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Thirdly, it is in comparatively few places, and those 
places chosen not with the smallest reference to this parti- 
cular problem, that we have anything like a trustworthy 
account of the rainfall throughout a considerable number of 
years. 
Fourthly, we have no information of any importance 
with respect to the rainfall at sea. 
3. Besides the formidable catalogue of difficulties now 
mentioned, we ought to bear in mind the following con- 
siderations. The convection currents of the earth are 
regulated by two things, one of which is constant, while 
the other may be variable. The constant element is the 
velocity of rotation of the earth on its axis, while the 
element of possible variability is the power of the sun. 
Hence it follows that if the sun be variable it will cause a 
variation in the direction as well as in the intensity of the 
earth’s convection currents, on the principle which tells us 
that the resultant of two forces, one constant and the other 
variable, must vary both in magnitude and direction. 
Now if it be true that we have a long period variation 
not merely of the intensity, but also of the distribution of the 
earth’s convection currents, and if we bear in mind the 
intensely local reference in rainfall, it would be too much to 
expect that the rainfall inequality should exhibit the same 
years of maximum and minimum at all places. It is even 
conceivable that some places might exhibit a maximum, 
when others showed a minimum, while others again might 
exhibit a double instead of a single period. 
4. It appears to me that if we bear' in mind these con- 
siderations it will not answer to add together the rainfall of 
a few selected stations as they stand, with the view of 
determining by this means whether there be a long period 
inequality in the rainfall of the whole Earth. We are not 
yet in a position to reply experimentally to this question. 
It does not however follow that nothing can be done. Dr. 
