262 'Translation of Key's Essai/s, 



Essay V. 



It is demonstrated that air and fire have Aveiglit, hxj the greater 

 celeritij with which heavy bodies viove, toward the end, than at 

 the beginning of their motion. 



An error, however small, committed in the beginning of any 

 doctrine, increases as we proceed, and most commonly leads to 

 very * serious difficulties. We experience it in regard to this 

 subject, for philosophers, having gone astray, almost on the very 

 threshold of natural science, ascribing levity and upward 

 motion to the two superior elements, saw themselves afterwards 

 much troubled to account for the natural descent of heavy 

 bodies being quicker towards the end than at its commence- 

 ment. The variety of opinions that we find in authors on this 

 question, sufficiently demonstrates their perplexity; I, who study 

 brevity, have no intention to bring them forward ; they who like 

 it, may read a good number of them, in the "Natural Principles" 

 of Pereriust, a judicious philosopher, in which, after having 

 quoted, he learnedly refutes them, and embraces one that he 

 professes to acquiesce in, till he finds a better: of this I shall 

 say something hereafter, as we go along, to shew that it is not so 

 true as it is plausible. I now present my own, which I have with 

 much study devised, in support of the preceding demonstrations. 

 The quickness of motion of a heavy body increases from the 

 beginning to the end, by the increase of elementary matter 

 which presses on it, and by the continual multiplication of the 

 impulse which it gives it in its descent. A figure will make my 

 assertion clearer, (see fig. 1.) Let A A be the heavens; B B 

 the earth; C its centre; D an iron ball, descending towards the 

 earth; E the same descending lower; F the same again, in the 

 middle of its descent; G the same, near the end of it; H H 

 two lines, drawn from the centre of the earth, to the heavens, 

 and touching the ball at D in the two extremities of its diame- 

 ter; II two similar lines, touching the ball at E; K K, two 



* Literally "tftoi'wy," "espineuses." 



t Benedict! Pererii de communibus omnium rerum naturaliun: prin- 

 cipiis et affectionibus. Libri XV. in 4to. Parisiis, \-,8Q- 



