241 
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 Jaws 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 
u2 
