THB GASEOUS POBTTOMS OF THE ATMOSPnERE. 



16 



part. But this mist being regularly formed by successive 

 condensations of vapour, and the vapour being as regularly 

 carried up in an ascending current that attains a great height, 

 is constantly heating and expanding the gases. Condensation, 

 therefore, may be confidently presumed to heat, expand, and 

 lighten the gases to so great an extent as to make their 

 pressure, even when added to that of the aqueous matter 

 that is present, less than the mean, and not more than it is 

 actually found at or near to the equator. 



The parts over the Atlantic Ocean, distant say about 30** 

 of latitude from the equator on each side, where the baro- 

 meter stands the highest, are occupied to a considerable 

 extent by the north and south-eastern trade winds, which 

 have but little condensation of vapour going on in them, and 

 the great mass of the air in these winds will consequently not 

 be heated by condensation. We may therefore conclude that 

 the cool state of the gases in these parts renders them heavy, 

 and produces the superior average atmospheric pressure that 

 is found in these latitudes. 



As we proceed farther from the equator, the general tem- 

 perature is lower and the tension of vapour is less ; the former 

 tends to increase and the latter to decrease the total pressure ; 

 and where the atmosphere is undisturbed by condensation, 

 the two causes would appear to produce nearly a mean atmos- 

 pheric pressure. This is seen principally in the northern 

 parts of America and Asia, where there is but little vapour, 

 and that little is not much disturbed by condensation. In 

 Iceland, however, where there is much condensation, the 

 pressure, as we have seen, is low. The same general facts are 

 traceable in this country and on the coast of Norway, all 

 those places shewing eflfects of condensation. 



In the southern hemisphere there are features not observ- 

 able in the northern, resulting apparently from the different 

 proportions of land and water surfaces, in the two parts. In 

 the northern the great extent of land leaves but a comparatively 



