The observations from balloons at great heights are neither sufficiently numerous nor accurate to enable us to form an opinion as to what is the temperature of interplanetary space, which the kinetic theory of gases places at 460∞ Fahrenheit below zero. This temperature is called "the absolute zero," and is calculated from the fact that air under a constant pressure contracts 1/490 of its volume for each degree Fahrenheit it is cooled below the temperature of freezing water, and consequently under no pressure it should have an infinite volume and a temperature of about 490∞ below freezing, or 458∞ below zero. There are other hypotheses regarding the temperature of space, but since it can never be measured directly, it will probably remain a matter of speculation. It is certain, however, that if the earth were deprived of its atmosphere, the temperature would fall very low, and even with our atmosphere as a blanket our earth would be uninhabitable were it not for the aqueous vapour which controls the selective absorption of the solar rays, transforming them into obscure rays so that they cannot escape from the atmosphere. Water-vapour, then, is a very important factor in the physics of the atmosphere, but it can only be considered briefly here. =Moisture of the Atmosphere.=--The air is constantly absorbing moisture from the water on the earth, but the tension of this aqueous vapour decreases with elevation much faster than does the atmospheric pressure. At the height of about a mile and a quarter half the quantity of water-vapour is below, while we must rise about three and a half miles to reduce the quantity of air one-half, as may be seen in Plate I. The relative humidity, or the percentage of moisture in the air, as compared to the amount which it could contain at that temperature, is nearly the reverse at low and at high levels. It is found from the kite-observations at Blue Hill, that up to the height of a mile or two the air is drier during winter and at night, and damper during summer and in the day-time than it is near the ground. At great heights probably the air is always very dry. The condensation of the invisible vapour into a visible form is considered in the next chapter on clouds.