«ct. in] DISSERTATION SECOND. 109 



relatively to one another ; that the question must be more 

 deeply inquired into; and that, without this, the evidence 

 on opposite sides could not be fairly and accurately corn- 

 pared. Thus it was, at a very fortunate moment, that 

 Galileo made his discoveries in Mechanicks, as they were 

 rendered more interesting by those, which, at that very 

 time, he himself was making in Astronomy. The system 

 of Copernicus had, in this manner, an influence on the 

 theory of motion, and, of course, on all the parts of na- 

 tural philosophy. The inertia of matter, or, the tenden- 

 cy of body, when put in motion, to preserve the quantity 

 and direction of that motion, after the cause which impress- 

 ed it has ceased to act, is a principle which might still 

 have been unknown, if it had not been forced upon us by 

 the discovery of the motion of the earth. 



The first addition which was made to the mechanical 

 discoveries of Galileo, was by Torricelli, in a treatise De 

 Motu Gr avium naturaliter descendentiwn et projectorum. ' 

 To this ingenious man we are indebted for the discovery 

 of a remarkable property of the centre of gravity, and a 

 general principle with respect to the equilibrium of bodies. 

 It is this : If there be any number of heavy bodies con- 

 nected together, and so circumstanced, that by their mo- 

 tion their centre of gravity can neither ascend nor descend, 

 these bodies will remain at rest. This proposition often 

 furnishes the means of resolving very difficult questions in 

 mechanicks. 



Descartes, whose name is so great in philosophy and 

 mathematicks, has also a place in the history of mechani- 

 cal discovery. With regard to the action of machines, 

 he laid down the same principle which Galileo had estab- 

 lished, — that an equal effort is necessary to give to a weight 



Vitae Jtalorum IHustrium, vol. J, p. 347. 

 14 



