12 



AMPLITUDE OF THE SOLAR-DIURNAL VARIATION 



the season for which the diagram is constructed. Thus, at the hour of 6 or 7 in 

 the morning, the annual variation is a maximum, disappearing at a quarter before 

 10 A. M., and reaching a second (secondary) maximum value at 1 P. M. It 

 almost disappears soon after 5 P. M., and a third still smaller maximum is reached 

 after 9 P. M. Half an hour before midnight, the annual variation again disap- 

 pears. At (and before and after) the principal maximum, between 6 and 7 in the 

 morning, the annual variation causes the north end of the magnet to be deflected 

 to the east in summer and to the west in winter; at 1 P. M., the deflections are to 

 the west in summer and to the east in winter. The range of the diurnal motion 

 is thus increased in summer and diminished in winter; the magnet being deflected 

 in summer more to the east in the morning hours, and more to the west in the 

 afternoon hours, or having greater elongations than it would have if the sun moved 

 in the equator. In winter, the converse is the case. The range of the annual 

 variation from summer to winter is about 3'.0, and its daily range about 2'. 6 at 

 Philadelphia. 



(C). Comparative Diagram op the Semi-Annual Deflection of the Solar-Diurnal Variation. 



i north 



1 south 



of the equator 



{=} 



9 Noon 15 

 Mean local time. 



9 Noon 15 

 Mean local time. 



The next diagram (C) has been projected in order to illustrate the semi-annual 

 inequality of the diurnal variation at four principal magnetic stations. 1 The 

 general features of the Philadelphia curve most nearly resemble those exhibited 

 in the St. Helena curve ; and, relatively, the Toronto and Hobarton curves appear 

 to represent rather extreme than normal shapes. The Philadelphia and St. Helena 



' The annual variation of the diurnal motion has been made the subject of a particular discussion by 

 General Sabine, in papers presented to the British Association and the Royal Society. See Reports of 

 the British Association, 1854, pp. 355-368, and Transactions of Royal Society, May 18, 1854, pp. 

 67-82; also, article XXVIII, Philosophical Transactions, 1851. 



