DISCOVERY OF THE LAWS OF MOTION. 33 



given time, without taking into account the Velocity 

 they have already. If we communicate velocity to 

 a body by the hand or by a spring, the effect we 

 produce in a second of time is lessened, when the 

 body has already a velocity which withdraws it 

 from the pressure of the agent. But it appears that 

 this is not so in the case of gravity ; the velocity 

 added in one second is the same, whatever down- 

 ward motion the body already possesses. A body 

 falling from rest acquirer a velocity, in one second, 

 of thirty-two feet ; and if a cannon-ball were shot 

 downwards with a velocity of 1000 feet a second, 

 it would equally, at the end of one second, have 

 received an accession of 32 feet to its velocity. 



This conception of Gravity as a Uniform Force, 

 as constantly and equally increasing the velocity 

 of a descending body, will become clear by a little 

 attention ; but it undoubtedly presents difficulty at 

 first. Accordingly, we find that Descartes did not 

 accept it. " It is certain," he says, " that a stone is 

 not equally disposed to receive a new motion or 

 increase of velocity when it is already moving very 

 quickly, and when it is moving slowly." 



Descartes showed, by other expressions, that he 

 had not caught hold of the true notion of accele- 

 rating force. Thus, he says, in a letter to Mer- 

 senne, "I am astonished at what you tell me, of 

 having found, by experiment, that bodies thrown up 

 in the air take neither more nor less time to rise 

 than to fall again ; and you will excuse me if I say 



VOL. II. D 



