Self-registering Rain-gauge

The rain-gauge can be combined with clock-work and other mechanism so as to be self-recording of the amount of rain, the time, and duration of its fall. For the details of construction the reader is referred to the next chapter, where he will find the instrument described in connection with Osler’s anemometer, as the “pluviometer.” To observe and duly record the times of commencement and termination of rain is very desirable. Scarcely any observer can attempt to do this ven approximately from personal observation. Hence the want of a cheap and simple self-recording rain-gauge is much felt, the present construction being too expensive for all but a few individuals.

In 1862, Mr. R. Strachan estimated the duration and amount of rain in London (Gray’s Inn Road) as follows:—

Months.	Inches.	Days.	Hours.	Months.	Inches.	Days.	Hours.
January.	1·86	19	88	July.	2·27	17	68
February.	0·37	9	25	August.	2·45	12	72
March.	3·40	22	130	September.	1·70	12	55
April.	2·34	14	80	October.	3·23	21	94
May.	3·04	16	90	November.	1·12	10	53
June.	2·45	20	83	December.	1·44	17	66

“During the year 1862, the rainfall amounted to 25·67 inches. Rain fell on 179 days, that is, on nearly every other day. The hours of rain were estimated at 904; therefore, if the rain had fallen continuously, it would have lasted nearly 38 days and nights.” The value of similar estimates of the rainfall by numerous observers would be very great to meteorology.

The principle of measurement in all these gauges is the relation existing between the areas of the collecting and receiving surfaces; that is, between the area of the funnel into which the rain falls, and the area of the cylinder which receives it. In Howard’s and Glaisher’s gauges, this cylinder is virtually the measuring glass itself; in the others, above described, the measuring scales show the same depth of water as in the cylinder of the gauge.

The cylinder being of less diameter than the funnel, and receiving all the rain collected by the funnel, it follows that its contents will have an increased depth. Now equal cylindrical volumes, having different diameters, are to each other in length inversely as the squares of the diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in diameter, a fall of 1 inch of rain will be represented in the gauge by 9 inches; for 3² : 9² : : 1 : x = 9. In this case, therefore, a length of nine inches of the measuring glass, tube, or scale, would represent an inch of rainfall, and be divided into tenths and hundredths of the artificial inch.