xxviii.] LOW PRESSURE NEAR PHOTOSPHERE. 405 



cannot get out of, and which has a definite relation to the mass of 

 our own earth. 



Now if these arguments are of any value we must concede 

 that the mean density of the sun is very low indeed, much 

 lower than that of any planet or satellite with which we are 

 acquainted ; so that we are perfectly justified in saying that 

 it is an enormous globe of gas, by which I do not mean that it 

 is absolutely and completely gaseous to the core. The gases of 

 the centre gases under very great pressure may put on the 

 appearance, if they do not put on all the physical properties, 

 of liquids. 



It will be seen that this revised value of the sun's mean 

 density is quite in accordance with the hypothesis. There is 

 another very important consideration which depends naturally 

 upon this revised value of the mean density, and which to clear 

 the ground we must refer to it in this place. 



The photosphere, as already stated, is about 400,000 miles 

 in round numbers from the sun's centre, and has an outer 

 atmosphere 500,000 miles high above it. If we take the lower 

 average density of the sun we note that the photosphere, assum- 

 ing it to be a shell, exists in a region of low pressure, and we 

 see in a moment that, unless we suppose the photosphere, or 

 something immediately inside the photosphere, to be solid, there 

 is no reason for supposing any sudden increase of pressure at the 

 photosphere itself. In fact, there are a great many reasons for 

 regarding this as improbable, not to say impossible, and the 

 lower the density the less likely is it. 



Hence, any changes at the photospheric level must be due to 

 something else than pressure, in all probability to temperature ; 

 still of course the pressure below will be higher than that above, 

 and the line of least resistance will be upwards. 



The ground then having been cleared, let us consider the 

 shells in a condition of perfect equilibrium ; there will be no 

 currents either up or down. 



