Excerpt from Observations Made at the Magnetical and Meteorological Observatory at St. Helena, Vol. 2: With Discussions of the Observations at St. Helena, the Cape of Good Hope, the Falkland Islands, Carlton Fort in North America, and Pekin; 1844 to 1849

Declinometer (continued from Vol. I. P. - The declinometer remained undisturbed in the double rectangular box in which the magnet was enclosed in July 1843, and with the adjustment made in March 1844, until the termination of the duties of the Observatory by the recall of the detachment of the Royal Artillery in May 1849. The hourly series of observations commenced in September 1842 was maintained until the end of August 1847, completing five years, during which the declinometer was undisturbed, except on two occasions, viz., firstly on July 24, 1843, when the cylindrical wooden case in which the magnet had been previously suspended was replaced by a double rectangular box, as a more effectual provision against currents of air; and secondly in January 1844, when the suspension thread which had been in use since February 1842 broke, and was replaced by a new thread, with which the observations were recommenced in March 1844. On the first of these two occasions, the continuity of the series was unbroken, great care having been taken to avoid any interference with the magnet or its suspension on the second, the continuity was interrupted, by reason of a half turn having been accidentally left in the suspension thread in the adjustment of March 1844. (vol. I. P. The hourly series of five years consists thereof of two independent portions, viz., from September 1, 1842, to the fracture of the suspension in January 1844, and from the replacement of the suspension thread in March 1844 to the termination of the series on August 31, 1847. To connect the mean monthly declinations derived from the scale readings during the second portion of the five years with those of the first portion, a constant correction of - 6'°o has been applied to the second portion, in compensation of a half turn in the suspension subsequent to march 1844. With this correction the mean scale readings, which are collected from the monthly tables, give the following values for the mean declination in each month those of January and February 1844, during which months the Observations were suspended, are marked in Table I. By an asterisk, having been interpolated from the values in December 1843 and March 1844.

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