348 THE PRINCIPLES OF SCIENCE. 



heights of the lunar and solar tides, Newton roughly 

 estimated the comparative forces of the moon s and sun s 

 gravity at the earth k . 



A few years ago it might have seemed impossible that 

 we should ever measure the velocity with which a star 

 approaches or recedes from the earth, since the apparent 

 position of the star is thereby unaltered. But the spec 

 troscope now enables us to detect and even measure such 

 motion with considerable accuracy, by the alteration which 

 it causes in the apparent rapidity of vibration, and conse 

 quently in the refrangibility of rays of light of definite 

 colour. And while our estimates of the lateral move 

 ments of stars depend upon our very uncertain know 

 ledge of their distance, the spectroscope gives the motion 

 in another direction in absolute quantity, irrespective of 

 all other quantities known or unknown, excepting the 

 motion of the earth itself 1 . 



The rapidity of vibration for each musical tone, hav 

 ing been accurately determined by comparison with the 

 Syren (p. 12), we can use sounds as indirect indications of 

 rapid vibrations. It is now known that the contraction of 

 a muscle arises from the periodical contractions of each 

 separate fibre, and from a faint sound or susurrus which 

 accompanies the action of a muscle, it is inferred that 



each contraction lasts for about &quot; of a second. Minute 



300 



quantities of radiant heat are now always measured indi 

 rectly by the electricity which they produce when falling 

 upon a thermopile. The extreme delicacy of the method 

 seems to be due to the power of multiplication at several 

 points in the apparatus. The number of elements or junc 

 tions of different rnetals in the thermopile can be increased 



Principia, bk. iii. Prop. 37, Corollaries, 2 and 3. Motte s trans 

 lation, vol. ii. p. 310. 



1 Roscoe s, Spectrum, Analysis, ist ed. p. 296. 



