8o Mr. H. H. Ho worth on 



Inasmuch as the earth is a sphere, with its furnace round the 

 equator and its refrigerator at each pole, each hemisphere 

 becomes covered with a funnel of air, with its terminus at the 

 pole closed, and thus it comes about that thepolar area cannot 

 find room for all the air which is heated at the equator. 

 Professor Tyndall puts the case clearly : — " On this terres- 

 trial globe," he says, " I trace with my hand two meridians. 

 At the equator of the globe they are a foot apart, which 

 would correspond to about 1,000 miles on the earth's 

 surface. But these meridians, as they proceed northwards,, 

 gradually approach each other, and meet at the north 

 pole. It is manifest that the air which rises between 

 these meridians, in the equatorial regions, must, if it went 

 direct to the pole, squeeze itself into an ever-narrowing bed, 

 and the space around the poles is unable to embrace the air 

 from the equator." {Heat as a Mode of Motion, p. 168-9.) 

 This is a most important fact, which has been overlooked in 

 some speculations on meteorology, and it interferes com- 

 pletely with those theories which look so symmetrical,- and 

 which postulate an exchange of air between the equator 

 and the poles, an exchange which cannot physically take 

 place on a considerable scale on a spherical body like the earth. 

 Another interference is caused by the cold of space, which 

 has the effect of cooling the upper air which has risen 

 in the equatorial region, and thus making it denser 

 after it has travelled some distance, until it becomes 

 heavier than the subjacent air, and must sink by the force of 

 gravity long before it reaches the two poles. The presence 

 of great continental areas separating the great oceans, where 

 heat is more quickly absorbed and more quickly radiated 

 than over water, must also interfere somewhat with the con- 

 tinuity of the circulation. We may, however, neglect this 

 at present, and consider what must be the result of the 

 operation of the other two causes. There can be little or no 

 doubt that the result must be to greatly shorten the journey 



