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 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 metals 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. 



