224 PROGRESS OF SCIENCE. 



wire, indicates on the dial the angle, the elasticity, and 

 the force of torsion of the wire facts of extreme importance 

 in metallurgy, in mechanics, and in physics. The torsion 

 balance speedily became essential in numerous branches of 

 inquiry. By means of it Coulomb determined the LAWS 

 OF THE TORSION OF WIRES (or elasticity of torsion), one 

 of the properties of solids which had not been investigated, 

 as elasticity of traction, of flexure, tenacity, ductility, and 

 hardness had been or were to be by Wollaston, Savart, 

 Kohlrausch, and others. By means of the torsion balance 

 Cavendish was able to discover the mean density of our 

 globe to be five and a half times that of water, thereby 

 practically determining the weight of the earth. The balance 

 of torsion applied in the galvanometer gives a delicate test 

 of electrical forces ; it is indispensable in the thermo-electric 

 pile ; the most delicate investigations in the theories of 

 magnetism, electricity, heat are due to it ; so that Coulomb, 

 by his invention, not only discovered three sets of important 

 laws himself, but caused rapid progress in several depart- 

 ments of physics. 



1737 98. Galvani discovered that the hind-legs of a 

 frog touched by two dissimilar metals were convulsed, and 

 as an interpretation of the result he believed it to be caused 

 by ANIMAL ELECTRICITY as distinct from ordinary electricity 

 (1789). Since then Nobili, Mateucci, Du Bois-Reymond, and 

 others have shown that in living animals' muscles and nerves 

 there exist electric currents (whose laws they investigated) ; 

 so that Galvani was not wholly mistaken. His error did not 

 consist in his belief that there was electricity in animals, for 

 there is, but that there were two essentially different kinds 

 of electricity, apart from what we call positive and negative 

 -electricity. 



17401810. Montgolfier (Brothers) invented AEROSTA- 

 TION 1783 (hot air balloons), soon made safer by hydrogen 

 gas and coal gas being substituted for hot air. One of the 

 most celebrated ascents was made by Mr. Glaisher, who rose 

 {1861) to 'an elevation of 36,000 feet. The balloon, which 

 is probably to become a new means of intercommunication, has 

 already proved useful in warfare, and has been serviceable 



