THE GASEOUS PORTIONS OF THE ATMOSPHEEE. 



13 



With reference to this subject Kaeratz says — " We may 

 admit that — 



inra. 



At the sea-coast the general mean pressure is 761 • 35 * 



At the equator it is only about 758 



At the latitude of lO'^ the pressure increases, 



and between 30*= and 40'' of latitude it 



attains its maximum 762 or 764=763 



Starting from this zone, it diminishes to 760 



And more north, it descends to about 756." 



This imperfect statement of atmospheric pressure in dif- 

 ferent latitudes is made up of averages, it presents a 

 range of nearly a quarter of an inch of mercury ; and Kaemtz 

 appears to think that the amount of pressure has some depen- 

 dence on mere latitude. In speaking, in the same part of his 

 book, of the total pressure as ascertained by the barometer, 

 he does not, however, deem it necessary to distinguish the 

 aqueous portion from the gaseous. 



But there are parts of the world where the average differ- 

 ence of pressure is more considerable than any of those just 

 given. In Schouw's table Tripoli is put down as having a 

 mean of 767*41'"™-, whilst Reikiavig, in Iceland, has one of 

 only 752^^-, which makes a difference between these two 

 places of more than half an inch of mercury. This difference 

 is, however, much exceeded by places in the southern hemi- 

 sphere. Captain Foster found that the average pressure near 

 Cape Horn, for a considerable period, was only about 29 '2^, 

 whilst at Valparaiso it is SO'l^-, and at the Cape of Good 

 Hope, 30* 2'"- Captain Ross also found that in the parts of 

 the antarctic seas which he visited, the pressure was very low, 

 even less than about 'Cape Horn. Now, in these localities, 

 the respective pressures found, though so unequal, are general 

 averages, and we have to endeavour to ascertain what can 

 cause such long continued unequal pressures, in a body 



* 762 millimetres are equal to 30 inches of mercury. 



