ATMOSPHERIC PRESSURE. 311 



and cooling of different strata of the air, of aqueous precipi- 

 tations, and even, as Dove has truly represented, the formation 

 of clouds and their shape and aspect. Thus the particular 

 form of cloud, which is often a characteristic and animating 

 feature of the landscape, announces the processes which are 

 taking place in the upper regions of the air ; and when the 

 air is calm, the clouds on a hot summer's day will sometimes 

 present a " projected image" of the diversities of form of the 

 heat- radiating surface of the Earth beneath them. 



In parts of the globe where radiation acts on very ex- 

 tensive continental and oceanic surfaces in certain relative 

 positions to each other, as between the east coast of Africa and 

 the west coast of the Peninsula of India, its effects are shewn 

 in the monsoons of the Indian Seas ( 389 ) the Hippalos 

 of the Greek navigators : as the direction of the monsoons 

 varies with the declination of the Sun, their periodical cha- 

 racter was early recognised and turned to the use of man. 

 In the knowledge of the monsoons, which has doubtless ex- 

 tended for ages over both Hindostan and China, and among 

 the Arabs to the west and the Malays to the east, as well 

 as in the still more ancient and more general knowledge of 

 land and sea breezes, there lay, as it were enveloped and con- 

 cealed, the hidden germ of that meteorological science which 

 is now making such rapid progress. The long chain of 

 magnetic stations extending from Moscow to Pekin through 

 the whole of Northern Asia, and which have also for their 

 object the investigation of meteorological relations, will 

 furnish data of great importance towards the investigation 

 of the Law of the Winds. Eor example, the comparison 

 of stations so many hundred miles apart will determine, 

 whether the same east wind blows from the elevated desert 



