MODERN SCIENCE. 223. 



electrified ; again, glass with a rugged surface becomes 

 negatively charged by rubbing with silk. Thus, electric 

 phenomena present a field of investigation absolutely bound- 

 less. It is seen by these examples that the terms adopted to 

 indicate the currents elucidate nothing as to the nature of 

 electricity, and are simply conventional labels used for our 

 convenience. It is needless to insist upon the importance of 

 Franklin's discoveries ; they caused GREAT STRIDES within a 

 short time. 



1732 181 1. Maskelyne determined, in 1774, the DENSITY 

 (or weight) OF THE EARTH to be five times that of water 

 by an experiment suggested by Newton, and which con- 

 sisted in ascertaining the attraction exerted by a mountain, 

 and comparing it with that of the earth as a whole. This 

 was effected by means of the plumb-line. Maskelyne found 

 the mountain . SCHIEHALLION (Perthshire) made the plumb- 

 line deviate from, instead of pointing straight to, the centre 

 of the earth. The deflection from the perpendicular being 

 calculated, and Hutton having determined the size and 

 weight of the mountain by a series of observations and 

 measurements of its materials, the density of the earth was 

 found out, and it has been confirmed by Cavendish, Baily, 

 Sir Henry James, and Sir Edward Sabine and others who 

 have investigated the question. But the latest verifications 

 (in 1893) give a mean of 5*4 instead of 5. 



17361806. Coulomb discovered the LAWS OF ELECTRIC 

 ATTRACTION and repulsion, and the method of determining 

 magnetic declination the law, that is, of decrease with 

 distance, f( Magnetic attractions and repulsions are inversely 

 as the square of the distances " ; proving it by two methods, 

 that of the TORSION BALANCE and that of oscillation. He 

 also showed (1802) that magnets act upon all bodies an 

 important discovery which Faraday will push much further. 

 Coulomb's main services to science lie in the invention of 

 the TORSION BALANCE just mentioned an apparatus of 

 extreme simplicity, consisting of a suspended wire, at the 

 lowest end of which a ball, to which an index (or hand) is 

 fixed, revolves over a graduated dial : the rotation of the 

 ball and hand, upon a slight twist being given to the 



