DIP AND INTENSITY. 247 



balances itself horizontally. As we proceed from this 

 lino towards either pole the dip increases, until, at the 

 north and south poles, the needle takes a vertical 

 position. The intensity of the earth's magnetism is 

 also found to vary with the position, and to increase in 

 a proportion which corresponds very closely with the 

 dip. But the intensity is not a function of the dip, 

 and the lines of equal intensity, isodynamic lines, are not 

 parallel to those of equal dip. We have already re- 

 marked on the diurnal variation of the declination of 

 the needle ; we know, also, that there exists a regular 

 monthly and daily change in the magnetic intensity. 

 The greatest monthly change appears when the earth is 

 in its perihelion and aphelion, in the months of Decem- 

 ber and June, a maximum then occurs ; and about 

 the time of the equinoxes a minimum is detected.* 



The daily variation of intensity is greatest in the 

 summer, and least in the winter. The magnetism is 

 generally found to be at a minimum when the sun is 

 near 'the meridian ; its intensity increasing until about 

 six o'clock, when it again diminishes.! 



What striking evidences all these well- ascertained 

 facts give of the dependence of terrestrial magnetism on 

 solar influence ! and in further confirmation of this 

 view, we find a very remarkable coincidence between 

 the lines of equal temperature the isothermal lines, 

 and those of equal dip and magnetic intensity. 



Sir David Brewster first pointed out that there were 

 in the northern hemisphere two poles of maximum 

 cold ; these poles agree with the magnetic points of 

 convergence; and the line of maximum heat, which 

 does not run parallel to the earth's equator, is nearly 

 coincident with that of magnetic power. Since See- 



* Hansteen ; as above. 



f On the effects of temperature on the intensity of magnetic forces, 

 and on the diurnal variations of the terrestrial magnetic intensity ; 

 by Samuel Hunter Christie, Esq. Philosophical Transactions, 

 vol. cxv. 1825. 



