As a reservoir of heat and a regulator of temperature, a rather dense atmosphere is a first necessity, in co-operation with the large quantity and wide distribution of water referred to in the last section. The very different character of our south-west from our north-east winds is a good illustration of its power of distributing heat and moisture. This it does owing to the peculiar property it possesses of allowing the sun's rays to pass freely through it to the earth which it warms, but acting like a blanket in preventing the rapid escape of the non-luminous heat so produced. But the heat stored up during the day is given out at night, and thus secures a uniformity of temperature which would not otherwise exist. This effect is strikingly seen at high altitudes, where the temperature becomes lower and lower, till at a not very great elevation, even in the tropics, snow lies on the ground all the year round. This is almost wholly due to the rarity of the air, which, on that account, has not so much capacity for heat. It also allows the heat it acquires to radiate more freely than denser air, so that the nights are much colder. At about 18,000 feet high our atmosphere is exactly half its density at the sea-level. This is considerably higher than the usual snow-line, even under the equator, whence it follows that if our atmosphere was only half its present density it would render the earth unsuitable for the higher forms of animal life. It is not easy to say exactly what would be the result as regards climate; but it seems likely that, except perhaps in limited areas in the tropics, where conditions were very favourable, the whole land-surface would become buried in snow and ice. This appears inevitable, because evaporation from the oceans by direct sun-heat would be more rapid than now; but as the vapour rose in the rare atmosphere it would rapidly become frozen, and snow would fall almost perpetually, although it might not lie permanently on the ground in the equatorial lowlands. It appears certain, therefore, that with half our present bulk of atmosphere life would be hardly possible on the earth on account of lowered temperature alone. And as there would certainly be an added difficulty in the needful supply of oxygen to animals and carbonic acid to plants, it seems highly probable that a reduction of density of even one-fourth might be sufficient to render a large portion of the globe a snow and ice-clad waste, and the remainder liable to such extremes of climate that only low forms of life could have arisen and been permanently maintained. THE GASES OF THE ATMOSPHERE