224 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
amounts to about a month at each change. So that the mon- 
soons of the Indian Ocean prevail really for about five months 
each way, viz., from May to September from the southv^^est, in 
obedience to the influence of the overheated plains, and from 
November to March inclusive from the northeast, in obedience to 
the trade-v^ind force. 
478. The monsoon season may be always known by referring 
to the cause which produces these winds. Thus, by recollecting 
where the thirsty and overheated plains are which cause the 
monsoons, we know at once that these winds are rushing with 
greatest force toward these plains at the time that is the hottest 
season of the year upon them. 
479. The influence of these heated plains upon the winds at 
sea is felt for a thousand miles and more. Thus, though the Des- 
ert of Gobi and the sun-burnt plains of Asia are, for the most 
part, north of latitude 30°, their influence in making monsoons is 
felt south of the equator (Plate VIII.). So, too, with the great 
Desert of Sahara and the African monsoons of the Atlantic ; also, 
with the Salt Lake country and the Mexican monsoons on one 
side, and those of Central America in the Pacific on the other. 
The influence of the deserts of Arabia upon the winds is felt in 
Austria and other parts of Europe, as the observations of Kriel, 
Lament, and others show. 
480. It would appear, therefore, that these desert countries ex- 
ercise a powerful influence in checking, and consequently in weak- 
ening, the force of the northeast trade-winds. There are no such 
extensive influences at work checking the southeast trades. On 
the contrary, these are accelerated ; for the same forces that serve 
to draw the northeast trade-winds back, or retard them, tend also to 
draw the southeast trade-winds on, or to accelerate them. Hence 
the ability of the southeast trade-winds to push themselves over 
into the northern hemisphere. 
481. Hence, also, we infer that, between certain parallels of 
latitude in the northern hemisphere, the sun's rays, by reason of 
the great extent of land surface, operate with much more intensity 
than they do between corresponding parallels in the southern ; 
and that, consequently, the mean summer temperature on shore, 
north of the equator, is higher than it is south : a beautiful phys- 
ical fact which the winds have revealed, in corroboration of what 
