The Low Barometer of the Antarctic Temperate Zone. 347 



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 between C and (7. 

 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 (7. There- 

 fore, precisely as the effect of the fluid envelope collecting 

 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 unshaded part is supposed to represent the 

 atmosphere. 



I have endeavoured to make the above explanation of my 

 theory in explanation of the low antarctic barometer as com- 

 plete and exact as possible ; but there is another way of pre- 

 senting the theory which, though less complete, may appear 

 clearer to some minds : — 



Variation of mean barometric pressure, as we proceed from 

 one place to another, may be due either to a variation of cir- 

 cumstances of heat, moisture, and other like relations, or 

 to a difference of level. Maury's explanation assigns to the 

 low antarctic barometer a cause or causes falling under the 

 former category. My theory amounts to the supposition that 

 the low barometer is due to an absolute difference of level. 

 I say that the sea-level, to which we refer barometric pressure, 

 is not a just level of reference when atmospheric pressure over 

 the whole globe is the subject of inquiry, because the southern 

 seas stand out to a greater distance than the northern seas 

 from the true centre of gravity of the earth's solid and fluid 

 mass. 



Assuming my theory to be correct, we have a means — 

 rough, it may be, but not uninstructive — of determining the 

 displacement of the centre of gravity of the earth's solid mass 

 from the centre of figure. For, accepting one inch as the 

 difference of barometric height at the two poles, it is easily 

 calculated that this difference amounts to a difference of level 

 of about 850 feet. In other words, the surface of the water at 



