306 LIGHT SCIENCE FOR LEISURE HOURS. 



placement of the centre of gravity of the solid portion. 

 But when we consider that the specific gravity of the 

 fluid envelope is less than one-fifth of that of the solid 

 globe, it is perfectly clear that the centre of gravity of 

 the entire mass will not be so far south as C'. For, of 

 the entire mass, the northern half is the heavier, and 

 therefore the centre of gravity must lie north of the 

 centre of the entire mass that is, north of C'. In 

 fact, it must lie much nearer to C than to C'. 



Thus, the centre of gravity of the solid globe, and 

 that of the entire mass, solid and fluid, both lie be- 

 tween C and C'. Now it is evident that the central 

 point, about which the earth's atmospheric envelope 

 tends to form itself as a spherical or spheroidal shell, is 

 the centre of gravity of the entire solid and fluid 

 terrestrial globe that is, is a point north of C'. There- 

 fore, precisely as the effect of the fluid envelope collect- 

 ing itself centrally about a point south of C is to cause 

 the mean depth of water to be greatest in the southern 

 hemisphere, so the fact that the atmospheric envelope 

 collects itself centrally about a point north of C' should 

 result in giving a greater mean depth of air (referred 

 to the sea-level) over the northern hemisphere. This 

 arrangement is represented in Fig. 3, in which the un- 

 shaded part is supposed to represent the atmosphere. 



I have endeavoured to make the above explanation 

 of my theory in explanation of the low antarctic baro- 

 meter as complete and exact as possible ; but there is 

 another way of presenting the theory, which, though 

 less complete, may appear clearer to some minds : 



