410 CLIMATE OF SIKKIM. ArpENDix F. 



much increased with the elevation. At 10,000 feet in December, 

 at 9 a.m., I saw the mercury mount to 132° with a diff. of -f 94°, 

 whilst the temperature of shaded snow hard by was 22° ; at 13,100 

 feet, in January, at 9 a.m., it has stood at 98°, diff. + 68° 2 ; and 

 at 10 a.m., at 114°, diff. + 81° 4, whilst the radiating thermometer 

 on the snow had fallen at sunrise to 0° 7. In December, at 13,500 

 feet, I have seen it 110°, diff. + 84° ; at 11 a.m., 11,500 feet, 122°, 

 diff. + 82°. This is but a small selection from many instances of 

 the extraordinary power of solar radiation in the coldest months, at 

 great elevations. 



Nocturnal and terrestrial radiation are even more difficult pheno- 

 mena for the traveller to estimate than solar radiation, the danger 

 of exposing instruments at night being always great in wild 

 countries. I most frequently used a thermometer graduated on the 

 glass, and placed in the focus of a parabolic reflector, and a similar 

 one laid upon white cotton,* and found no material difference in 

 the mean of many observations of each, though often 1° to 2° in 

 individual ones. Avoiding radiation from surrounding objects is 

 very difficult, especially in wooded countries. I have also tried the 

 radiating power of grass and the earth ; the temperature of the latter 

 is generally less, and that of the former greater, than the thermo- 

 meter exposed on cotton or in the reflector, but much depends on 

 the surface of the herbage and soil. 



The power of terrestrial, like that of solar radiation, increases 

 with the elevation, but not in an equal proportion. At 7,400 

 feet, the mean of all my observations shows a temperature of 

 35° 4. During the rains, 3° to 4° is the mean maximum, but the 

 nights being almost invariably cloudy, it is scarcely on one night out 

 of six that there is any radiation. From October to December the 

 amount is greater = 10° to 12°, and from January till May greater 



* Snow radiates the most powerfully of any substance I have tried ; in one 

 instance, at 13,000 feet, in January, the thermometer on snow fell to 0*2°, which 

 was 10*8° below the temperature at the time, the grass showing 6*7° ; and on 

 another occasion to l - 2°, when the air at the time (before sunrise) was 21 "2°; the 

 difference therefore being 20°. I have frequently made this observation, and 

 always with a similar result ; it may account for the great injury plants sustain 

 from a thin covering of ice on their foliage, even when the temperature is but 

 little below the freezing-point. 



