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II.

Method to be pursued during the terms of Observation.

The six appointed terms every year fall towards the end of the months January, March, May, July, September, and November; they commence at noon, Göttingen mean time, on the last Saturday in each of those months, and terminate at noon on the following day; the sub-terms, which have hitherto been added to the principal terms (from 8 to 10 in the evening on the Tuesday and Wednesday of the following week) will in future be discontinued. According to the rule, the position of the magnet needle is in each term determined every 5 minutes, so that one term affords 289 results. The clock is regulated, in Göttingen, previous to the commencement of each term, accurately to mean time. As a near coincidence in the time of the individual determinations at various stations is highly desirable, most of the observers at other places are accustomed to regulate their clocks also to Göttingen mean time. Where this cannot be done, it is recommended that such whole minutes be chosen for the moment of observation as approximate nearest to the times of observation at Göttingen. If, for instance, it had been found, previously to the commencement of the term, that the clock-time required for the observations was about 13′ 48″ in advance of Göttingen mean time, the needle should be observed at 0h 14′, 0h 19′, 0h 24′, 0h 29′, and so forth of the clock. In every case, it is best to choose the full minutes.

The position of the needle to be determined for any instant of time is not that position which the suspended magnet bar actually has at the instant, but that which it would have were its magnetic axis at that instant exactly in the magnetic meridian. This distinction was unnecessary, as long as such needles only were employed as were not susceptible of very great accuracy: with them it was only requisite that the needle at the time of observation should not be in perceptible vibration, and the observation might at once be made. The instruments now in use are