Page:Scientific Memoirs, Vol. 2 (1841).djvu/61

Rh to the following scheme,—taking the approximate value at 43ˢ, dividing it into four parts, and deriving the final result from five partial results. The first column contains the times of notation; the second the times to which the partial results severally correspond: it is obviously unimportant that the final result, if accurately taken, falls at 0ʰ 5′ 0″·1. If the final result is based on six partial results, then the following scheme is adopted: The advantage of this modification in the mode of observing is most evident, when it is desired to follow the course of the magnetic declination more closely than at intervals of 5 minutes. These intervals, sufficient for the ordinary progress of the changes of declination, are in fact too large for the examination of the greater and more rapid changes; and it was in this view, and because shorter intervals could scarcely be generally adopted throughout the terms of 24 hours, that subordinate terms were added, each of two hours' duration, in which the observations were to be made at intervals of 3 minutes. As, however, the subordinate terms occasioned some difficulties, and, as they have hitherto brought to light but few phenomena of corresponding importance, it has been decided to discontinue them. The same object can be attained even more effectually in another manner. The rule of observing at every 5 minutes is retained; but if at any time rapid changes of declination occur, the