250 A SHORT HISTORY OF SCIENCE 



vibrating cords and plates, discovering the dependence of harmony 

 on the ratio of the rates of vibration, and the relations of length, 

 thickness, and tension of cords. He explains resonance and dis- 

 sonance. He assumes light to have a finite velocity, but does 

 not succeed in measuring it. 



Let each of two persons take a light contained in a lantern, or 

 other receptacle, such that by the interposition of the hand, the one 

 can shut off or admit the light to the vision of the other. Next let 

 them stand opposite each other at a distance of a few cubits and prac- 

 tice until they acquire such skill in uncovering and occulting their 

 lights that the instant one sees the light of his companion he will un- 

 cover his own. After a few trials the response will be so prompt that 

 without sensible error the uncovering of one light is immediately 

 followed by the uncovering of the other, so that as soon as one exposes 

 his light he will instantly see that of the other. Having acquired skill, 

 at this short distance, let the two experimenters, equipped as before, 

 take up positions separated by a distance of two or three miles and 

 let them perform the same experiment at night, noting carefully 

 whether the exposures and occultations occur in the same manner as 

 at short distances ; if they do, we may safely conclude that the prop- 

 agation of light is instantaneous ; but if time is required at a distance 

 of three miles which, considering the going of one light and the coming 

 of the other, really amounts to six, then the delay ought to be easily 

 observable. If the experiment is to be made at still greater distances, 

 say eight or ten miles, telescopes may be employed, each observer 

 adjusting one for himself at the place where he is to make the experi- 

 ment at night ; then although the lights are not large and are 

 therefore invisible to the naked eye at so great a distance, they can 

 readily be covered and uncovered since by aid of the telescopes, once 

 adjusted and fixed, they will become easily visible. . . . 



He seeks to apply to astronomical phenomena the new discoveries 

 in magnetism. 



Everywhere the mathematical and inductive method became 

 manifest in this man. Almost all domains of science received there- 

 from the most powerful impulse. And above all the whole field of 

 science was freed from the outgrowths of metaphysical modes of 

 thought with which it had been previously so overrun. Galileo's 



