ON THE EAETH’S MAGNETISM. 
393 
27. The principal features pointed out by Messrs. De La Rue, Stewart, and Loewy* 
of the growth and decadence of sun-spots were of a simple character ; the spots acquired 
a minimum magnitude at a heliocentric longitude a little greater than that of the planet, 
and a maximum at a heliocentric longitude a little more than 180° greater than that of 
the planet ; and there was a gradual progression in the change from minimum to maxi- 
mum and vice versa in the intervening periods. 
28. It must be admitted that the curves which we have found of magnetic variation 
in planetary periods do not possess the same simple character ; but if we confine our 
attention to those of them which have been yielded by the largest number of individual 
observations of disturbance, viz. to the curves of Easterly disturbance of Declination 
and to the curves of disturbances decreasing the Horizontal Force, we shall find in them 
definitiveness of character and some remarkable points of correspondence and difference 
that would seem to be deserving of attention. We first note that, for the synodic period 
of Venus, the curves of Declination and Horizontal Force have their principal inflections 
alike, and that this likeness attaches, though in a less degree, to the curves for the 
synodic periods of Mercury and Jupiter, in common with those for Venus; secondly, 
that whilst the curves of Venus are strikingly bolder and more definite for the synodic 
period than for the sidereal period, there is no very marked difference in the case of 
the curves of Mercury. Again, we note the close resemblance in the two curves of the 
Earth and in the two of Mercury for its sidereal period — in the latter case of so precise 
a kind that, keeping in mind that the curves are derived from independent observations 
with instruments of different construction, it is difficult to suppose that they do not 
indicate a real periodicity in nature. 
29. It is not claimed for these investigations that they account for any substantial 
part of the so-called decennial variation of magnetic disturbance, but only that there 
may be, and probably are, subordinate planetary variations of the kind described, which 
are superimposed upon the more strongly marked decennial variation, and that if they 
are, they are included with the variations that have been deduced from the observations. 
It must be allowed, too, that, until the character of the decennial variation be brought 
out more fully than as yet (by a great extension of the period of observation), doubt 
must remain as to whether these apparent variations which follow the periods of the 
planets may not be due, wholly or in part, to the imperfect elimination of the decennial 
variation. The irregularities observed in the duration of the sun-spot period, with 
general correspondence in magnetic disturbance, as far as observation permits the 
comparison, would seem to indicate that the decennial period itself must be regarded 
as subordinate to some more ' extended period, in the recurrence of which the irregu- 
larities alluded to would be repeated in the same order. It is for this reason that we 
have not attempted, from the twenty-six years of observations available, to determine 
the duration and character of the decennial variation, considering that such an under- 
taking would, with present data, be to a great extent labour in vain. 
* Proceedings of the Eoyal Society, vol. xx. page 210. 
3 Gr 
MDCCCLXXV. 
