country as in continental climates. The latter appears in a very marked way in tropical countries, where its connection with the ascending currents of air is unmistakeable. In some parts of Brazil the clouds at certain seasons collect every afternoon to form a nimbus, resulting in a thunder-storm, and the same phenomenon is said to occur frequently at the Lake of Como.

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CHAPTER VIII.

RAIN, SNOW, AND HAIL.

Rain. We now come to the most important mode of restoration to the earth of the moisture which is abstracted from its surface by evaporation, and which returns to it most frequently in the liquid, but occasionally in the solid, state.

It is comparatively an easy matter to collect rain, but the case is different as regards hail and snow. Indeed the drifting of snow produces such irregularities in the distribution of the fall that any estimation of the general amount collected over a given area must be made more or less at random.

The simplest form of rain-gauge is Howard's consisting of a funnel to catch the rain and a Winchester quart bottle to receive and collect it. The only objection to this is the liability of the bottle to break in frost or with hard usage. The water collected is always measured in a graduated glass, which shows true inches for the aperture of the gauge. gauge. Glaisher's gauge is a modification of Howard's, with a copper can, instead of a bottle, to collect the rain as it flows from the funnel, the delivery-tube being, moreover, bent in a U form, in order to check evaporation from the receiver below. The chief improvements recently effected in the construction of these gauges are the

straightening of the small tube just mentioned, to prevent its being choked with dirt, and the addition of a vertical cylinder on the top to retain snow when it falls, and prevent its being blown out of the gauge, as would be very likely to happen if the funnel were

FIG. 23.

Rain Gauge with
Measuring Glass.

shallow. This improvement was apparently first proposed by Quetelet in 1852.1 Such gauges are now known as Snowdon gauges.' Fig. 23 shows this gauge in its most recent form. The diameter of the cylinder is 8 inches.

In addition to the simple gauges just mentioned there are endless other patterns. We have float-gauges adapted for use at mountain stations which are rarely accessible; various forms of au

tomatic gauges, which are moved either by the wind or by a clock, so as to separate the amount collected either according to the direction of the wind or to the hour at which the fall took place; and finally, we have gauges, like Beckley's, which show continuously the rate at which the rain comes down.

The measurement of rain seems an easy matter, but practically it requires considerable precautions.

Firstly we infer the amount collected over a large district from that caught in a small funnel, so that great care is requisite to ensure that we know exactly the sectional area of the funnel at its top. Mr. Symons and others have recommended the use of 5-inch gauges. The Meteorological Office, however, prefers the 8-inch gauge the size originally adopted by Mr. Glaisheras affording greater security against error in measure

Sur le Climat de la Belgique, cinquième partie, p. 2. Brussels, 1852.

ment, the amount to be measured being larger than if the diameter were only 5 inches.

If we know the size of our gauge we must ensure that this size will not be changed, as would be the case if the shape of the aperture were altered. Most gauges are circular, and, as a circle encloses the greatest area of all closed curves of the same perimeter, the least dint, or squeezing in, of the sides of the funnel must make it indicate too small an amount of rain.

Secondly, precautions must be taken that there shall be no loss, by evaporation, of the water collected in the gauge. This is very likely to occur with passing showers on a hot day, and Mr. Glaisher attempted to meet the difficulty by curving the delivery-tube.

Thirdly, the greatest care is required as to the exposure of the gauge, both in regard of its being sheltered by trees or walls and in regard of its height above ground. It is unnecessary to insist on the importance of a perfectly open site. Every one takes advantage of a tree or wall for shelter from a shower. Such obstacles, even at some distance, produce irregular currents of air and disturb the equable fall of the rain. No object ought to subtend a greater angle from the horizon than 20° in any direction from the gauge. The gauge must be firmly fixed, so that it shall not blow over, and its rim must be perfectly level.

The height above the ground is a matter of no less importance in rain measurements. It has been generally decided in this country to adopt one foot above the ground for the height of the gauge, but this rule has not been followed abroad. It has long since been established that more rain is collected at the surface of the ground than at any elevation above it. The

reason of this is now clearly understood, owing to the researches of Mr. Symons and Mr. Dines.1 The difference is simply due to the mechanical effect of a building or other support, on which the gauge is placed, in producing eddies, and it increases with the force of the wind.

In connection with the placing of gauges on roofs or at a height above the ground, it is of importance to draw attention to the great influence exerted by a vertical wall on the indications of a gauge placed upon it. If you are standing on the edge of a cliff against which a violent wind is blowing, you find yourself in a calm, owing to the upward blast produced by the impact of the wind on the cliff face, and interfering with the current, which would otherwise sweep over the top of the cliff. It is a well-known fact that sheep will crowd to the edge of a weather' cliff in a gale, as they know that a few yards back from the edge they will feel the wind with increased force. The rain drops will accordingly be carried over and past a gauge on a wall when the wind is blowing against that wall.

The late Rev. Dr. Robinson, of Armagh, has told the writer that he once had a very perfect illustration of this fact. He had constructed a peculiar gauge, consisting of a sphere, erected over a funnel, which, therefore, exposed the same section to the rain at whatever angle it might be driving. Under ordinary circumstances this gauge collected more than one of the ordinary pattern. It was, however, erected on the parapet of the flat roof of the Observatory. The distance from the dormer window, which gave access to

› Report of the British Association, 1881, p. 551. British Rainfall, 1880, p. 13; 1881, p. 41.