264 Trtwiliitkm o/' Key's Essays. 



lines L L presses on it, whence the quickness of its motion 

 must increase; to which is to be added the impulse which this 

 matter is continually giving it, as it still keeps falling down 

 on the ball. The opinion of Pererius somewhat resembles this 

 idea of an impulse ; for he thinks that the air which follows 

 pushes the ball; but he is mistaken in this, that air being- 

 light, and naturally tending upwards, cannot push the ball 

 downwards, any more than a boat, towed against the stream 

 is impelled up the river by the water, which meeting the prow, 

 divides, and passing the sides runs continually downwards ; 

 for how can it, following this course, strike the* stern 

 above? The other part of his assertion is no better, con- 

 tending that the air, agitated by the motion, yields more readily 

 to the thing moved. It is just the reverse, for air and water, 

 when agitated, are capable of supporting larger weights. Ashes 

 are suspended in water, and feathers in the air, when they 

 are agitated, and fall down when the fluids are at rest. Surely, 

 according to this reasoning, the motion should be slower towards 

 the end, the agitation being then greater. 



Essay VI. 



t Gravity is so intimately united to thejirst matter of the elements, 

 that when they change from one into another, they always re- 

 tain the same weight. 



My principal object, hitherto, has been to fix in the minds of 

 all, the persuasion that air has in itself a principle of gravity, 

 since it is from this that I purpose to derive the increased weight 

 of tin and lead when they are calcined. But before I shew 

 how that happens, I must extend my observation, and add, that 

 the weight of any body is examined in two ways, by reason 

 or by the balance. It is by reason that I have found weight in 

 all the elements ; it is reason which now induces me to deny 

 that erroneous maxim, which has obtained from the birth of 



* Frapper en haut la pouppe. 



t P^anteur, I have translated this word by the term gravity, though 

 r believe it was not used in this sense before the time of Newton; in 

 like manner prso-, is srenerallj rendered srrtrritnfe. 



