Magnetic Declination and Horizontal Force and Solar Spots. 71 



the little drop shown in all the curves in 1882 and 1883 is also 

 striking. 



Observation has been supposed to indicate that the magnetic effect 

 follows the sun-spot effect, so that a retardation, or what has been 

 called a "lagging," of the magnetic effect exists, but the evidence for 

 this has never appeared to me to be quite satisfactory. The fewer 

 comparisons of sun-spot and magnetic epochs given in my first paper 

 seemed to give support to such supposition. But taking the more 

 extended comparison contained in the preceding table of epochs, it is 

 seen that the declination epoch is on the whole retarded by Q'lSy (frac- 

 tion of year), whilst that of horizontal force is accelerated by O'QSy. 

 Mean retardation = 0'Q6y. Looking at the irregularities in the num- 

 bers on which these means are founded, it seems doubtful whether, 

 without a yet more extended comparison, any real difference or 

 definite lagging can be said to exist. Two things, however, may be 

 noted. One is, that the differences in the early part of the table are 

 inclined to positive, and in the later portion to negative values. 

 What will happen when further results are included ? Considering, 

 however, the strength of the evidence for some form of relation that 

 has been shown by what has preceded to exist, it seems in every way 

 probable that the individual differences of epoch represent in great 

 part accidental residuals. Rather, indeed, is it not likely that to 

 some extent small differences or irregularities may be expected to 

 appear, when it is remembered that we are comparing together solar 

 and terrestrial phenomena, to us probably only incomplete manifest- 

 ations of involved actions, about which little is known and of which 

 the cause is obscure. Especially, too, when, as regards the terres- 

 trial effect, the magnetic variations are deduced from the observa- 

 tions at a single station, Greenwich. Consider also the composite 

 character of the sun-spot phenomena at about the time of minimum 

 epoch. After a maximum, as the following minimum epoch is 

 approached, sun-spots become in successive years fewer in number 

 and smaller in magnitude, the regions north and south of the solar 

 equator in which spots appear tending, as the spots become less 

 numerous, more and more towards the equator, until at the minimum 

 epoch the spots disappear. At the same time, spots of, as it were, a 

 new cycle come into view in high latitudes, and become rapidly more 

 numerous and important until another maximum is passed, after 

 which the spots become again less numerous, the regions in which 

 they appear approaching as before the solar equator, until at the 

 following minimum again the spots disappear. Thus the curve at 

 the minimum epoch seems to be produced by the junction of an 

 expiring cycle with a new outburst becoming rapidly active. 



In tabulating the magnetic records at Greenwich, ifc has been the 

 practice to include all days (except those of extreme disturbance), 



