MODERN SCIENCE. 241 



174,800 miles per second, slightly less than the velocity of 

 light. Wheatstone also invented the polar clock, for as- 

 certaining the hour by observing the amount of polarisation 

 of the sky. 



i8o4(?) Steinheil discovered THE EARTH to be a 

 CONDUCTOR of electric currents a discovery which saved 

 a back- wire in electric telegraphy; constructed (1837) a 

 telegraph (the current of which was produced by an electro- 

 magnetic machine) just after Wheatstone's telegraph had 

 come out. Steinheil was the first to make a three-prism 

 spectroscope. Fraunhofer had made his with one prism 

 only ; Bunsen and Kirchhofif, after Steinheil, used four, 

 arranged at convenient angles between two telescopes an 

 arrangement which constitutes the spectroscope, although 

 improved forms have come out since. We may conclude 

 our notice of Steinheil by stating that he should be re- 

 membered as one of the great physicists of Germany, by 

 the side of Jacobi, Bunsen, Kirchhoff, and Helmholtz. 



1810 1878. Regnault found the method of estimating 

 the density of gases; showed four methods of ascertaining 

 their expansion. an investigation for which he will always 

 be remembered; determined the COEFFICIENTS OF EXPAN- 

 SION another inquiry involving years of patient and skilful 

 labour ; devised a method, and made an apparatus for 

 determining specific heat ; found the specific heat of air to 

 be 0*2377 ; found that air, nitrogen, hydrogen, carbonic acid 

 deviate from Boyle's law, and become less and less com- 

 pressible beyond a given pressure a fact which Despretz, 

 Dulong, Arago also noticed ; so that the law is only ap- 

 proximately true, though true up to a certain pressure in 

 all cases. Regnault also devised a method of discovering 

 the tension of aqueous vapour. He invented a BAROMETRIC 

 MANOMETER for measuring pressures of less than one atmos- 

 phere, and invented also a hygrometer. As a chemist, 

 he investigated the action of chlorine upon chlorhydric 

 ether, and carried out countless successful experiments. In 

 him the physicist and the chemist were blended in an equal 

 degree. 



1811 1 8 . Bunsen, in collaboration with KIRCHHOFF, 



R 



