British Rainfall 1896, p.18
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British Rainfall 1896 page 18
Tarn (1472ft.), four gauges - A, B, C, D - on The Stye (1077ft.), and one at Seathwaite (422ft.) So much for the history. Before giving the results we must ask attention to Tables I. and II., and to the accompanying maps. Table I. contains the early observations, 1847-1857; at its foot will be seen two lines of means, marked respectively "Arithmetical" and "Reduced." Reduced, in this case, does not mean made less. Very often the "Reduced" is more than the "Arithmetical," and as this may puzzle some readers, it had better be explained. Everybody knows that an "Arithmetical" mean is the sum of the entries divided by their number; the total for 5 years at Sca Fell (Table I.), 372.08 divided by 5 gives 74.42 in. That is clear enough. But the years 1849-53 might have been wet ones, like 1850-52, and if so that "Arithmetical" average, although correct as a matter of calculation, would not correctly represent the average rainfall - and that is why "Reduction" comes in. In British Rainfall, 1895, we gave the rainfall in every one of the 50 years at Seathwaite, and we gave the ratio which the fall in each individual year bore to the average of the 50 years. A wet year - for instance, 1872 - had 182.05 in., whereas the average for 50 years was 137.31 in., so that in 1872 the rainfall was 44.74 in. above the average, or an excess of about 1/3, therefore the ratio of that year was given as 133, and the ratios were similarly given for every year. The "Reduction," then, is merely correcting of the "Arithmetical" mean for the effect of the group of years being wet or dry. As it happens, in the case we have quoted, the "Arithmetical" and the "Reduced" mean are nearly identical; we had better explain why. The last column in Table I. gives the Seathwaite ratios (from p.25 of Brit. Rain., 1895), and it will be found that for the years above mentioned (1849-53) the average ratio was 99, therefore the "Arithmetical" mean has to be increased by only one per cent., and so the "Reduced" becomes 75 inches. This process has been adopted, because it is without doubt the right one; but, as already implied, there is probably no area of the same size in the British Isles in which the application of this method is, owing to the deflection of the air currents by the mountains, equally unsatisfactory. The construction of Table II. is precisely similar to that of No.1. It is necessary to say a few words respecting the maps, also, before we proceed to consider the results. The area is, as already stated, |
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