38
THE SUN.
which represents the total area of Sun -spots clay by clayobserved, they are found to be most remarkably similar. Anysudden irregularity in one of them is answered by a corre-sponding irregularity in the others, although the oscillationson the Sun -spot curve may often slightly precede those in themagnetic curves.
In a very important memoir by Mr. W. Ellis, Superintendentof the magnetical and meteorological department of the BoyalObservatory, Greenwich, published in part ii. of the Philo-sophical Transactions for 1880, the following conclusions arededuced from the observations of the preceding thirty-sevenyears:—
1. That the diurnal ranges of the magnetic elements ofdeclination and horizontal force are subject to a periodicalvariation, the duration of which is equal to that of the knowneleven year Sun -spot period.
2. That the epochs of minimum and maximum of magneticand Sun -spot effect are nearly coincident, the magnetic epochson the whole occurring somewhat later than the correspondingSun -spot epochs. The variations of duration in differentperiods appear to be similar for both phenomena.
3. That the occasional more sudden outbursts of magneticand Sun -spot energy, extending sometimes over periods ofseveral months, appear to occur nearly simultaneously, andprogress collaterally.
The close agreement of the three curves shown in a diagramfor the whole period of thirty-seven years is most interesting.We append a reduced copy for the ten years from 1867 to1876 inclusive.
It should be mentioned that the numbers in the scale onthe right hand correspond to Dr. Wolf’s determination ofthose which best represent the relative frequency of the spots,observed during the years in question. For the curves Band C, twice one of the upper graduations (each of which cor-responds to ten of Sun -spot number), or one of the lowergraduations of double width, is equivalent, respectively, to oneminute of arc of declination, or 0 - 003 of the horizontal force.