in the Magnetic Forces of the Earth. 39 



Diurnal Variations of the Horizontal Magnetic Intensity. 



The formula which the Thermal Theory of Magnetism has 

 furnished for the horizontal magnetic intensity of a place is 



H«CT 



in which C is a constant, and T the mean annual temperature of 

 the place. This formula is equivalent to the statement that the 

 mean horizontal magnetic force is proportional to the mean tem- 

 perature. We have therefore to compare the diurnal variations 

 of the horizontal force with the diurnal variations of the tern- 

 perature of the place. The theory strictly requires that the com- 

 parison should be with the daily variations in the absolute amount 

 of sensible heat near the earth's surface, but we know, from the 

 laws of the heating and cooling of bodies, that when the tem- 

 perature is rising at its surface the earth is, in general, receiving 

 more heat than it loses, and that when the temperature at the 

 surface is falling, it is losing more heat than it receives — so that 

 a rise or fall of surface temperature will in general indicate an 

 increase or decrease of the total amount of heat. This consider- 

 ation suffices for the enquiry which first arises, viz. : whether the 

 horizontal force increases and decreases with the total amount of 



heat. A good set of observations of the daily variations of the 



temperature below the surface would be required for a thorough 

 and minute discussion of the subject before us, but the facts 

 already known and the established theory of the heating and 

 cooling of bodies appear to supersede the necessity of such obser- 

 vations, except when the attempt is to be made to obtain precise 

 quantitative determinations. 



We will begin by comparing the curve showing the mean daily 

 variation of the horizontal intensity at Philadelphia for the year 

 1844 (fig. 3), with the curve showing the mean daily variation of 

 temperature for the same year (fig. 8). 



It will be observed that "the horizontal intensity attains its max- 

 imum at from 15^ to 16 h , or from 3 to 4 p. m., and that the max- 

 imum temperature occurs at the same hour; — also that the hori- 

 zontal intensity increases with the temperature in the forenoon 

 (after 10h), and decreases with it in the afternoon and evening. 

 The same correspondences are observable in the curves for the 

 other years and for the quarters of years, with the single qualifi- 

 cation, that the maximum of horizontal intensity sometimes occurs 

 an hour or two later than the maximum of temperature. They 

 ar e an indication that the daily variation of temperature is, in all 

 probability, at least one cause of the variation of horizontal inten- 

 sity. When we compare the curves still farther we notice the 

 following points of difference between them. 1. The horizon- 

 tal force increases during the latter half of the night until 5 to 

 8 a. m., and then decreases until 10 a. m., whereas the temperature 



