1848.] 



Observations made on a Botanical Excursion. 



377 



Table-land of Birbhoom and Behar. 



Nocturnal Radiation. 









CS 









C3 



Sunrise. 





£ 





9 P. M. 





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M 









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o 









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o5 



Et: jT 



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$c 



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a 



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£ 



d 



. £ 



Is' 



S 8 



S 



S3 £ 

 eo o 



a 

 2 



F- 



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£ o 



Exposed Th 



51.1 



4. 



9.0 



6 



56.4 



5.3 



7.5 



7 



On Earth 



48.3 

 46.6 



2.5 

 6.2 



3.7 

 9.0 



3 

 5 



53.8 

 54.4 



4.9 

 7.2 



5.5 

 10.0 



6 



On Grass 



7 



On one occasion, and that at night, the dew point was as low as 9°.l, 

 with a temperature of 66°, a depression rarely equalled at so low a 

 temperature ; this phenomenon was transient and caused by the passage 

 of a current of air loaded with dust, whose cooling particles possibly 

 absorbed the atmospheric humidity. I neglected to collect any of the 

 powder. From a comparison of the night and morning observations of 

 Thermometers laid on grass, — the earth, — and freely exposed, it appears 

 that the grass parts with its heat much more rapidly than the earth, 

 but that still the effect of radiation is slight, lowering its temperature 

 but 2° below that of the freely exposed thermometer. 



As compared with the climate of Calcutta these flat hills present a 

 remarkable contrast, considering their proximity in position and 

 moderate elevation. 



The difference of temperature, deduced from the sunrise morning and 

 afternoon observations, amounts to 4°, which, if the mean height of the 

 hills where crossed by the road, be called 1133 feet, will be equal to a 

 fall of one degree for every 288 feet. This is below the usual equiva- 

 lent for that height : Play fair assuming, 1° equal to 270 feet of elevation, 

 and more recent observers 1° as equal to 250 feet. A comparison of 

 the solitary temperature taken at the top of Paras Nath with the 

 cotemporaneous one at Calcutta, gives 1° of temperature for every 211 

 feet, which is again much above the assumed standard. 



In the dampness of the atmosphere Calcutta contrasts very remarka- 

 bly with these hills ; the dew point on the Hooghly averaging 51°.3, 



