163 PROF. B. STEWART ON THE LONG-PERIOD 



Fourthly, we have no information of any importance 

 with respect to the rainfall at sea. 



3. Besides the formidable catalogue of difficulties now 

 mentionedj 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 while 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. Meldrum and others appear to have achieved good 

 preliminary work in the direction of indicating the exist- 



