TABLE XXII.

FACTORS FOR MULTIPLYING THE EXCESS OF THE DRY BULB THERMOMETER OVER THAT OF THE WET BULB, TO FIND THE EXCESS OF THE TEMPERATURE OF THE AIR ABOVE THAT OF THE DEW-POINT.

TABLE XXIII.

WEIGHT IN GRAINS OF A CUBIC FOOT OF VAPOUR, UNDER THE PRESSURE OF 30 INCHES OF MERCURY FOR EVERY DEGREE OF TEMPERATURE FROM 0° TO 100°. THE TEMPERATURE IS THE DEW-POINT, AND THE WEIGHT OF VAPOUR IS THE WEIGHT WHICH CAN BE SUSTAINED AT THAT TEMPERATURE WITHOUT BECOMING VISIBLE.

(81) Measurement of Rain-Rain Gauges.

A rain gauge may be constructed of any open vessel of known area, the rain collected being afterwards measured in a common ounce measure. A fluid ounce contains 1.733 cubic inch of water. An easy calculation will then show how much this is per square inch of surface.

Accurate observations can however only be made by having a proper rain gauge, as represented in the diagram (see Fig. 24), which is what is known as the "Meteorological Office Rain Gauge." It has a high rim to secure the more correct measurement of snow

In this is a funnel directing the rain into a graduated glass vessel, (Fig. 25), by the aid of which the rainfall is estimated directly.

Mainly through the efforts of Mr. G. J. Symons, F.R.S., the great practical importance of an accurate knowledge of the amount of rain falling in different localities has been increasingly recognized during the last twenty-five years. The general result of experiments has been to show that the size of the gauge is not very important, but that practically from 3 to 8 inches in diameter is the most expedient. In exposed positions 5-inch gauges collect rather too little; for such stations 8-inch gauges are therefore

.

preferable. If the orifice of a gauge be nearly level with the ground it will collect an undue amount, as the water will splash in off surrounding grass, leaves, or soil; on the other hand, if the gauge be raised above the ground, it will collect less the higher it is raised. It has therefore been decided that the orifice should in all cases be 1 foot above the surface of the ground.

Mr. Symons' rules as to placing rain gauges are as follows:Rules for Observers.-A rain gauge should not be set on a slope or terrace, but 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. Tall-growing flowers, vegetables, and bushes must be kept away from the gauge. 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.

The funnel of a rain gauge must be set quite level, 1 foot above ground, and so fixed by three or four pegs that it will remain firm in spite of any gale of wind.

The gauge should be emptied regularly each day, at 9 a.m., and the amount then measured entered to the day before that of

measurement.

The measurement simply consists in emptying the contents of the bottle into the glass measure, which must be held upright, and noting the number of the division to which the water rises. Each division equals one hundredth of an inch; the fifty divisions, therefore, equal half an inch-that is to say, one division should be entered as 01; 25 as 25; and 50, or half an inch, as 50; of course, if there is more than that, it must be measured separately; for instance, twice full up to the 50 and once to the 6 would be

*50
50

·06

1.06 or one inch and six hundredths.

The amount should always be written down before the water is thrown away.

All columns should be cast up twice. When there is no rain, a line should be drawn rather than cyphers inserted.

When very heavy rains occur, it is desirable to measure on their termination, and enter the particulars as remarks, and it will be found a safe plan after measuring to return the water to the gauge, so that the morning registration will not be interfered with. Of course, if there is the slightest doubt as to the gauge holding all that falls, it must be emptied, the amount being previously written down.

In snow, melt what is caught in the funnel and measure that as rain, and select a place where the snow has not drifted, measure with a rule the average depth, and enter it in the remarks.

A fall of rain measuring the tenth of an inch in depth, corresponds to the deposit of 2,262 gallons, or about 40 hogsheads, or 10 tons weight of water per acre.

Rainy days. In recording meteorological observations it is of value to know the number of rainy days in any particular place, the accepted definition of a rainy day is a day on which falls at least 01 inch of rain.

(82) Registration of Sunlight.

In the interests of health the number of hours direct sunshine that falls on any particular locality is perhaps more important than any other single factor.

The registration of sunlight of late years has been simplified by the invention of several ingenious appliances. One of the best of these is the Whipple-Cassella universal sunshine recorder (see Fig. 26).