88 USES OF DESERTS. 



hours indeed, the whole period required for one of 

 these vesels to run through the tropics from Lat. 23 

 30' N. to Lat. 23 30' S., does not exceed eight days. * 

 These few facts probably place the revolution effected 

 by steam over sails in a clearer light than any more 

 detailed description could do. 



Space, and the nature of our subject, will of course 

 preclude any attempt at a description of winds in 

 general; we therefore confine our remarks to the great 

 systems of the trade winds prevailing upon the 

 ocean, because of their extreme importance upon the 

 subject of climate, and because their action may be 

 witnessed at sea in a way which it cannot be on 

 land. 



A variety of causes combine to modify the course 

 of the winds on land. Wherever the earth, for example, 

 becomes highly heated by the sun's rays, the atmo- 

 sphere near its surface becomes rarefied, and ascends, 

 thus creating an indraught to supply its place. The 

 question which often arises in enquiring minds What 

 is the use of deserts? may here perhaps receive some 

 attempt at solution. Deserts have their uses in mo- 

 difying the action of the winds, and it may be are 

 created as one of Nature's great mediums of atmo- 

 spheric circulation. There can be no doubt that by 

 deserts, and other extensive dry plains, the natural 

 action of the trade winds is disturbed, and what 

 is known as a " Monsoon," or deflected trade wind, 

 takes their place. It is, however, on great continents 

 like Central Asia, Africa, and Australia, where these 

 phenomena are best witnessed. 



* New Zealand Shipping Company's Handbook for Passengers, 

 1889, p. 13. 



