Rain is of all meteorological phenomena the most capricious, both as regards its frequency and the amount which falls in a given time. In some places it rarely or never falls, whilst in others it rains almost every day; and there does not yet exist any theory from which a probable estimate of the rainfall in a given district can be deduced independently of direct observation. But although dealing with one of the most capricious of the elements, we nevertheless find a workable average in the quantity of rain to be expected in any particular place, if careful and continued observations are made with the rain-gauge. G. J. Symons, the meteorologist, to whose continued investigations we are indebted for our most reliable data upon the subject of rainfall, gives the following practical instructions for using a rain-gauge;—
“The mouth of the gauge must be set quite level, and so fixed that it will remain so; it should never be less than 6 inches above the ground, nor more than 1 foot except when a greater elevation is absolutely necessary to obtain a proper exposure.
“It must be set on a level piece of ground, at a distance from shrubs, trees, walls, and buildings, at the very least as many feet from their base as they are in height.
“If a thoroughly clear site cannot be obtained, shelter is most endurable from N.W., N., and E., less so from S., S.E., and W., and not at all from S.W. or N.E.
“Special prohibition must issue as to keeping all tall-growing flowers away from the gauges.
“In order to prevent rust, it will be desirable to give the japanned gauges a coat of paint every two or three years.
“The gauge should, if possible, be emptied daily at 9 A.M., and the amount entered against the previous day.
“When making an observation, care should be taken to hold the glass upright.
“It can hardly be necessary to give here a treatise on decimal arithmetic; suffice it therefore to say that rain-gauge glasses usually hold half an inch of rain (0·50) and that each 1⁄100 (0·01) is marked; if the fall is less than half an inch, the number of hundredths is read off at once, if it is over half an inch, the glass must be filled up to the half inch (0·50), and the remainder (say 0·22) measured afterwards, the total (0·50 + 0·22) = 0·72 being entered. If less than 1⁄10 (0·10) has fallen, the cipher must always be prefixed; thus if the measure is full up to the seventh line, it must be entered as 0·07, that is, no inches, no tenths, and seven hundredths. For the sake of clearness it has been found necessary to lay down an invariable rule that there shall always be two figures to the right of the decimal point. If there be only one figure, as in the case of one-tenth of an inch, usually written 0·1, a cipher must be added, making it 0·10. Neglect of this rule causes much inconvenience.
“In snow three methods may be adopted—it is well to try them all. 1. Melt what is caught in the funnel, and measure that as rain. 2. Select a place where the snow has not drifted, invert the funnel, and turning it round, lift and melt what is enclosed. 3. Measure with a rule the average depth of snow, and take one-twelfth as the equivalent of water. Some observers use in snowy weather a cylinder of the same diameter as the rain-gauge, and of considerable depth. If the wind is at all rough, all the snow is blown out of a flat-funnelled rain-gauge.”