82 CLIMATIC CYCLES AND TREE-GROWTH. 
From the description above it is easily seen that the sunspots are 
not likely to be in themselves the fundamental solar activity, but rather 
an index of something else, and possibly a very sensitive index, for the 
percentage change in spot numbers is hundreds of times as great as the 
percentage changes in measured radiation between sunspot maxima 
and minima. 
Suggested causes of sunspots.—The cause of sunspots is still a matter 
of conjecture, and there is no generally accepted hypothesis to explain 
them. ‘There is analogy to our clouds in that both indicate decreased 
temperature. In their limitation to certain latitudes they resemble the 
belts of Jupiter. The belts of Jupiter are roughly the lines of division 
between the powerful easterly equatorial current and the slower moving 
zones on either hand; and indeed this has been suggested as an expla- 
nation of the particular location in latitude of the sunspots, for there 
is an increase in speed of rotation of the sun’s surface as the equator 
is approached. Their periodic character is very difficult to explain. 
Fundamental periodic changes in the body of the sun have been sug- 
gested and, in the absence of better explanations, some such statement 
hazily indicating the direction in which explanation is to be sought, is 
perhaps the best that we can do. Planetary influence, however, has 
often been proposed as the cause. The near agreement between the 
revolution period of Jupiter and the sunspot period has naturally 
attracted attention. Stratton (1911-1912) has made a very interest- 
ing study of the appearance, continuance, and disappearance of spots 
on portions of the sun facing toward or away from Jupiter and Venus. 
A few per cent more spots do originate and disappear on the “‘after- 
noon”’ of the side facing Venus than on other longitudes, but he con- 
siders the case of physical relationship not proven. 
Planetary influence is sought in a theory proposed by W. J. Spill- 
man (1915). In this theory gravitation is assumed to be due to 
pressure variations in the ether arising from electronic rotation in the 
attracting body. The varying speed of a planet in its orbit between 
perihelion and apheljon, involving varying quantities of energy, requires, 
he says, an interchange between the kinetic energy of the plant and 
the atomic energy of the central attracting body. This atomic energy 
is in the vibrations of the electrons, but he thinks it is likely to affect 
both the temperature and the electric activity of the central body. 
The effect in this way of Jupiter and Saturn would exceed the sum of 
all the other planets combined and is therefore the only one considered. 
The effect of Jupiter with its substantial variations in distance between 
perihelion and aphelion predominates, and we have a marked resem- 
blance between the sunspot curves since 1770 and the differential 
planetary effect. One notices that this interchange of energy would 
presumably affect all parts of the sunalike and that therefore we could 
not expect an excess of sunspots on the side facing Jupiter. 
