166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



We see that the most active cause for a tidal wave lies in Jupiter, 

 followed closely by Venus. Mercury and the earth come next, the 

 remaining planets being much less active. 



Although the earth comes only in the fourth rank, we will con- 

 sider it first because we are better situated for examining its effects. 

 At each instant we can consider the sun as divided into two equal 

 hemispheres, one visible, the other not. The limiting meridians turn 

 uniformly over the surface of the sun in 27.35 days, the synodical 

 period. 



Let us first suppose that the earth has no physical influence on the 

 development of the spots. The ratio between the total sun-spot 

 areas in the two hemispheres may happen to have any value what- 

 ever; but the mean value taken over a long period of time embrac- 

 ing many synodical rotations, say for a whole solar-spot cycle, should 

 differ very little from unity. 



We can not at any given moment count or measure the spots on 

 the invisible hemisphere. But we can count the spots which appear 

 en the eastern border and compare these with those which disap- 

 IDear in the corresponding time limit at the western border. The ratio 

 of the two numbers would have a tendency to surpass unity if it is 

 at a time of decrease in spots and to be less than unity if in the in- 

 creasing phase. But taken over a whole cycle, the mean value should 

 differ very little from unity. 



Now, let us suppose that the earth does have a physical influence, 

 for instance, to fix our attention, that the presence of the earth above 

 the horizon of some point on the sun favors the development of a 

 spot at that point. As this development is certainly not instanta- 

 neous, any more than is its disappearance, more spots will be born in 

 the visible hemisphere than in the opposite one. Consequently, more 

 spots will disappear over the western border than appear at the 

 eastern. The inverse inequality will be observed, provided we observe 

 over a sufficiently long period, if the presence of the earth causes the 

 disappearance of spots. 



Instead of comparing the eastern with the western border we could 

 compare the two halves of the visible disk, the right with the left, 

 and the result would be equally decisive. Practically, if the action 

 of the earth on the solar surface is real, the action will necessarily 

 take a certain time to become manifest. Considerable masses must 

 be moved, masses doubtless subject to interior friction. It is so rela- 

 tive to terrestrial tides which at any point of the earth suffer a 

 variable retardation, but always very marked with reference to the 

 passage of the moon over the meridian. If the earth has no influence, 

 the two halves — the right and left — would, if considered over a suffi- 

 cient time, show the same number and same area of spots. If the 

 earth has a real influence there will be found a persistent and sys- 

 tematic inequality. 



