclined plasie. When, moreover, the 

 circumferences roll upon furfaces perfectly folid and 

 level, there is indifpenfably a confiderabie friction be- 

 tween the axle-tree and the nave. 



The little elevations and deprefTions in roads change 

 alfo the direction of the power. A boric placed higher 

 or lower, by the difpofition of the ground, inftead of 

 making his effort in th? line C P, parallel to the por- 

 tion of the plane which fupports the wheels, makes it 

 often by the line C S or C R, that is, obliquely to the 

 direction of the refiftance CM, and confequently with 

 difadvuntage ; for a carriage which may be moved 

 eafily by one horfe only upon a horizontal plane, often 

 requires many horfes to move it up a rifing ground. 



In general, the mod advantageous mode of moving 

 burdens in carriages over rough and uneven ground, 

 as roads are for the moft part, is, according to Meflrs. 

 Stevin, Wallis, and Deparcieux, to draw in a rifing 

 line, as C R j for this purpofe it is neceffary that the 

 axle of the wheels fhould be a little lower than the 

 breads of the horfes, by which means the direction of 

 the power approaches more to the parallelifm of each 

 of the final! inclined planes which form die inequalities 

 of the .roads. 



But 



