INTRODUCTION. 



eastern by '013 in the northern hemisphere ; but in the southern, it 

 is -047 in favour of the eastern. The annexed diagram will serve to 

 illustrate this irregular outline, which is produced by projecting the 

 corresponding mean heights of the barometer for every 5 of latitude. 

 Although it is difficult to account or assign a cause for this pheno- 

 menon, yet it would seem to result from the difference or unequal 

 distribution of the land and water in the two 

 hemispheres. And this opinion is somewhat 

 strengthened from the same, though a much 

 less difference, occurring in longitude, as it 

 will be perceived that the western section, 

 comprising the Pacific Ocean, is almost desti- 

 tute of land, when brought into comparison 

 with the eastern on the same parallels. The 



bulging of the outline within the north and south temperate zone is 

 different on whatever meridian a section of the globe may be made ; 

 the former being greater than the latter: and here again we see the 

 great disparity of land that exists. 



Will not this irregular outline of the atmosphere readily account 

 for many of the oscillations, or wave-like undulations, to which our 

 atmosphere is subject, and its constant variations, which are found to 

 be progressive over large areas ? Must not the inequality in height at 

 all times tend to cause action and reaction ? Hence the atmospheric 

 tides. In different latitudes these are seen by the oscillations of the 

 barometer to be extremely unequal, and the greatest inequalities cor- 

 responding to those latitudes when the column stands highest ; while 

 within the tropics, where the pressure is less, the oscillations are more 

 regular, and free from the disturbances and sudden changes which so 

 frequently take place in the temperate zone. The changes of pressure 

 proceeding north and south on a meridian, have been, in some cases, 

 imputed to the action of the trade winds, from its occurring within 

 their limits. From what has been already shown in these pages, it is 

 evident that this cannot hold good in the Southern Pacific ; for it will 

 be recollected that they are confined with narrow limits in latitude, 

 as well as being intermitted for a part of the year, and that this 

 depression extends through the zone of variable winds near the 

 equator. 



The degree of moisture is greater in the southern hemisphere than 

 the northern, as also in the western section than the eastern. To 



15 



