J 04 DISSERTATION SECOND. [paht i. 



and geometrical reasoning to explain the phenomena of 

 nature. 



As early as 1592 he published a treatise, della Sciensa 

 Mechanica, in which he has given the theory, not of the 

 lever only, but of the inclined plane and the screw ; and 

 has also laid down this general proposition, that mechani- 

 cal engines make a small force equivalent to a great one, 

 by making the former move over a greater space in the 

 f*ame time than the latter, just in proportion as it is less. 

 No contrivance can make a small weight put a great one in 

 motion, but such a one as gives to the small weight a velo- 

 city which is as much greater than that of the large weight, 

 as this last weight is greater than the first. These gene- 

 ral propositions, and their influence on the action of ma- 

 chinery, Galileo proceeded to illustrate with that clearness, 

 simplicity, and extent of view, in which he was quite un- 

 rivalled ; and hence, I think, it is fair to consider him as 

 the first person to whom the mechanical principle, since 

 denominated that of the virtual velocities, had occurred 

 in its full extent. The object of his consideration was the 

 action of machines in motion, and not merely of machines 

 in equilibrio, or at rest ; and he showed, that, if the effect 

 of a force be estimated by the weight it can raise to a 

 iven height in a given time, this effect can never be in- 

 creased by any mechanical contrivance whatsoever. 



In the same treatise, lie lays it down as a postulate (snp- 

 posisione,) that the effect of one heavy body to turn an- 

 other round a centre of motion, is proportional to the per- 

 pendicular drawn from that centre to the vertical passing 

 through the body, or in. general to the direction of the 



rcc. This proposition he states without a demonstration, 

 and passes by mean3 of it to the oblique lever, and thence 

 to the inclined plane. To speak strictly, however, the 



inonstrations with respect to both these last are incom- 



