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It is, however, not at all an easy matter to ensure that the observations of terrestrial radiation shall be strictly comparable at all stations and at all seasons of the year. The rule given is to place the thermometer on greensward with its bulb just on the level of the tops of the grass. In many places, even in this country, it is not easy to ensure that grass shall be always available in winter, and that in summer it shall not be allowed to grow too high.

When speaking of temperature I have said that the effect of solar radiation varies with the nature of the surface on which the rays fall, being least on ice or water and greatest on a sandy soil, and instances of the extraordinary temperatures attained under the lastnamed conditions have been cited. In proportion as the ground is covered with, or permeated by, water, is the effect of solar radiation, in warming it, reduced, for a large proportion of the heat which reaches it is employed in evaporating some of the water, and is, therefore, rendered latent.

The facility with which heat is radiated from the earth into space is affected to a great extent by the nature of the covering with which the earth is provided.

Grass and herbage are better radiators than earth or gravel, because the heat they lose is not made good by conduction, and so the temperature at night will fall far lower over a meadow than over a road. The action of trees is more energetic than that of grass.

The indications of a thermometer placed just over grass will, therefore, as a general rule, range, during the night time, when radiation from the earth is most

active, several degrees below those of a similar instrument placed in a screen above the grass, and protected from the action of radiation. As a general rule, the grass minimum thermometer reads several degrees below the ordinary minimum at the height of 4 feet above the ground. In some instances, however, as, e.g., during very wet fogs, it has been found that the temperature on the grass has been higher than that in the screen; but this phenomenon is very rare.

In order to secure a surface of uniform radiating power in all climates and at all seasons, it has been suggested to lay the thermometers on a piece of cloth and dispense with the use of grass altogether. The question of nocturnal radiation is, however, very complicated, and requires much further investigation; we shall see its importance when treating of dew and hoar-frost in Chapter VII. A careful and elaborate discussion of the subject will be found in a paper by Mr. Jas. Glaisher, Phil. Trans.,' 1847.1

A very practical use of nocturnal radiation has been made from time immemorial in India in the preparation of ice, and on such a scale that about 10 tons of ice can be procured in a single night from twenty beds of the dimensions about to be given, when the temperature of the air is 15° or 20° above the freezingpoint. An account of the practice followed will be found in a paper by Mr. T. A. Wise, given in 'Nature,' vol. v., p. 189. The locality referred to is the immediate neighbourhood of Calcutta. A rectangular piece of ground is marked out, lying east and west, and

1 'On the Amount of the Radiation of Heat at Night from the Earth, and from various bodies placed on or near the surface of the Earth.' By Jas. Glaisher, Phil. Trans. 1847, p. 119.

measuring 120 by 20 feet. This is excavated to the depth of two feet, and filled with rice straw, rather loosely laid, to within six inches of the surface of the ground. The ice is formed in shallow dishes of porous earthenware, and the amount of water placed in each is regulated by the amount of ice expected.

In the cold weather, when the temperature of the air at the ice-fields is under 50°, ice is formed in the dishes. The freezing is most active with N.N.W. airs, as these are the driest; it ceases entirely with southerly or easterly airs, even though their temperature may be lower than that of the N.N.W. wind.

No ice is formed if the wind is sufficiently strong to be called a breeze, for the air is not left long enough at rest, above the bed, for its temperature to fall sufficiently, by the action of radiation.

The rice straw, being kept loose and perfectly dry, cuts off the access of heat from the surface of the ground below it, and, when the sun goes down, the temperature of the air in contact with the dishes is reduced some 20° below that prevailing two or three feet above them. The rapid evaporation of the water into the dry air above creates also an active demand for heat to be rendered latent in the formation of steam, and the result of all these agencies is the formation of ice, under favourable circumstances, on the extensive scale above mentioned.

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CHAPTER V.

PRESSURE.

THE pressure of the atmosphere comes next in importance to its temperature as the cause of many phenomena of great consequence to human life.

This property of the atmosphere was first discovered by Torricelli, a pupil of Galileo's, who also devised the means of measuring it. Galileo had noticed that

water would not rise in a pump over a certain height, eighteen cubits (diciotto braccia), above the level of the well; and he endeavoured to explain this fact by comparing the column of water in the pump-bore to a cylindrical rod held by its upper end, which by its own weight lengthens itself and at last breaks.

Torricelli, in 1643, however, devised the following experiment which decided the question. He took a tube about 3 feet long, closed at one end, and filled it with mercury. He then inverted this, and plunged its lower end into a basin filled half with mercury and half with water. When the open end of the tube was immersed in the mercury, the level of the mercury in the tube sank until the length of the column, measured from the surface of the mercury in the basin, was about 30 inches, leaving the space in the tube above the column empty. When, however, the open end of the tube was raised above the level of the mercury, but so

as to be still under the surface of the water, the contained mercury immediately fell out of the tube, and its place was taken by the water, which rushed in with great violence and entirely filled the tube. This experiment proved that the height of the column of liquid which would stand in any tube depended on the specific gravity of the liquid of which the column was composed.

The reason why a fluid stands at all in such a tube is that the air presses downwards with a uniform force on all parts of a free fluid surface. If, then, we relieve a portion of that surface from the pressure by inverting an exhausted tube over it, the fluid will be forced up in a column over the space whence pressure has been removed, i.e. into the tube, until the weight of that column exercises a pressure equal to that of the air outside.

This pressure is, speaking generally, about 14.7 lbs. on each square inch, and as the specific gravity of mercury is 13.59, and the weight of a cubic inch of water is 252.5 grains, an easy calculation will show that about 34 feet of water, or 30 inches of mercury, will counterbalance the pressure of the atmosphere.

Use is made of the pressure of the atmosphere in the construction of the pump. The air is drawn out of the bore by the sucker, and the water rushes up after it to fill the vacuum produced. In the case of the barometer, we do not draw the air out of the tube, but we fill the tube with mercury and invert it in a basin of the same liquid, when, as above explained, the length of the column supported in the tube will be about 30 inches.

I have said that the space left in the tube above the

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