1865.] 159 [Chase. 



A. P. S., vol. xiii, Art. VI), the modifying magnetic influence of 

 rotation (ibid., pp. 120, 129), barometric tides (ibid., pp. 128-7), 

 winds (ibid., p. 121, and Proc. A. P. S., x, 104), thermal changes 

 (Gen. Sabine, loc. cit.), and lunar attraction (Proc. A. P. S., ix, 

 434-8; Trans. A. P. S., xiii, 129; and Gen. Sabine's Diagrams, 

 Toronto Obs., vol. iii, plate 2), and the probable, if not certain, de- 

 pendence of the variations of long period upon trade-winds (supra, 

 Exp., 19-24) and planetary positions (Sabine and Wolf, loc. cit.), 

 the hypothesis of any peculiar magnetic aether, electric currents, or 

 specific solar and lunar magnetism, to explain the normal perturba- 

 tions of the needle, appears to be entirely superfluous and unphilo- 

 sophical. Every particle of the earth's atmosphere is continually 

 receiving and imparting the heat which is radiated from the earth 

 and sun, its specific gravity constantly changing in such manner as 

 to produce incessant rapid and short oscillations, both in the planes 

 of the earth's thermal meridians* and in the great circles which 

 pass through the centres of the earth and sun. The consequent dis- 

 turbance of equilibrium, which is still farther increased by the con- 

 densation of vapor, the sun's direct attraction, and the earth's rota- 

 tion, is counterbalanced by terrestrial attraction, acting most forcibly 

 where the sun is in the horizon, and with the least relative eflfieiency 

 when the sun is in the zenith (or at noon in the summer solstice, 

 provided the station is extra-tropical). 



The pressure thus exerted varies from lb. to 151b. per square inch. 

 Taking the mean (7jlb. per square inch or 10801b. per square foot) 

 as the average equilibrating tendency, we have a force nearly fifteen 

 times as great as that which produces, and more than twenty-eight 

 times as great as that which is produced by, a violent hurricane. 

 (Enc. Britan., 8th edit., xiv, 647.) Only an insignificant portion of 

 this mighty energy is exerted in the production of the various at- 

 mospheric currents, the remainder being quietly transmitted from 

 molecule to molecule, and manifesting itself in barometric, f magnetic, 

 and other meteorological perturbations. The adequacy of our sup- 

 posed cause will therefore hardly be doubted ; and, since its pene- 

 trating, pervading influence can be impeded by no material shield or 

 screen, the demonstration of a correlation of heat and magnetism 



* The earth's most powerful radiation is vertical, or in radial lines; next in 

 intensity is the radiation towards the thermal pole.s, or along the thermal meri- 

 dians; on the isothermal parallels, the radiation is comparatively insignificant. 



•j- The morning and evening maxima and the noon minimum of eifective 

 pressure, combined with rotation, are the principal causes of the daily barometric 

 tides. 



