It will be seen from the table that air on a warm day in summer, with a temperature of 77° F., can hold nearly five times as much moisture as air at the freezing point, or 32° F. The amount actually present, however, is usually below the maximum, and is recorded for meteorological purposes as a percentage of the maximum. Thus if the “relative humidity” at 77° F. were 70 per cent., it would imply that the weight of moisture in 1 cubic metre was 70 per cent. of 22·8 grammes; that is, nearly 16 grammes. If 1 cubic metre of air at 77° F., containing 16 grammes of moisture, were cooled to 50° F., a quantity of water equal to (16-9·3) = 6·7 grammes would separate out, as the maximum content at the lower temperature is 9·3 grammes. Precipitation would commence at 66° F., at which temperature 1 cubic metre is saturated by 16 grammes. And similarly for all states of the atmosphere with respect to moisture, cooling to a sufficient extent causes deposition of water to commence immediately below the saturation temperature, and the colder the air becomes afterwards the greater the amount which settles out. The temperature at which deposition commences is called the “dew point.” Whenever atmospheric moisture assumes the liquid form, drops are invariably formed. These may vary in size, from the tiny spheres which form a mist to the large raindrops which accompany a thunderstorm. But in every instance it is necessary that the air shall be cooled below its saturation point before the separation can commence; and keeping this fact in mind we can now proceed to demonstrate the production of mists and fogs. Here is a large flask containing some water, fitted with a cork through which is passed a glass tube provided with a tap. I pump some air into it with a bicycle pump, and then close the tap. As excess of water is present, the enclosed air will be saturated. Now a compressed gas, on expanding into the atmosphere, does work, and is therefore cooled; and consequently if I open the tap the air in the flask will be cooled, and as it was already saturated the result of cooling will be to cause some of the moisture to liquefy. Accordingly, when I open the tap, the interior of the flask immediately becomes filled with mist. If we examine the mist in a strong light by the aid of a magnifying glass, we observe that it consists of myriads of tiny spheres of water, which float in the air, and only subside very gradually, owing to the friction between their surfaces and the surrounding air preventing a rapid fall. The smaller the sphere, the greater the area of surface in proportion to mass, and therefore the slower its fall. And so in nature, the mists are formed by the cooling of the atmosphere by contact with the surface, until, after the saturation point is reached, the surplus moisture settles out in the form of tiny spheres, which float near the surface, and are dissipated when the sun warms up the ground and the misty air, and thus enables the