Volume 1

A treatise on hygiene and public health / edited by Thomas Stevenson and Shirley F. Murphy.

Date:
1892-1894
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on hygiene and public health / edited by Thomas Stevenson and Shirley F. Murphy. Source: Wellcome Collection.

    187/1056 (page 167)
    EAINFALL This subject has on account of its great engineering importance received much attention, and there are now few countries in the world without special organisations for its registration. As the natural result thereof, considerable ap- proach has been made towards uniformity in the pattern of rain gauge and in the mode of registration. It used (fifty years since) to be not unusual to put rain gauges on roofs (I suppose that they might be the nearer to where the rain came from), but long before that it had been shown (but forgotten) that gauges on elevated buildings collect much less than those near the ground.1 In the British Isles, in India, Ceylon, Canada, and in nearly all our Colonies, the gauges are one foot above ground. On the con- tinent of Europe, in Algeria, Java, and Sumatra, they are generally from three to five feet above the ground, and there- fore record about three per cent, less than they would do if at one foot. In the United States, owing to the Signal Office stations being mostly in the heart of cities, a great many of the rain gauges are understood to be on house tops. No satisfactory discussion of American rainfall can be based on such observations. The pattern of gauge generally used in this country is shown in fig. 69 ; this represents one five inches in diameter, but there are also many eight- inch ones at work, and those established in France by the Association Erancaise of about eight and a half inches diameter are very similar in pattern. The vertical portion of the funnel, i.e. that above the cone, is known as a Snowdon rim (from having in England been adopted first for gauges intended for use near Snowdon), and is intended to secure the accurate measurement of sligbt falls of snow. Having entered the funnel, the rain passes down through a tube, made long and narrow to check evaporation, into the bottle, where it remains until measurement. If the fall be exceptionally great, or if a sudden frost sets in, freezes the water, and cracks the bottle, the record is not spoiled, for the water is saved by the can, and may be measured almost as well as if nothing had happened. The measuring jar is divided propor- tionally to the area of the gauge, the diameter of which should always be an exact dimension, 5-00 inches or 8-00 inches, as it is then easy, if the original measuring jar is broken, to obtain a new one precisely adapted to the funnel. The principle of graduation is so obvious a*? scarcely to need mention. Take a 5-inch gauge ; if the diameter be 5 inches the area is 19*64 inches, therefore a rainfall of an inch, i.e. 1 inch deep over the whole of a parish or town, would in this rain gauge deposit 19*64 cubic inches, or 4958 grains of water. It is found in practice most convenient to make the jar hold half an inch. Therefore 2479 grains are poured in and the jar is marked with a line repre- senting 0*50 inch or h inch ; subdivisions are similarly marked, and so finally 1 This deficiency has heen proved to be due to the wind blowing the rain over, and out > of, the funnels of gauges in exposed positions.