E. Loomis on Solar Spots, Magnetic Declination, ete. 157 
but almost blended with the preceding maximum of 1787, in 
consequence of an unusual prevalence of spots during the in- 
termediate interval. From 1750 to 1860, Dr. Wolf reckons only 
10 periods, which would make the average length of one period 
11 years; but if we reckon 11 periods (as I think we ought) we 
shall find the average length of one period to be 10 years. 
If now we attempt to explain the fluctuations in the sun’s 
surface by ascribing them to the influence of the planets, we 
find that the planet whose period approaches nearest to the pe- 
riod of 10 or 11 years, is Jupiter, whose time of revolution is 
11-86 years, which is nine months greater than Wolf's period, 
and almost two years greater than the period above found. But 
the interval between two successive heliocentric conjunctions of 
Jupiter and Saturn is 19°86 years. Once, therefore, in 9°93 
tion; and if we suppose that the action of these planets upon the 
sun has some analogy to that of the moon upon the earth in 
raising a tide, then we shall have a cause whose period corres- 
1 i the maxima of 
the solar ains, however, a cause why 
these periods are alternately increased and diminished by 3 or 
an. the successive riable in 
amount. This cause may, perhaps, be found in part in the po- 
sition of Jupiter and Saturn in their orbits at the time of con- 
junction an ee but to render the explanation c 
in successive revolutions, and these differences go on steadily 
increasing for several revolutions of Jupiter; in other words the 
