This applies to the so-called “troposphere”--mixing-zone. One of the most remarkable discoveries in recent times, made by Teisserenc de Bort and Assman, is the fact that the decrease of temperature with height does not continue indefinitely but only up to a certain elevation,--in middle Europe about 11 km. (7 miles), in Lapland about 7 km. (4.5 miles), and at the equator about 15 km. (10.5 miles)--and above this point the temperature remains constant. We now meet the peculiar condition that the temperature of this upper layer, which is called stratosphere--“film-zone”--is lowest over the equator, because it commences at a great height there, and lowest over the polar regions, where it extends farther down. The stratosphere has received its name from the fact that it consists so to speak of lamellæ almost parallel to the earth’s surface and moving in a horizontal direction while vertical motions are absent. The winds in these strata have a marked westerly direction (_i. e._ they are east winds) and they become stronger the higher the stratum--at an altitude of 83 km. (52.5 miles) their velocity is about 100 m. (330 ft.) per second. In the troposphere on the other hand west winds predominate. The wind direction in the stratosphere was observed on the so-called luminous night-clouds which were found as high as 80 km. (50 miles) above the Earth. These strata consequently revolve slower around the earth’s axis than the solid body of the planet itself. At an elevation of 80 km. (50 miles) the rotational speed has decreased to 65 per cent. of the angular velocity of the earth’s surface. We have reason to believe that the very highest strata stand still, that is do not take part in the earth’s rotation on its axis. This would follow if outside space were not entirely devoid of vapour so that our atmosphere would merge imperceptibly into the exceedingly attenuated gas masses of interplanetary space. As high as the mixing-zone extends, so high also is the composition of the air constant and like that at the surface of the earth. But above this limit--in Scandinavia, we might say above an elevation of 10 km. (6.2 miles)--commences a rapid tapering of the heavy gases, while the percentage of the light ones correspondingly rises. Foremost among the latter is hydrogen, with only half the weight of helium. The presence of hydrogen in the atmosphere has been shown by Boussingault, and the proportion in which it occurs has later been measured by Armand Gautier. It is about one three hundredth part of one per cent. It increases extremely rapidly with height in the stratosphere so that 80 km. (50 miles) above the earth and upward hydrogen is more abundant than all other known gases of the atmosphere at the same altitudes.