588 MR ALEX. BUCHAN ON THE MEAN PRESSURE OF THE ATMOSPHERE 



ing air is the chief cause of the cooling effect being so much less than that which 

 would be experienced by the dry air. 11 * 



The influence of vapour in lowering the pressure is well illustrated by the 

 low pressure in the tropics towards which the trades blow, this belt being 

 characterised by a highly saturated atmosphere and heavy rains. 



Again, much more vapour is observed in the air at places near the shores of 

 the north of the Atlantic in winter than at places in the same latitudes in the 

 interior of continents. In Great Britain, as compared with the interior of Asia, 

 the excess is great ; and in the former case the skies are generally cloudy, and 

 in the latter clear. Also, over the same region, the atmosphere of which abounds 

 in vapour, the mean winter temperature is much higher than it is on the continent; 

 and from the conclusion arrived at by Sir William Thomson regarding the 

 temperature of an ascending column of saturated air, the relatively higher 

 temperature over such regions must continue to prevail up to very great heights. 

 Hence, owing to the presence of a larger amount of vapour, and to a higher 

 temperature, the air resting on the north of the Atlantic and regions adjoining is 

 specifically lighter than in the continents which surround it; consequently the 

 charts show an enormous diminution of pressure over this region, as compared 

 with the continents. Similar depressions from like causes occur in the north of 

 the Pacific and in the Antarctic regions. 



Since dry and cold air is, on the other hand, specifically heavy, we should 

 expect that in the interior of continents, where temperatures are low and the air 

 is dry in winter, that pressures would be high ; and observations show (see the 

 Charts) that the highest mean pressures occur in Asia and North America at this 

 season. For the same reason, pressures are also highest in Australia, South 

 Africa, and the south of South America in the winter months. 



There is another source from which atmospheric pressure is increased. It has 

 been shown from the Charts that the tendency of the prevailing winds on the surface 

 of the earth is to blow round and in upon the space where pressures are low, and 

 out of the space where pressures are high. Now, since in this way vast volumes of 

 air are poured into the space where pressure is low without increasing that pressure, 

 and vast volumes flow out of the space of high pressure without diminishing 

 that pressure, it follows that the air poured in is not allowed to accumulate 

 over this space, but must escape into other regions ; and that the air which 

 flows out from the place of high pressure must have its place supplied by fresh 

 accessions from above. The exchange indicated here is probably brought about 

 in this way :— Since in winter, over the north of the Atlantic, the atmosphere is 

 specifically lighter than in surrounding regions, there are here the conditions of 

 an ascending current ; and it may be inferred that the ascent will continue until 



* Sir William Thomson in Mem. Lit. and Phil. Soc. Manchester, vol. ii. 3d series, p. 131. 



