CHARACTERISTICS AND DISTRIBUTION OF LAKES 525 
of Australia, the Kalahari Desert of Africa, and the Atakama Desert 
of South America. These inland drainage areas are estimated by 
Murray^ to occupy 11,486,000 English square miles, or about three- 
seventeenths of the total land-surface of the globe. 
It was shown by very numerous observations during the Challenger 
Expedition that in the open ocean far from land the daily fluctuations 
of temperature in the surface waters do not exceed one degree 
Fahrenheit. Hence the atmosphere over the ocean may be said to 
rest on a surface the temperature of which is practically uniform 
at all hours of the day. This is in striking contrast to what obtains 
on land-surfaces towards the centres of the continental masses, where 
the air is often dry, and solar and terrestrial radiation produce a very 
wide daily range of temperature, possibly one hundred degrees Fahren- 
heit from 3 p.m. to 3 a.m. In the temperature conditions here 
indicated we have one of the prime factors of meteorology, — a factor 
which determines the position of the great permanent anticyclonic 
areas over the oceans. Air with a large quantity of water- vapour 
absorbs more of the sun's rays, becomes in consequence more heated, 
and is specifically lighter than dry air ; hence moist air ascends in 
cyclonic areas, is deprived of its moisture in ascending, becomes cool, 
and spreading laterally descends as heavy dry cool air in anticyclonic 
areas. The redistribution of the mass of the atmosphere is brought 
about in this manner. Numerous observations show that winds are 
simply the movements of the atmosphere that set in from where 
there is a surplus to where there is a deficiency of air. Isobaric 
maps and maps showing the prevailing winds are in accordance with 
each other. 
Now, the two belts of inland drainage areas and low rainfall above 
referred to are likewise the regions of high annual atmospheric 
pressure, which pass in two belts completely round the globe. The belt 
of high pressure in January in the northern hemisphere broadens as 
it passes over land and contracts as it crosses over the ocean. Its 
greatest breadth is over Asia, and its least over the Pacific, that is, 
where land and ocean attain respectively their maximum dimensions. 
Similar relations exist in the southern hemisphere. In the zvinter months 
of each hemisphere these areas on land are occupied by anticyclones, 
in which heavy, dry, cold air descends and flows out on the surface 
all round. In the summer mo?iths of each hemisphere these same areas 
become cyclonic, and the winds are drawn in at the surface from sur- 
rounding regions and are deprived of their moisture before reaching 
the centre of the desert regions, where they ascend as warm currents 
to the higher regions of the atmosphere. 
The primary cause of the rainless, desert, salt-lake, and inland 
1 See Scott. Geogr. Mag., vol. ii. p. 552, 1886. 
