6-being about the rate of 42 miles an hour. Henry Stephens informs me that, in an earlier part of the same day, when the force of the wind was greater, he made similar observations at Red braes Cottage, near Edinburgh, on the rate of motion of the clouds, which he determined, from twenty observations, to be at the rate of 109 miles an hour. And as the velocity did not even in this case appear to be at all striking, or greater than is often seen, it cannot be doubted that the rate of motion of the upper currents of the atmosphere far transcends the limits usually assigned to them.

CHAPTER X.

RAIN, SNOW, AND HAIL.

392. WHATEVER lowers the temperature of the air may be considered as a cause of rain.

Various causes may conspire

to effect this object, but it is chiefly brought about by the ascent of air into the higher regions of the atmosphere. Moist air-currents are forced up into the higher parts of the atmosphere by colder, drier, and therefore heavier, wind-currents getting beneath them, and thus wedgeways thrusting them upwards; and the same result is accomplished by ranges of mountains opposing their masses to the onward horizontal course of the winds, so that the air forced up their slopes is cooled, and its vapour condensed into showers of rain or snow. Moist air-currents are also drawn up into the higher regions of the atmosphere over the area of least pressure at the centre of storms; and in such cases the rainfall is generally very heavy. The temperature of the air is lowered, and the amount. of the rainfall increased by those winds which convey the air to higher latitudes. This occurs in temperate regions, or in those tracts traversed by the return trade-winds, which in the north temperate zone blow from the south-west, and in the south temperate zone from the north-west. The meeting and mixing of winds of different temperatures is also a cause of rain, since the several portions, when combined into one, are incapable of holding in suspension the same quantity of vapour that each could hold separately. The rainfall is also increased if the prevailing winds arrive immediately from the

sea, and are therefore moist; but diminished if they have previously passed over large tracts of land, particularly mountain-ranges, and are therefore dry. Since the quantity of rain is evidently much modified by the temperature of the earth's surface over which the rain-producing winds blow, it follows that sandy deserts, by allowing solar and nocturnal radiation to take immediate effect in raising or depressing the temperature, and forests, by delaying, if not in many cases counteracting, the effects of radiation, have each a peculiar influence on the rainfall.

393. The rain-cloud has been already generally described in paragraphs 385, 386; but the more specific conditions under which rain is precipitated are the following, as stated by M. E. Renou:-1. Two layers of clouds at least; an upper layer, the cirrus, which being at a great height is composed of minute ice-particles at a very low temperature, probably not higher than -40°; and a lower layer, the cumulus, or cumulostratus, which has its density increased and its temperature diminished by the descent of the ice-crystals of the cirrus. 2. The temperature of the air at the earth's surface as high as possible. 3. The atmospheric pressure notably lower than in surrounding regions. 4. Regular horizontal currents of air allowing the atmosphere to remain a sufficiently long time in a state of unstable equilibrium. 5. A rapid movement of the air tending to re-establish the equilibrium of temperature and pressure, by mixing together the different layers of the atmosphere. The geographical distribution of rain over the globe is proportioned to the temperature, the humidity, the mean depression of the barometer, the fluctuations in the temperature, and the configuration of the earth's surface.

394. Rain sometimes falls from a cloudless sky, and is then called Serein. Sir J. C. Ross thus describes a case which occurred near Trinidad on the 25th December 1839: "It was a beautiful clear night, not a cloud to be seen in any part of the heavens, yet we had a light shower of more than an hour's continuance. The temperature of the dew-point was 72°, and that of the air 74°." Many similar cases have been recorded.

395. The instrument for ascertaining the quantity of rain which falls is called a Rain-gauge. Rain-gauges are of vari

Fig. 29.

what is still simpler, by a hole, a, fig. 28, pierced
in the funnel at the top. As this form of gauge
is objectionable from the fre-

quent breaking of the glass
tube in time of frost, a float
is used instead, which is
raised by the water, and a
scale is attached to it pro-
jecting above the gauge, by
which the quantity of rain
is measured. This is the
gauge, fig. 29, commonly
known as Fleming's gauge,
which is used extensively in
Scotland. Since this gauge
does not admit of very nice
measurement, another sort
is frequently used, consisting

Fig. 30.

of a receiving-vessel, and a glass measure of much smaller diameter, which admits of as nice graduation as may be de

sired. A good specimen of this class is the gauge recommended by G. J. Symons, London, fig. 30, in which bis the vessel which receives the rain, and e the graduated vessel which measures the amount. There being often great difficulty or trouble experienced in replacing the glass measure when it chances to get broken, the late G. V. Jagga Rao,

Y

Fig. 31.

a wealthy zemindar of Vizagapatam, proposed a gauge (fig. 31) in the form of a funnel, having a diameter of 4.697 inches, or a receiving area of 17.33 square inches. Now, since a fluid ounce contains 1.733 cubic inches of water, it follows that for every fluid ounce collected by this gauge the tenth of an inch of rain has fallen. The measure can of course be graduated to any degree of nicety; and it may easily be reproduced if required. It is also the cheapest rain-gauge, costing only 7s. 6d. when made of copper, and 4s. 6d. when made of tin. Self-registering rain-gauges have been invented by Osler and Crosley; but, being too expensive for general use, they need not be described here.

396. As regards the size of the receiving surface of a raingauge, or its diameter, series of observations, with gauges of different diameters, have been conducted since the beginning of 1864 by Colonel Ward, Castle House, Calne, Wilts, and by the Rev. J. Chadwick Bates, Catleton Moor, Manchester. Mr Symons has discussed these three years' observations in his 'British Rainfall, 1866,' and drawn the conclusion that "so far as size is concerned, no difference exceeding 1 or 2 per cent exists between the indications of gauges whose apertures have areas from 12 to 452 square inches." Gauges 1 inch or 2 inches in diameter register too little. The 3-inch gauge is not included in the list, which is an unfortunate oversight, considering its extensive use in certain parts of Great Britain; but, judging from the returns of the other sizes, it is probable that it would have measured about the same amount as the larger gauges. We may therefore conclude that gauges, from 3 inches diameter and upwards, are all nearly equally good for