39 
out even in the annual averages a discrepancy of more than an inch, while 
the differences "between the monthly results are in some cases surprisingly 
large. Thus the mean rainfall of January, which, deduced from the first 
ten years, is a trifle over two and a half inches, deduced from the second ten 
years is upwards of four inches ; and the mean of February, which in the first 
period, is not much more than an inch and a third, becomes in the second 
period two inches and six tenths. Clearly, therefore, ten years is too short a 
period for obtaining even appro: tely the average rainfall of a particular 
month, while even for a whole year the possible error of a ten years' 
average is not unimportant; it may indeed be greater than appears by the 
able, for it is possible to select two decennial periods of which one 
(IS55 -1864) shall give an annual average of 30*735 inches, the other 
(1859 — 1868) an average of 34 - 155 inches. Results showing an even 
greater discordance than this would be obtained if the calculations were 
based upon the two eleven-yearly periods 1854 to 1864, and 1859 to 1869, 
The averages drawn from these two periods are 30*092 inches and 34-338 
inches respectively. Hence it follows that two observers might have 
recorded the rainfall of Clifton with perfect accuracy for eleven years each, 
and yet their annual averages should differ by more than four inches, — a 
consideration which must certainly enhance the value of a connected series 
of observations such as that now submitted. 
Assuming then a twenty years' average to be fairly true, it may be said 
that the mean annual rainfall at Clifton is 32 inches, the range extending 
from less than 23 to more than 42 inches. The three driest months on an 
average are February, March, and April. In the table it appears as if 
February were the driest of all, but when allowance is made for the different 
lengths of the months, it turns out that the driest, on an average of twenty 
years, is the proverbially " showery " month of April. The three wettest 
months of the year are, in the order of their wetness, October, January, 
and August. Looking at this matter from a more general point of view, 
it is worthy of note that the three driest months occur consecutively at the 
time of year when winter is gradually yielding to spring; and that the 
wettest three-monthly period is that which includes the transition from 
summer into autumn. The difference between these two periods is 
striking. In the three months of which the early spring is the centre, 
- namely, -February, March, and April, the average aggregate rainfall is 
6*142 inches. In the three months of which the early autumr ' the 
centre, namely, August, September, and October, the average is 9*876 
inches. 
