4.56 THE POPULAR SCIENCE MONTHLY 



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 propa- 

 gated, 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 ex- 

 plained 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 tempera- 

 ture 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 at- 

 mosphere 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 diam- 

 eter 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 diam- 

 eter. 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- 



