Page:Scientific Memoirs, Vol. 2 (1841).djvu/66

54 clock, the name of the observer, and remarks on such observations as may be somewhat doubtful. An early communication is always greatly to be desired. 2em

III.

Extract from the daily Observations of Magnetic Declination during three years at Göttingen.

To discriminate the regular changes of declination, amidst those incessant changes of greater or less amount, which we call irregular, in so far as their occurrence seems unconnected with any periodical rules, requires a great number of observations on a fixed plan, persevered in for a length of time, in order to deduce, by suitable combinations, mean values, freed as far as possible from the influence of those anomalies by which the individual declinations are affected. In general, in this part of the globe, the declination increases during the forenoon, but the increase is unequal on different days; it even sometimes happens, though rarely, that at the usual hour of maximum, the declination is less than it was during the earlier part of the same day. The cause of the morning increase may be in operation every day; but its influence is sometimes increased, sometimes diminished, and sometimes entirely masked, by other irregular intervening forces. Observations on a single day, or continued for a few days only, cannot therefore determine either the amount of the effect due to the regular cause, or its inequalities at different seasons. For this, mean values, taken from a great number of days, are required. The same is the case with those progressive changes which take place in one direction for a very long time; these we call secular, because they require a long series of years to amount to many degrees. Single observations, repeated after an interval of only a few years, even though performed on the same day, in the same month, and at the same hour, can afford us no certain knowledge respecting them; but mean numbers, obtained by continued observations, allow us to anticipate, at the end of very few years, what it would otherwise take many tens of years to fix with any considerable degree of approximation.

With this view, from the very commencement of the observations to be performed at our Magnetic Observatory, I have included among them the daily determination of the absolute