Chase.] ^'JQ [February 



In the daily relations of heat to magnetism, there is an 

 anomaly, analogous to that which I pointed out in the bar- 

 ometric tides. For if we take the natural division of the daj^, 

 the variations of declination are greatest when the sun is above 

 the horizon, or during the warmest half of the twent^^-four hours, 

 but if we divide bj^ the meridian, they are greatest in the cool- 

 est half, or the morning hours. So in the j^ear, if we divide at 

 the equinoxes, they are greatest at aphelion, when the least heat 

 is received from the sun, but if we divide at the solstices, 

 they are greatest in summer, when the local heat is greatest. 

 It may be impossible to explain these discrepancies so simplj' 

 as the analogous ones in the barometric tides, but the same ex- 

 planation will doubtless serve, to some extent, in both cases. 

 The following points of coincidence maj^, perhaps, contribute 

 towards a proper reference of the several disturbed motions to 

 the disturbing body. 



At syzygy and at high tide, the disturbing attraction is di- 

 rectly opposed to terrestrial attraction ; at summer and during 

 the day-hours, there is the same opposition, intensified by the 

 accompanying heat disturbance ; at the equinoxes the change 

 of gyroscopic plane is most rapid ; at sunrise, and at the sec- 

 ond equinox, the motion (of rotation or of revolution, respect- 

 ively), is added to that which is due to solar heat and attrac- 

 tion ; in the aphelion semester, the land hemisphere being 

 most directly exposed to the sun, the dail}^ fluctuations of 

 temperature are consequently greater, and the relative effect 

 of those fluctuations is greater, from the fact stated by Sabine 

 (he. cit.)^ that the earth's total magnetic force is then more fee- 

 ble ; the expansion of vaporization, and the collapse of conden- 

 sation are both sources of sudden and violent agitation ; at full 

 moon the night disturbance is greater than at new moon, and 

 the average disturbance of the entire twent3^-four hours is 

 therefore greater. 



The following summary gives the principal points of the 

 generally accepted theory of terrestrial magnetism, together 



.030 = . 067 in. ; and an annual range from + .082 to — .053 = .135 in. 

 These values give the barometrically formulated distance of the sun 

 (•oM) "^ X 22,738,900 = 92,313,146 miles, a curiously close approximation 

 to the 92,380,000 miles (with a probable error of ± 136,000 miles) of 

 Newcomb's recent estimate (App. II, "Washington Ast. Obs. for 1865). 



