with Geographic Botany. — Temperature. 313 



Among the Himma-leh mountains Mr, Royle considers he 

 has obtained results similar to those of Saussure, inferred 

 from the small deposition of snow in some locahties, and its 

 very speedy removal. 



By radiation*, then, the depression of temperature in high 

 latitudes and on ascent is in some measure counteracted, 

 since the temperature of radiation and of the atmosphere are 

 inversely different. The former carries with it light, and its 

 operation is powerful for a time, as in the polar regions, 

 where its duration is considerable ; and on high mountain 

 chains, where it bursts through the rarefied air, and lasting 

 only for a few hours disappears. Can any circumstances of 

 difference in polar and alpine floras be traced to this ? The 

 most probable answer will be found in the duration of life 

 among plants of the same species growing naturally in both 

 regions, or by a comparison between two nearly alhed species 

 of the same genus. Any inquiry on this subject should be 

 directed to the periods occupied from the fall of the seed to 

 germination, thence to flowering, to defloration, and to the 

 shedding of the seeds. 



Many circumstances are continually developed Avhich must 

 be attributed to the power of radiation, though, being so 

 closely connected with light, the latter must be allowed a por- 

 tion of the agency. Plants transferred from bright clear cli- 

 mates lose much of the brilliancy of their colours in a clouded 

 one like our own ; many of our garden favourites have thus 



* In experiments on radiation the bulb of the thermometer exposed to 

 the sun's rays is covered with cotton or wool dyed black, and the instru- 

 ment is fixed on a surface admitting free movement in two directions, ver- 

 tically and sideways, thus allowing the thermometer to be placed at all 

 times to receive the direct rays. 'J'o ensure this more completelj-, a style 

 about two inches long is attached, and when the surface is so moved that 

 this throws no shade, the sun's rays impinge directly on the covered bulb. 

 The bulb may be covered with black paint, or any other substance of this 

 colour, and it is to be regretted that a particular material has not been 

 generally adopted. Another thermometer for comparison should be placed 

 in an imexceptionable situation in the shade. 



The amount of radiation varies so rapidly from trifling causes, that it is 

 very necessarj- to rejiister the exact circumstances under which the experi- 

 ments are conducted. A small difference in inclination — a passing cloud 

 over the sun — the accession of a breeze — may make a difl^erence of some 

 degrees. Sometimes the thermometer is placed in a tube of white paper, 

 which also has an important influence. 1 have also observed, that after a 

 short exposure the mercury rises to a certain height and soon falls again 

 two or three degrees. I believe this to be invariable, and must be attri- 

 buted to the unequal expansion of the mercury and the glass at the com- 

 mencement of the observation. It is therefore requisite to wait till the 

 mercury has become settled to a certain point before the instrument is read 

 ofl". No observations require more minute attention or a greater regard to 

 circumstances than those of radiation. 



Ann. 6i' Mag. N. Hist. Vol. ix. Y 



