204 



MB. T. HOPKINS ON THE INFLUENCE OF SUN-HEATED LAND 



evidently determined by the large amount of condensation 

 of vapour that is taking place at the time against the 

 Himalaya Mountains. We have then evidence, in the whole 

 range of country we have traversed, that dry, solar heating 

 of air does not produce ascending atmospheric currents and 

 their consequent horizontal winds. We also see what does 

 produce them. We perceive that they are produced by 

 condensation of vapour; the evidence may therefore be con- 

 sidered conclusive as far as respects the parts that have 

 been passed under review. 



But the great desert of Northern Africa is the part 

 generally pointed out by those who say that the heat of 

 dry land produces important ascending aerial currents. 

 Respecting this desert, " Dr. Oudney, in the course of his 

 long journey from Tripoli to Lake Tchad, estimated the 

 elevation of the southern Sahara at 1,637 feet. The French 

 engineer, Fournel, has by careful barometric measurement 

 based on corresponding observations, made it probable that a 

 part of the northern desert is below the level of the sea. 

 Between Biscara and Sidi Ocba the ground is only 243 feet 

 above the level of the sea."* And Milner says that "from 

 the west coast of Africa, and between Morocco on the north 

 and the Senegal river on the south, this wilderness extends 

 easterly to the Red Sea. It embraces a space of more than 

 46° of longitude and 15° of latitude, or a length of 3,000 

 miles by a breadth of 1,000. A large extent of the Sahara 

 is a dead level ; but low sand hills, wadys or valleys, and 

 projecting rocks, are frequent." — (p. 226.) It is stated by the 

 same writer to be "like an immense furnace, warming all 

 the regions on the Mediterranean Sea, in the south of Europe 

 and the west of Asia." It will however be found on inquiry, 

 that the meteorological facts relating to this desert present no 

 evidence to sustain such an opinion. There is no reason to 

 suppose that the heated air of this desert rises and flows over 



* See Hvunboldt'a Aspedi of Nature, p. 115. 



