Page:Scientific Memoirs, Vol. 2 (1841).djvu/69

Rh {| align=center
 * || 1834–1835. || 1835–1836. || 1836–1837. || Mean.
 * April || 10 56·9 || 13 33·9 || 17 9·7 || 13 53·5
 * May || 10 47·2 || 13 3·7 || 16 36·4 || 13 29·1
 * June || 10 18·8 || 11 44·9 || 15 17·3 || 12 27·0
 * July || 10 21·5 || 10 34·8 || 15 31·8 || 12 9·4
 * August || 10 22·9 || 12 44·4 || 16 2·6 || 13 3·3
 * September || 9 33·9 || 11 6·4 || 14 45·0 || 11 48·4
 * October || 7 28·8 || 9 42·3 || 12 58·8 || 10 3·3
 * November || 25·9 || 7 34·2 || 7 33·3 || 6 51·1
 * December || 3 37·9 || 4 53·2 || 6 33·1 || 5 1·4
 * January || 4 22·9 || 5 32·2 || 10 10·9 || 6 42·0
 * February || 5 25·9 || 7 48·5 || 8 52·7 || 7 22·4
 * March || 10 7·7 || 12 15·0 || 13 20·0 || 11 54·2
 * Mean || 8 14·2 || 10 2·8 || 12 54·3 || 10 23·8
 * }
 * October || 7 28·8 || 9 42·3 || 12 58·8 || 10 3·3
 * November || 25·9 || 7 34·2 || 7 33·3 || 6 51·1
 * December || 3 37·9 || 4 53·2 || 6 33·1 || 5 1·4
 * January || 4 22·9 || 5 32·2 || 10 10·9 || 6 42·0
 * February || 5 25·9 || 7 48·5 || 8 52·7 || 7 22·4
 * March || 10 7·7 || 12 15·0 || 13 20·0 || 11 54·2
 * Mean || 8 14·2 || 10 2·8 || 12 54·3 || 10 23·8
 * }
 * February || 5 25·9 || 7 48·5 || 8 52·7 || 7 22·4
 * March || 10 7·7 || 12 15·0 || 13 20·0 || 11 54·2
 * Mean || 8 14·2 || 10 2·8 || 12 54·3 || 10 23·8
 * }
 * Mean || 8 14·2 || 10 2·8 || 12 54·3 || 10 23·8
 * }

It will be perceived that, not only in the mean values, but also in each of the separate years, the difference has been smallest in December; and this is what we might expect, as those changes which vary according to the time of the day must necessarily be ascribed to the action of the sun, although as yet we know not how this action is effected. It may at first appear surprising, on the other hand, that the differences are not greatest at the time of the summer solstice, but appear smaller in June and July than in April, May, and August, especially as the coincidence of all three years in this circumstance affords a presumption that it is not accidental. It must not, however, be overlooked, that in the months immediately following the solstice, the time of the minimum of the declination is earlier, and therefore the whole increase would be sensibly greater than the change reckoned from 8 o'clock.

It is further observable that in each month the differences are greater in the second year than in the first; and again, in the third year greater than in the second. But these differences are by far too great in amount to admit of our considering them as parts of a secular increase, and it is rather to be expected that by continuing the observations for several years we shall not fail to discover a fluctuation. But, in any case, we hereby learn that one year may differ from another in respect to the effect of the sun on the earth's magnetism, somewhat in the same way that one summer or one winter differs from another in temperature. On this account also we shall only arrive at an accurate determination of the mean values by observations continued for several years.