110 
column representing observations at 2 p.m. would be much 
higher than the mean of the whole. On the other hand, in 
the 26 hourly series, provided it be sufficiently extensive, 
we should perceive no such differences. Thus in the 24 
hourly series the differences of the means of the various 
vertical columns from the mean of the whole would be 
much greater than in the 26 hourly series, and the mean 
amount of these differences might be taken to form a 
numerical criterion of the presence or absence of an 
inequality. 
6. This method therefore, applied to the subject in hand, 
might be expected to reveal the presence or absence of 
inequalities in rainfall, provided we have observations 
sufficient for the purpose. It is clear that the successful 
application of this method does not require a previous 
knowledge of the exact form of the inequality. Whether a 
maximum rainfall occurs at epochs of maximum, or at 
epochs of minimum sun spot frequency ; whether there be 
only one rainfall maximum corresponding to the solar period, 
or two, or even three, is a matter of no consequence, as far 
as this method is concerned. All that is necessary is that 
the rainfall should always be similarly affected by similar 
states of the sun. Here however we must bear in mind 
that this method of detecting inequalities by summing up 
and averaging the departures from the mean caused by the 
inequality, likewise sums up and averages the accidental 
fluctuations. Now these accidental fluctuations are parti- 
cularly large for rainfall, and it is therefore desirable to 
lessen their disturbing effect as much as possible. This can 
only be done by conflning ourselves to long series of obser- 
vations in which the accidental fluctuations may be supposed 
o counteract each other to a great extent, while the long 
period fluctuations will remain behind. 
7. Through the kindness of Mr. Whipple, Director of the 
Kew Observatory, I have received copies of those catalogues 
