HEIGHT OF THE SNOW-LINE.] 



METEOROLOGY. 



1193 



generally speaking, as the sea is approached, the snow- 

 level falls ; but still numerous other local interfering 

 causes must also be taken into account ; and such may 

 easily be studied by reference to an ordinary map, in 

 conjunction with the latitudes given in the table. 

 Another instance of curious variation of the snow-level, 

 in respect of the latitude, is furnished by the Carpathian 



range, the highest of which, exceeding 9,500 feet, and in 

 north latitude 49, is still below the snow-line, although 

 the Alps, in 45, have it at an elevation of about 8,500 

 feet. The increased altitude in respect to the Carpathian 

 range, is due to the great convection of heat from the 

 plains belo\v, which, of course, are powerful radiators and 

 absorbers. 



ELEVATION OF rue SNOW-LINE ABOVE THE LEVEL OF THE SEA. 



We have frequently remarked that extreme dryness 

 is characteristic of certain regions ; and Thibet furnishes 

 an instance of it, and also of excessive temperature ; for 

 the climate is exceedingly cold and dry, owing to the 

 great elevation of the plateau on which it is situated 

 (10,000 to 15,000 feet) ; yet, although its latitude is from 

 30 to 38 north of the equator, the snow-line (20,000 

 feet) is higher by nearly 5,000 feet. So deficient is the 

 moisture, that few vegetables of any kind can be culti- 

 vated ; and in most parts, trees and herbage are entirely 

 absent. The radiation from the earth is enormous ; for 

 whilst, during the day, the thermometer will frequently 

 indicate a temperature of 150, at night it falls below 

 freezing point. This affords us another proof of the 

 truth of Dr. Tyndall's deduction that to the power 

 possessed by suspended aqueous vapour of retaining heat 

 by absorption, is due the warmth and genial temperature 

 of .such climates as our own. 



The causes of excessive climates, and excessive changes 

 occurring in comparatively uniform climates, may be 

 readily traced, in many cases, to arise from rapid radia- 

 tion of heat into space, uninterfered with by strata of 

 superincumbent aqueous vapour suspended in the atmo- 

 sphere. Amongst the excessive class, we have already 

 instanced Thibet ; but we may add another illustration 

 in that of the Sahara, the great desert of northern 

 Africa. It is comprised between latitudes 16 and 33 

 N., and longitudes 17 W. and 23 E., and chiefly con- 

 sists of table-lands, varying in height from 1,000 to 

 2,000 feet above the sea-level, with occasional mountain 

 groups attaining an elevation of from 3,000 to 6,000 

 feet. The temperature varies from 50 to 130, during 

 the day ; whilst, at night, water freezes when exposed to 

 tin: wind. In some parts no dew falls, and snow is 

 unknown, owing to the entire absence of aqueous vapour. 

 Kain falls only, in quantity, at intervals of some years ; 

 for during nine mouths, annually, easterly winds prevail, 



VOL. I. 



varied, for three months only, by those from the west, 

 or the regions bordering on the Atlantic. Generally, no 

 plant, except an occasional palm, is met with, which 

 derives its nourishment from springs resting beneath the 

 surface ; the only variation from a perpetual death-like 

 aspect being the oases so sparsely scattered, and situated 

 in depressions below the ordinary level of the country. 

 The simoom here spreads its destructive effects with the 

 fullest energy, loaded, as it is, with fine sand, and pos- 

 sessing an intense and suffocating heat, which is destruc- 

 tive of both animal and vegetable life. 



A great portion of the Russian empire, in Europe and 

 Asia, has a climate of the excessive type. Situated 

 remote from extensive ocean tracts, the entire area 

 experiences only winds which have been robbed of their 

 moisture before arriving in most parts of the country. 

 Again, the cold, dry, Arctic winds freely course over the 

 surface, from the Frozen Ocean to the southern limits of 

 the empire, unchecked by any mountain ranges of 

 sufficient height to afford due protection to the interior. 

 Hence, so far north as St. Petersburg, the heat in 

 summer is great ;* whilst in the Crimea, which has tho 

 same latitude (45) as the south of France and the north 

 of Italy, snow lies often for five months on the ground ; 

 and the Sta of Azov, the northern parts of the Black 

 Sea, and the adjacent rivers, are all frozen. The 

 sufferings of our troops during the Russian war 

 (1854 '56), iu this region, afforded a painful illustration 

 of the severity of the climate in that country. Located 

 on the same parallel as Bordeaux, Milan, Venice, &o., 

 where a genial warmth is rarely disturbed by great 

 variations of temperature, our armies were decimated, 

 near Sebastopol, by a winter scarcely inferior to an 

 Arctic one in its rigour. At Odessa, one of the chief 

 Crimean ports on the Black Sea, the thermometer 



The annual range of the thermometer, at St. Petersburg, is between 

 22 and 90 Fah. 



7N 



