109 
Meldrum and others appear to have achieved good prelimi- 
nary work in the direction of indicating the existence of a 
rainfall inequality depending upon the state of the sun. 
Dr. Meldrum began by pointing out that in a good many 
places there is a greater rainfall during years of maximum 
than during years of minimum sun spots, and that this 
phenomenon repeats itself from one solar cycle to another. 
Again, Governor Rawson has pointed out the existence of 
certain localities where the rainfall inequality appears to be 
of a precisely opposite character, while Dr. Hunter has 
showed the practical importance of the investigation with 
reference to certain tropical stations. The subject has 
likewise been discussed by Smyth, Stone and others. 
5. The question has arisen whether it might be possible 
to throw any light on this problem by the method of 
detecting unknown inequalities, proposed by Mr. Dodgson 
and myself (see Proceedings of the Royal Society, May 29, 
1879). The essence of this method consists in a way by 
which we may numerically estimate the indications of an 
inequality. Let us suppose for instance that in ignorance of 
the diurnal range of temperature we try to find whether 
there be a temperature inequality of 24 hours or whether 
there be not rather one of 26 hours. We should begin by 
taking a large number of hourly readings of temperature 
and we should group these into two series, the one 
containing 24 numbers in each horizontal row, and the other 
26. We should thus have 24 vertical columns from the one 
series and 26 from the other, and we should take the mean 
of each vertical column of each series as well as the mean of 
the whole. 
Now it would speedily be found that an inequality was 
indicated by the 24 hourly series and none by that of 26 
hours. For in the first series the mean of that vertical 
column representing observations at 5 a.m. would be greatly 
less than the mean of the whole, while the mean of that 
