tions of these changes, is now become essential to the successful prosecution of magnetic discovery, for the following reasons:—
"1st. That the progressive and periodical being mixed up with the transitory changes, it is impossible to separate them so as to obtain a correct knowledge and analysis of the former without taking express account of and eliminating the latter, any more than it would be practicable to obtain measures of the sea-level available for an inquiry into the tides, without destroying the irregular fluctuation produced by waves.
"2ndly. That the secular magnetic changes cannot be concluded from comparatively short series of observation, without giving to those observations extreme nicety, so as to determine with perfect precision the mean state of the elements at the two extremes of the period embraced, which, as already observed, presupposes a knowledge of the casual deviations.
"3rdly. It seems very probable that discordances found to exist between results obtained by different observers, or by the same at different times, may be, in fact, not owing to error of observation, but may be due to the influence of these transitory fluctuations in the elements themselves.
"4thly and lastly, Because the theory of these transitory changes is in itself one of the most interesting and important points to which the attention of magnetic inquirers can be turned, as they
