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of the gases merely expands them, without making them rise and 

 overflow to other parts. The theory of Halley, once generally adopted, 

 represented that the air was greatly heated in the particular part 

 where the sun was nearly vertical, which made the air rise in that 

 part alone, admitting cooler air to flow into the place of that which 

 had ascended, and produced an influx of cool air below, from all parts 

 around, to the heated part, and an overflow above from it. But in 

 time experience showed that this hypothesis was not in accordance 

 with facts, and it was abandoned. The theory of Haclley, which 

 has been since adopted, substitutes the whole tropical belt, for the 

 heated locality of Halley, which travelled with the sun in his daily 

 course ; but the supposed rise of air in the tropical belt, with an 

 overflow above and an influx below, was asserted to be equally un- 

 supported by experience, and, being unproved, may be fallacious. 

 The rise of heated air in a chimney, sometimes pointed at as an illus- 

 tration, was shown to be not analogous to that which takes place 

 when the sun heats the air unequally in different latitudes ; if it 

 were, the theory of Halley would be true, and cool air would flow 

 from all parts around to the greatly heated locality, just as cool air 

 passes to a fire, and, when heated, up a chimney. It was then shown 

 that it is gravitation which establishes an equilibrium of pressure in 

 the atmosphere, and that direct solar heating of the surface of the 

 earth and the air near to it, does not destroy that equilibrium. The 

 sun by heating the gases merely expands them, in proportion to the 

 increase of temperature in the part near the surface, and the gases 

 over every portion of the hemisphere that is exposed to the action of 

 the sun is proportionally heated, expanded and raised without any 

 overflow of air taking place ; leaving the equilibrium of pressure un- 

 disturbed by such heating. The solar heat merely raises the air that 

 is near the surface, over the most heated latitudes, a little higher than 

 the adjoining less heated, the difference in the rise in the various 

 latitudes, from the polar to the tropical regions, being successively 

 small ; and as there is no alteration produced in the weight of any 

 vertical column of the atmosphere, in any latitude, there is neither 

 overflow of air above, nor disturbance of the equilibrium of pressure. 

 The great disturbances that take place in the atmosphere were then 

 maintained to be caused by the heat which is conveyed, from the 

 surface of the globe, in vapour to different parts of the atmosphere 



