218 THE GENESIS OF SCIENCE. 



does not habitually supply the means, and suggest ih( 

 modes of investigation, as in the sciences dealing with time 

 extension, and force ; and partly to the fact that the great 

 majority of the materials with Avhich chemistry deals, in 

 stead of being ready to hand, are made known only by the 

 arts in their slow growth ; and partly to the fact that even 

 when known, their chemical properties are not self-exhibit 

 ed, but have to be sought out by experiment. 



Merely indicating all these considerations, however, let 

 us go on to contemplate the progress and mutual influence 

 of the sciences in modern days ; only parenthetically no 

 ticing how, on the revival of the scientific spirit, the suc 

 cessive stages achieved exhibit the dominance of the same 

 law hitherto traced how the primary idea in dynamics, a 

 uniform force, was defined by Galileo to be a force which 

 generates equal velocities in equal successive times how 

 the uniform action of gravity was first experimentally de 

 termined by showing that the time elapsing before a body 

 thrown up, stopped, was equal to the time it took to fall 

 how the first fact in compound motion which Galileo ascer 

 tained was, that a body projected horizontally will have a 

 uniform motion onwards and a uniformly accelerated mo 

 tion downwards ; that is, will describe equal horizontal 

 spaces in equal times, compounded with equal vertical in 

 crements in equal times how his discovery respecting the 

 pendulum was, that its oscillations occupy equal intervals 

 of time whatever their length how the principle of virtual 

 velocities which he established is, that in any machine the 

 weights that balance each other, are reciprocally as their 

 virtual velocities ; that is, the relation of one set of weights 

 to their velocities equals the relation of the other set of 

 velocities to their weights ; and how thus his achieve 

 ments consisted in showing the equalities of certain magni 

 tudes and relations, whose equalities had not* been pre 

 viously recognised. 



