Laws Of Rain-fall

Tropical countries have a dry and a wet season during the year: dry, when the sun is at the opposite side of the equator; wet, when the sun is overhead. With reference to the British Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st. The stations of least rain are inland, or on the east or south-east coasts; the stations of greatest rain are on the western coasts. 2nd. The rain-fall is very large in the vicinity of mountain chains or groups, unless t e station happens to be some miles to the north-eastward.

It may be well to illustrate these remarks by quoting the average fall at a few places, grouping them as—

Westerly.		 Â	Central.		 Â	Easterly.
Â	Inches.	Â	Â	Inches.	Â	Â	Inches.
Bodmin	43	Â	Enfield	23	Â	Witham (Essex)	21
Bolton (Lancashire)	44	Â	Epping	23	Â	Patrington (Hull)	21
Coniston (Windermere)	71	Â	Derby	24	Â	Sunderland	17
Seathwaite	127	Â	York	22	Â	Inveresk (Edinburgh)	25
Torosay (I. of Mull)	75	Â	Stirling	39	Â	Pittenweem (Fife)	24
Killaloe (Limerick)	38	Â	Perth	29	Â	Dublin	22

Mr. Green, the celebrated aeronaut, has asserted from his experience, “that whenever a fall of rain happens, and the sky is entirely overcast, there will invariably be found to exist another stratum of cloud at a certain elevation above the former;” and the recent scientific balloon ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher says, “It would seem to be an established fact, that whenever rain is falling from an overcast sky, there is a second stratum above.” “It would also seem that when the sky is overcast without rain, that there is no stratum of cloud above, but that the sun is shining on the upper surface. In every instance in which I have been up under these circumstances, I have found such to be the case, agreeing in this respect also with Mr. Green’s observations.”

The amount of rain collected in a gauge placed near the surface of the earth is larger than in any gauge placed above it; and the higher the gauge is placed, the less water is collected. Mr. Glaisher contends that his balloon experiments corroborate this law.

120. Utility of Statistics of Rain-fall.—The utility of knowing the rain-fall of any locality is sufficiently obvious, and little need be said upon the subject. The rain-gauge should be in the hands of every gardener and farmer. In the management of out-door plants and crops, as well as in the construction of cisterns and tanks for the supply of water, a rain gauge is a valuable assistant. By its use, the gardener will be guided in judging how far the supply of moisture to the earth is needed; and he will also see how beneficial is even a hasty shower to growing plants, when he considers that a fall of rain measuring the tenth of an inch in depth, corresponds to the deposit of about forty hogsheads per acre. The study of the rain-fall of a country is of considerable interest to agriculturists. The health and increase of domestic animals, the development of the productions of the land, as well as the daily labours of the farmer, are dependent upon the excess or deficiency of rain. “It must be a subject of great satisfaction and confidence to the husbandman to know at the beginning of a summer, by the certain evidence of meteorological results on record, that the season, in the ordinary course of things, may be expected to be a dry and warm one; or to find, in a certain period of it, that the average quantity of rain to be expected for the month has fallen. On the other hand, when there is reason, from the same source of information, to expect much rain, the man who has courage to begin his operations under an unfavourable sky, but with good ground to conclude, from the state of his instruments and his collateral knowledge, that a fair interval is approaching, may often be profiting by his observations; while his cautious neighbour who waited ‘for the weather to settle’ may find that he has let the opportunity go by. This superiority, however, is attainable by a very moderate share of application to the subject; and by the keeping of a plain diary of the barometer and rain-gauge, with the hygrometer and vane, under his daily notice.” The statistics of rain-fall are not only valuable and interesting in a meteorological point of view, and for agricultural purposes, but are also highly important in connection with sanitary arrangements for towns, and engineering operations. This is especially evident to the hydraulic engineer. As rain is an important source of water-supply to rivers, canals, and reservoirs, it is evident that a knowledge of the probable fall for any season or month, at a given place, as furnished by averages of the observations of former years, will be the data upon which the engineer will base his plans for providing for floods or droughts; while the measurement of the actual quantity which has just fallen, as gathered from the indications of a series of gauges, will suggest to him the precautions to adopt either to economise or conduct away the in-pouring waters.

“When a canal is conducted across an undulating country, its course is necessarily governed by the accidents of the ground, and it alternately rises and falls. In this case, rising by a succession of levels, it necessarily arrives at a certain highest level, which is called by engineers the summit level. From this it again descends by a corresponding series of levels. Now, it is evident that, supposing the locks to be all equal in magnitude, the ascent of a vessel will require the descent of as much water from the summit to the lowest level as would fill a single lock; for this quantity of water must be discharged from each lock of the series when the vessel passes through it.

“The same may be said of the process by which the vessel descends along the series of locks on the other side of the summit. It appears, therefore, that a supply of water must always be maintained on the summit level sufficient to fill a single lock twice for each vessel which crosses the summit.

“It happens, fortunately, that by the laws of natural evaporation, rain is precipitated in greater quantities on elevated summits than on the intermediate valleys, so that the moving power, in this case, accommodates itself to the exigencies of intercommunication.”—Dr. Lardner’s “Handbook of Natural Philosophy.”

121. New Form of Rain-Gauge.—Since the foregoing pages were in type, a modification of Howard’s rain-gauge has been arranged by Mr. Symons, which is compact in design, convenient in use, and low in price. It combines the advantages of most gauges; having solidity, and facility of measurement. The bottle is placed in a tin case, to the bottom of which are attached stout spikes, which, when forced into the earth, prevent its being upset either by wind or accident. The bottle being transparent, and slits made in the case, the fall of rain is seen at a glance, or with a race-glass, from a window. The funnel being attached to the cover of the case is thereby kept strictly horizontal, and the depth of rain can be accurately measured by lifting the bottle from its case and emptying it into a graduated glass jar.

The funnel of this gauge is a very deep cone, to prevent the rain drops outsplashing. When properly placed, the receiving surface will be twelve inches above the ground, which experience has shown to be the most advantageous height.