ch. viit.] ORGANIZATION OF THE SCIENCES. 205 



tion of light enabled him to make the first step towards 

 ascertaining the motions of the stars. It was thus when 

 Cavendish's torsion-balance experiment determined the specific 

 gravity of the earth, and so gave a datum for calculating the 

 specific gravities of the sun and planets. It was thus when 

 tables of atmospheric refraction enabled observers to write 

 down the real places of the heavenly bodies instead of their 

 apparent places. It was thus when the discovery of the 

 different expansibilities of metals by heat, gave us the means 

 of correcting our chronometrical measurements of astronomical 

 periods. It was thus when the lines of the prismatic 

 spectrum were used to distinguish the heavenly bodies that 

 are of like nature with the sun from those which are not. It 

 was thus when, as recently, an electro-telegraphic instrument 

 was invented for the more accurate registration of meridional 

 transits. It was thus when the difference in the rates of a 

 clock at the equator and nearer the poles, gave data for 

 calculating the oblateness of the earth, and accounting for 

 the precession of the equinoxes. It was thus — but it is 

 needless to continue. We have already named ten cases in 

 which the single science of astronomy has owed its advance 

 to sciences coming after it in Comte's series. Not only its 

 secondary steps, but its greatest revolutions have been thus 

 determined. Kepler could not have discovered his celebrated 

 iaws, had it not been for Tycho Brahe's accurate observations ; 

 £.nd it was only after some progress in physical and chemical 

 science that the improved instruments with which those 

 observations were made, became possible. The heliocentric 

 theory of the solar system had to wait until the invention of 

 the telescope before it could be finally established. Nay, 

 even the grand discovery of all — the law of gravitation — 

 depended for its proof upon an operation of physical science, 

 the measurement of a degree upon the earth's surface. Now 

 this constant intercommunion, here illustrated in the case of 

 one science only, has been taking place with all the sciences 



