74 Mr. Quinby^s reply to Mr. Blake. 



satisfactory if nothing had been said about the space through 

 which the power moves ; for since the problem is reduced to 

 the case of the Pully, and the mechanical power of the Pully is 

 known, it was unnecessary, (for the purposes of the demon- 

 stration,) to mention the space through which the power 

 moves. My reason for mentioning the space through which 

 the power moves, was merely to show, to those who are not 

 well acquainted with mechanics, what is understood to be a 

 case in which there is no loss of power. When a machine is 

 reduced to any of the simple mechanical powers, it is then 

 knoivn that there cannot be any loss of power; and it would 

 be unnecessary to repeat what every tyro in mechanics al- 

 ready knows, that in all the simple mechanical powers, what 

 is lost in force is gained in space ; and what is lost in space is 

 gained in force. 



A. B. Q.UINBY. 



June 14, 1827. 



Note. — In the demonstration which Mr. Blake has given, 

 he has compared the " whole degree of force exerted by the 

 shackle bar on the crank," with the " mean tendency to rota- 

 tion in the crank which that force produces." The " tenden- 

 cy of a force to produce rotation," is the product of that force 

 and the distance at which it acts from the centre of motion. 

 The things, therefore, which Mr. Blake has compared are not 

 of the same kind ; and he has fallen into the very error of 

 which he accuses me. The quantities which are compared 

 in the demonstration which I gave are, " tendency of P to 

 produce rotation," and u mean tendency of P to produce rota- 

 tion." These are of the same kind. At page 339 he says, 

 " Another [power] will drive a body against a resistance ten 

 feet ;" this he states contains but two attributes ; but can a 

 body move ten feet without occupying some portion of time ? 

 and if it occupy any portion of time, does not this example 

 contain the three attributes ? How then does Mr. Blake find 

 the same difference between this example and the third, that 

 there is between a square foot and a cubic foot ? 



A. B. QuiNBY. 



