British Rainfall 1867, p.43
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British Rainfall 1867 page 43
Indeed, they are in most cases so well concealed that they cannot easily be found by a stranger. Thus my excellent and indefatigable friend Mr. Symons, in the autumn of 1866, when on a tour of inspection and verification among the contributory gauges to this volume, cruized for hours among the rocks and defiles of Wasdale Head and Styehead Pass in search of my gauges, and he must forgive me for expressing my satisfaction at the total failure of his expedition. He could not find one of them! As somewhat connected with the subject of these remarks, I wish to add a few words on the subject of evaporation, a subject surrounded with difficulties of all kinds, so much so as to call from Mr. Symons the remark, "I don't believe in evaporators!" The only important experiments on this subject that have been published, so far as I am aware, are those of Mr. Luke Howard and Dr. Miller, the former being made in London, and the latter in Whitehaven. Mr. Howard found the annual evaporation about 20 inches, but as the vessel was covered it is obvious that this amount must be far too small. Dr. Miller found the average evaporation, by uninterrupted daily observation for 12 years, to be 29.664. He employed a dish 8 in. in diameter, which during the day was exposed to wind and sun, though too much sheltered by surrounding walls. During rain and at nights it was placed under a capacious shed 9 ft. high, open at front. In this manner it may at first sight seem that all legitimate influences are fairly at work: but it seems to be open to two grave objections. The immediate removal to a place of shelter when rain commences, is dependent on the vigilance of the person in charge, and when under cover during rain, evaporation must be greatly impeded, and at nights this will continually operate to the diminishing of the true amount. The apparatus I have employed is a dish 8 inches diameter, so contrived as always to have in it half an inch of water. The dish is accurately bored and turned to a knife edge; it is four and a half inches deep, and has a tap in the bottom, through which the water is discharged into a measuring tube. It is five feet from the ground, alongside a rain gauge at a similar elevation. Half an inch of water (additional to the initial half inch) is poured in every morning, and re-measured the following morning, allowance being made for the rain (if any) in the interval. Thus it will be seen that the water in the dish is exposed freely at all times to all the influences which either promote or retard evaporation; nevertheless it is open to three objections, two operating unduly to increase the apparent amount, and one to diminish it; and for these I can find no remedy. The material of the dish becomes unduly heated by the sun in warm weather, and in heavy rains there is some little loss by |
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