111. Glaisher’s Rain-Gauge.—The rain-gauge designed by Mr. Glaisher, the well-known meteorologist, and used by most observers of the present day, is arranged for the reception of the water which falls upon its receiving surface only, and for the prevention of loss by evaporation. The rain is first collected in a funnel, B, (fig. 84,) the receiving surface of which is turned in a lathe. The conical surface of the funnel slopes to the pipe, E, at an angle of 60° from the horizontal receiving surface. The tube, E, is of small aperture, and is bent up, in order to retain the last few drops of rain, so that the only opening for the escape of vapour may be closed as long as possible. The funnel, B, fits upon the cylinder, A, tightly in the groove, D. A copper can is placed inside the cylinder, A, to receive the rain from the funnel. Once or twice a day, or after a shower, this can should be taken out, and the water measured in the glass measure, C, which is graduated to hundredths of an inch, according to the calculated quantity of water, determined by the area of the receiving space. In use, this gauge should be partly sunk in the ground, so that the top may be about five inches above it. Thus situated, there will be little or no evaporation from it during any month of the year; and the readings need not be taken daily, although desirable.
112. Rain-Gauge with Float.—In this construction the graduated glass measure is dispensed with. The cylinder of the gauge is made less in diameter than the funnel, and a hollow, very flattened spheroid of copper forming a float, and carrying a vertical graduated boxwood scale which moves through the orifice of the funnel, is placed in it. As the rain accumulates the float rises, and the amount of rain in the gauge is read upon the scale from the top of the gauge, a bar, having a hole at the centre for the passage of the scale, being fixed diametrically across the receiving space of the funnel. The gauge is provided at the bottom with a brass cock, by which the water may be allowed to flow out of it whenever necessary.
This form of gauge is not very suitable for the measurement of small quantities; but is admirably adapted for localities where the rainfall is excessive.
Fig. 85.
113. Rain-Gauge with Side-Tube.—This instrument, as represented in fig. 85, is a cylindrical vessel, mounted on a base shaped as a frustum of a cone. This base may be filled with sand or gravel to make the instrument stable, so that when placed upon a lawn or in a garden it may have an ornamental appearance. The funnel for collecting the rain is larger in diameter than the cylinder. Parallel to the cylinder, and communicating with the lowest part of the interior and extending to its top, is a graduated glass tube, open at both ends. The rain collected will rise as high in this tube as in the cylinder, and its amount can therefore be read off without any trouble. The gauge is emptied by the brass tap at the bottom of the cylinder.
114. Admiral FitzRoy’s Rain-Gauge.—A form of rain-gauge, very well adapted for expeditious observation at any time, has been designed by Admiral FitzRoy, and extensively employed by his observers. It is cylindrical in shape, with the funnel let into the top; and the rainfall is collected in an inner and much smaller cylinder, so that a small fall is represented by a considerable depth of water in the gauge. The amount of rain which has fallen is ascertained by a dipping tube, similar in principle to the dipping syphon used by gaugers for taking out specimens of wines or spirits from casks by simply removing the bung. A short, vertical, tubular opening provided with a cap, which is attached to the instrument by a chain that it may not be lost, is formed in the funnel. The measuring tube, which has a small hole at each end, should be placed upright in the gauge; then the thumb should be pressed over the upper aperture, while the tube is lifted gently out, holding in the lower part a quantity of water representing the depth of the rain in the gauge, the upper edge of which is at the mark to be read off. The glass tube is graduated to inches and tenths; hundredths of an inch can be readily estimated by the eye. The marks are fixed by actual trial with a standard gauge, and are artificial, not true, inches.
115. 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 even 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:—