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sured by the sine of that deviation, — or, since the deviation is 

 small, by the angle of deviation itself, or by the ordinate 

 of the diurnal curve ; and the sum of all these forces 

 throughout the day, or the integral of the diurnal action, is 

 measured by the area of the diurnal curve. Dr. Lloyd has 

 computed this area for the several months of the year; and, 

 on comparing it with the corresponding area of the diurnal 

 curve of temperature, he finds that there is a marked agreement 

 in the course of the two functions. The slight dissimilarities 

 which exist between them may be accounted for by the cir- 

 cumstance, that it is to the heating power of the sun, exerted 

 upon the earth's surface, and not upon its atmosphere, that 

 we must ascribe the changes of declination ; and the author 

 feels assured, that as soon as we are in possession of data, re- 

 specting the diurnal changes of temperature of the earth's 

 surface, sufficient to institute a comparison similar to that 

 now made with the temperature of the air, the agreement of 

 the laws will be found to be still more complete. 



Annual Variation. 



The annual variation of the declination was discovered by 

 Cassini, in 1786. It appeared from the observations of Cas- 

 sini, that the north pole of the magnet moved to the east 

 during three months, viz., from the vernal equinox to the 

 summer solstice ; and, consequently, the declination dimi- 

 nished. During the remaining nine months, viz., from the 

 summer solstice to the vernal equinox, it moved to the west, 

 and the declination increased. The increase, during the nine 

 months, preponderated over the decrease, which took place 

 during the remaining three ; and thus the declination was 

 greater at the close of the year than at the commencement. 

 This excess is the yearly amount of the secular change, 

 which was then additive. 



Although the law of the annual variation may be traced 

 u 2 



