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British Rainfall 1867 page 19
Determination of the ratio of the rainfall in each of
22 years to the mean of the whole period 1845
to 1866.
I do not intend, on the present occasion, to print the falls
in the whole 23 years at all stations, but simply the
results obtained by means of the ratios in the last column.
When the new gauges have been at work for a year or two, we
may perhaps be able to discontinue the more difficultly
accessible gauges, and review in detail the accuracy, or
otherwise, of our present deductions.
It may be well to give in detail a specimen of the mode by
which the approximate means have been obtained. The fall at
Wastdale Head was as follows:


     
 in.   in.   in. 
1845 ...  108.55  1848
...  115.32  1851
...  97.94 
1846 ...  105.93  1849
...  107.22  1852
...  109.58 
1847 ...  96.34  1850
...  108.76  1853
...  83.39 


Mean of these nine years, 103.67
By rerefence to the last column of the ratio table, it will
be found that the mean ratio of those nine years was 102.9;
that is to say, the rainfall in those years was 3 per cent.
above the average. It only remains, therefore, to divide the
mean observed fall by the mean ratio to deduce the
approximate true mean, e.g. 103.67÷1.029=100
inches.






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