perature gradient, and induces a more lively movement in the lower levels. In certain localities, in order to keep up the vertical continuity of barometric pressure, the warm air slides out radially in all directions, where conditions are right, and this movement first induces the vortical action in the upper sections of the hurricane which is gradually propagated, when it is highly developed, to the surface. Tornadoes are formed in somewhat a similar way, but in this case the cold and warm masses lie side by side on the same level, though there is a tendency for the cold air to overflow the warm air. The sliding action of the warm air against the cold sheet is the first incentive to the curling-up process which culminates in a tornado. In the ordinary cyclones the temperature distribution is such that the vertical gradient is about the same in the cold as in the warm mass, taken separately, though there are moderate variations in the different quadrants surrounding the high and the low areas of pressure. The warm air then overflows the cold air in two branches, and the cold air underflows the warm air in two branches. This tends to induce vortical action, but as already explained it is retarded, and the development is very imperfect on account of its intrusion into the eastward drift.
While our knowledge of the distribution of velocity and temperature in the atmosphere of the sun is much less perfect than it is of the atmosphere of the earth, we have yet definite knowledge regarding several important features. Apparently the sun's atmosphere does not operate in the same way that we have found to be the method of the circulation of the atmosphere of the earth. It is quite easy to see that these two atmospheres should work in a very different way. The atmosphere of the earth is really a thin shell of air heated in one zone by the solar radiation falling upon it, and then this thin shell simply slides around over the surface of the earth according to the laws which have been described. In the case of the atmosphere of the sun we have no definite knowledge as to its depth. It is proper to infer, from the law of pressure and mass, that the density near the center is such that the interior of the solar mass consists of a nucleus in a highly viscous or even solid state. Such a nucleus may be only one third of the diameter of the sun, but as the radius of the sun is 694,800 kilometers it would make a nucleus of about 400,000 or 500,000 kilometers in diameter. Above this the shell of the sun would be something like 400,000 kilometers thick, that is, about 250,000 miles. Our observations can not penetrate below the surface of the solar photosphere, and of course it is impossible to trace out the great currents which are undoubtedly operating within this enormously thick mass. On the surface we know from various sources that at the equator the solar mass drifts from east to west as we look at the sun's disc with a velocity such that the sun turns on its axis, as we see it, once in 26.68 days. This rota-
