the atmosphere has any defined upper limit, yet the kinetic theory of gases seems to afford evidence that the molecules of oxygen and nitrogen do not escape from the earth's attraction, and therefore the hypothesis of Professor Fˆrster is unwarranted, that interplanetary space is filled with _Himmelsluft_, or very thin air. =Temperature of the Atmosphere.=--The warmth of the atmosphere is derived chiefly from the sun's rays which, arrested by the earth's surface, are partly reflected and partly radiated back through the atmosphere. Not more than seventy-five per cent.--Professor Langley says only sixty per cent.--of the heat of the sun, which is received vertically on the upper surface of the atmosphere, penetrates to the earth, and very much less than this when the angle of the sun is low. The reason why temperature diminishes as we ascend, is partly owing to the greater loss of heat by radiation through the thinner envelope of the upper strata, and partly owing to the greater absorption of the heat given off from the earth by the lower and denser strata. In general, it may be said that there is a diminution of 1∞ Fahrenheit for each three hundred and thirty feet that we rise vertically, but, this rate varies greatly at different heights, places, and times. For instance, the decrease is not the same on mountains as it is in the free air, and in the northern hemisphere it is greater on the south than on the north sides of mountains; it is usually greatest near the ground, and is faster in summer than in winter. But in the average, the temperature falls as much for three hundred and thirty feet of elevation as it does for a change of seventy miles on the earth's surface north or south of the equator. When dry air rises, because it is heated and thereby is made lighter, the laws of thermo-dynamics show that, by reason of its expansion, its temperature is decreased 1∞ Fahrenheit for each one hundred and eighty-three feet that it ascends, and, by compression, its temperature is increased as much if it is made to descend the same distance. This is called the "adiabatic rate of change of temperature," because it is produced by an alteration in the density of the air, due to variation in pressure, without the addition or loss of heat. In the course of this book there will be occasion frequently to refer to this law of heating and cooling. The adiabatic rate of change is seldom observed on mountains because of their influence upon the currents of air in contact with their flanks, or even in balloons, on account of imperfect measurements, but, as will be explained in the closing chapter, the adiabatic change of temperature is confirmed by the observations with kites, which furnish the best method of obtaining the temperature of the free air up to moderate heights. The adiabatic cooling of rising currents of air is another reason for the rapid decrease of temperature with height up to a mile or more. The upper air alters its temperature from diurnal and