Correspondence. 



457 



aspects are raised with much more difficulty than on northern. In 

 the Mauritius, the forest is nearly all on northern aspects ; the oak 

 decidedly prefers it ; the pine also seems to prefer it ; and the birch 

 which grows at a higher elevation on the Borendo than the oak or 

 pine, grows also much larger at an elevation on the northern side at 

 least 2000 feet higher than where it grows, weakly and stunted on 

 the southern side at Setee. If I am right, I hope Captain Hutton 

 will correct the above error into which he may have been led inad- 

 vertently. I recollect in 1839, I was surprised to see the moun- 

 tains above Algiers covered with snow on the northern aspect, when 

 there was none visible on the southern aspect of the mountains of 

 Arragon in Spain, though much higher. The sketch I send you of 

 the Borendo, represents a snow cloud which commenced falling while 

 I was making the sketch about half an hour after we had crossed the 

 ridge. The forest on Mount Jacho is greatest on the northern 

 aspect, principally oak and pine. The magnificent forest of Muhas- 

 soo, which is quickly disappearing to make way for potatoes, is 

 principally on the northern aspect ; there I measured one pine which 

 had been blown down, whose base from its position had never seen 

 the sun; it measured as it lay, straight trunk 160 feet, the top of it 

 had disappeared, but it might have been 10 or 20 feet more. 



The two mountains near Simla called great and little Lhally, are 

 wooded to their crests on the northern aspects, and bare and rocky 

 on the southern ; these are familiar instances of the naked appearance 

 of the southern aspects; few may have seen the northern, but all 

 must recollect the appearance of the crests fringed with tall pine 

 trees, giving an idea that they only grow on the top of the mountains. 

 Suppose that forests grow most luxuriantly where moisture is most 

 abundant, as in the swamps of Surinam : it is proved by chemical 

 analysis, that in the formation of woody fibre water is decomposed ; 

 that trees decompose water, and assimilate hydrogen and carbonic 

 acid; that thereby 72.35 parts of oxygen by weight must be separa- 

 ted as a gas, for every 27.65 parts of carbon which are assimilated 

 by the plant; or what is much more probable, plants under the same 

 circumstances may decompose water, the hydrogen of which is assi- 

 milated along with the carbonic acid, while oxygen is separated ; if 

 the latter . change takes place, 8.04 parts oxygen must unite with 



3 M 



