Chase.] ^-^Q [May 19. 



miles more distant from the earth's centre than the other. If the 

 differences of vapor, temperature, barometric pressure, force and di- 

 rection of wind, atmospheric electricity, &c., did not so complicate 

 the problem as to discourage even the most sanguine experimenter 

 from any attempt at solution, any result that could be obtained under 

 such circumstances would give little general satisfaction. 



It is possible, however, that the end, which we should vainly 

 strive to reach directly, may be indirectly attained. Indeed, the 

 various stages of an indirect road have long been known, but we 

 have not been able to compare them by any common measure. The 

 motion of gravity, by percussion or the obstruction of simple fall, has 

 been repeatedly converted into the motion of heat; and the motion 

 of heat, by the thermo-electric pile, has been converted into the mo- 

 tion of magnetism. The experiments of Barlow, Coulomb, Kupffer, 

 and Christie,* on the influence of heat upon the magnet, furnish 

 data that may lend some aid to any investigator who seeks to ascer- 

 tain the precise value and modification of each force, in these succes- 

 sive conversions. 



But I look most hopefully to researches that are based upon dif- 

 ferences of specific gravity. Even the experiments of Barlow and 

 others, to which I have just referred, as well as the electro-magnetic 

 currents which are generated by chemical solution, involve such dif- 

 ferences; the thermal aerial currents which harmonize with and in- 

 crease the effects of simple gravitation towards the sun, are caused 

 solely by the greater centripetal tendency of the cold, dense air 

 which has the greatest specific gravity; and the recent investigations 

 in thermo-dynamics, together with the experiments of Fusinieri and 

 Peltier,"!" confirm the natural conviction that the imponderable agents 

 can only be manifested through their influence on ponderable mat- 

 ter, and, therefore, under tendencies to equilibrium with the force of 

 gravitation. I already find a curious approximate coincidence, to 

 which I attach little importance so long as it is unsupported by cor- 

 roborative evidence, but I refer to it as an indication of the very 

 character that we might reasonably expect, and one that may possi- 

 bly become valuable in the course of future research. The last 

 edition of the Encyclopaedia Britannica, Article "Heat," gives for 

 the expansion and consequent diminution of specific gravity between 

 32° and 212° Fahr., of 



Iron, g|^ 



Air, g 



* See Enc. Britan., 8th ed., XIV, 35—39. 

 f Taylor's Scientific Memoirs, III, 394. 



