.sVA' \\'11.LIAM .SY/.. !///. Y.V, l-.R.S. 297 



(han towards the floor, and that upon plunging a ther- 

 nio u tank of heated water the surface temperature is 

 found slightly superior to that near the bottom. 



Tin's.- ronviH'tiou currents owe their existence to a preponde- 

 rance of the cooled descending over the ascending current ; but 

 this difference being slight, and the ascending and descending 

 currents intermixing freely, they are, generally speaking, of a 

 .slavish character ; hence in all heating apparatus it is found 

 essential to resort either to artificial propulsion, or to separating 

 walls between the ascending and the descending currents, in order 

 to give effect to the convective transfer of heat. 



In the case of a fluid sun another difficulty presents itself 

 through the circumstance that the vast liquid interior is en- 

 veloped in a gaseous atmosphere, which, although perhaps some 

 thousands of miles in depth, represents a relatively very small store 

 of heat. Convection currents may be supposed active in both 

 the gaseous atmosphere and in the fluid ocean below, but the 

 surface of this fluid must necessarily constitute a barrier between 

 the two convective systems, nor could the convective action of 

 the gaseous atmosphere, that is to say, the simple up and down 

 currents caused by surface refrigeration, be such as to disturb 

 the liquid surface below to any great extent, because each de- 

 scending current would have had plenty of time to get inter- 

 mixed with its neighbouring ascending current, and would, 

 therefore, have reached its least intensity on arriving on the 

 liquid surface. 



As regards the liquid, its most favourable condition for heating 

 purposes would be at the critical point, or that at which the 

 slightest diminution of superincumbent pressure would make it 

 flash off into gas ; but considering that, by means of conduction 

 and convection, the liquid matter must have assumed in the 

 course of ages a practically uniform temperature to a very con- 

 siderable depth, it follows that the liquid below the surface, with 

 fluid pressure in addition to that of the superimposed gaseous 

 atmosphere, must be ordinary fluid, the critical condition being 

 essentially confined only to the surface. 



Conditions analogous to those here contemplated are met with 

 in a high-pressure steam boiler, with its heated water and dense 

 vapour atmosphere. Suppose the fire below such a boiler be 



