The systematic measurement of rainfall is one of those pursuits which prove more interesting in the doing than in the prospect. It enables us to compare one season or one year with another; tells us what the weather has been while we slept; affords a little mild excitement when thunderstorms are about; and compensates to a limited extent for the disadvantages of a wet day.
The general practice is to examine the gauge daily (say at 10 a.m.); to measure the water, if any, collected during the previous twenty-four hours; and to enter the record at once. Gauges are made which record automatically the rainfall on a chart or dial, but these are necessarily much more expensive than those which merely catch the water for measurement.
This last class, to which our attention will be confined chiefly, all include two principal parts--a metal receiver and a graduated glass measure, of much smaller diameter than the receiver, so that the divisions representing hundredths of an inch may be far enough apart to be distinguishable. It is evident that the smaller the area of the measure is, relatively to that of the receiver, the more widely spaced will the graduation marks of the measure be, and the more exact the readings obtained.
The gauge most commonly used is that shown in Fig. 151. It consists of an upper cylindrical part, usually 5 or 8 inches in diameter, at the inside of the rim, with its bottom closed by a funnel. The lower cylindrical part holds a glass catcher into which the funnel delivers the water for storage until the time when it will be measured in a graduated glass. The upper part makes a good fit with the lower, in order to reduce evaporation to a minimum.
Such a gauge can be bought for half a guinea or so, but one which, if carefully made, will prove approximately accurate, can be constructed at very small expense. One needs, in the first place, a cylindrical tin, or, better still, a piece of brass tubing, about 5 inches high and not less than 3 inches in diameter. (Experiments have proved that the larger the area of the receiver the more accurate are the results.) The second requisite is a piece of stout glass tubing having an internal diameter not more than one-quarter that of the receiver This is to serve as measuring glass.
The success of the gauge depends entirely upon ascertaining accurately how much of the tube will be filled by a column of water 1 inch deep and having the same area as the receiver. This is easily determined as follows:--If a tin is to be used as receiver, make the bottom and side joints watertight with solder; if a tube, square off one end and solder a flat metal to it temporarily. The receptacle is placed on a perfectly level base, and water is poured in until it reaches exactly to a mark made 4 inches from the end of a fine wire held perpendicularly. Now cork one end of the tube and pour in the water, being careful not to spill any, emptying and filling again if necessary. This will give you the number of tube inches filled by the 4 inches in the receiver. Divide the result by 4, and you will have the depth unit in the measure representing 1 inch of rainfall. The measuring should be done several times over, and the average result taken as the standard. If the readings all agree, so much the better.
The next thing is to graduate a scale, which will most conveniently be established in indelible pencil on a carefully smoothed strip of white wood 1 inch wide. First make a zero mark squarely across the strip near the bottom, and at the unit distance above it a similar mark, over which "One Inch" should be written plainly. The distance between the marks is next divided by 1/2-inch lines into tenths, and these tenths by 1/4-inch lines into hundredths, which, if the diameter of the receiver is four times that of the tube, will be about 3/16 inch apart. For reading, the scale is held against the tube, with the zero mark level with the top of the cork plugging the bottom. It will, save time and trouble if both tube and scale are attached permanently
Fig. 151. Standard rain-gauge.
Fig. 152. Section of homemade rain-gauge.
to a board, which will also serve to protect the tube against damage.
A tin funnel, fitting the inside of the receiver closely, should be obtained, or, if the exact article is not available, a longer one should be cut down to fit. Make a central hole in the bottom of the receiver large enough to allow the funnel to pass through up to the swell, and solder the rim of the funnel to the inside of the receiver, using as little heat as possible.
If you select a tin of the self-opening kind, you must now cut away the top with a file or hack-saw, being very careful not to bend the metal, as distortion, by altering the area of the upper end of the tin, will render the gauge inaccurate.
The receiver should be supported by another tin of somewhat smaller diameter, and deep enough to contain a bottle which will hold 3 or 4 inches of rainfall. In order to prevent water entering this compartment, tie a strip of rubber (cut out of an old cycle air tube) or other material round the receiver, and projecting half an inch beyond the bottom (Fig. 152).
All tinned iron surfaces should be given a couple of thin coats or paint.
The standard distance between the rain gauge and the ground is one foot. The amount caught decreases with increase of elevation, owing to the greater effect of the wind. The top of the gauge must be perfectly level, so that it may offer the same catchment area to rain from whatever direction it may come.
To simplify measurement, the receiver and tube may be arranged as shown in Fig. 153. In this case the water is delivered directly into the measure, and the rainfall may be read at a glance. On the top of the support is a small platform for the receiver, its centre directly over the tube. The graduations, first made on a rod as already described, may be transferred, by means of a fine camel's hair brush and white paint, to the tube itself. To draw off the water after taking a reading, a hole should be burnt with a hot wire through the bottom cork. This hole is plugged with a piece of slightly tapered brass rod, pushed in till its top is flush with the upper surface of the cork.
If the tube has small capacity, provision should be made for catching the overflow by inserting through the cork a small tube reaching to a convenient height-say the 1-inch mark. The bottom of the tube projects into a closed storage vessel. Note that the tube must be in position before the graduation is determined, otherwise the readings will exaggerate the rainfall.
A rain-gauge of this kind requires protection against frost, as the freezing of the water would burst the tube. It will be sufficient to hinge to the front of the support a piece of wood half an inch thicker than the diameter of the tube, grooved out so as to fit the tube when shut round it (Fig 154).
Fig. 153. Self-measuring gauge.
Fig. 154. Gauge in case.