1857.] Beport on the Progress of the Magnetic Survey. 59 



This anomalous phenomenon seems to be due to the following 

 causes. 



The plateau of Umerkuntuk itself has comparatively little jun- 

 gle, and the ground is exposed freely to the action of the sun. 

 Bound the base of Umerkuntuk, and especially on the slopes of the 

 hills, very thick and extensive jungles prevail, which keep the 

 ground moist and cool, as they prevent the heating of the surface 

 by the powerful tropical sun. 



The meteorological observations which I had occasion to make 

 in many parts of the jungles of Central India, as well as those of 

 my brother Hermann in Assam, and of my brother Adolphe in the 

 Godavery jungles, offer many striking examples of the cooling 

 influence which extensive jungles exercise in India on the temper- 

 ature of the ground, as well as on the atmosphere. 



The anomalous cooling of the ground by jungles is still more 

 apparent than the difference of temperature in the air of jungles 

 when compared with open districts, since the differences in the 

 temperature of the air are much more equalized by winds and atmo- 

 spheric currents. 



At Agra I was enabled, through the kind assistance of Colonel J. T. 

 Boileau, the Chief Engineer, to ascertain the temperature of the earth 

 nearly 50 feet below the surface. The insensible thermometer was 

 sunk in a small shaft dug for the purpose, and remained buried there 

 for a fortnight before it was taken up. Th9 temperature was, from 

 the 22nd of February to the 4th of March, 28° 3' centigrade, equal 

 to 82° 9' Fahrenheit. The temperature thus observed accords, 

 within one degree, with the temperature of the earth at about 60 

 feet below the surface, ascertained at Benares in April 1855. The 

 temperature at one metre below the surface at Agra was only 25 

 degrees centigrade, as the upper strata had been cooled by the 

 influence of the past cold season. 



The temperature of the wells at Agra was always a little lower 

 than that of the ground, thus directly ascertained. As an average 

 of several wells from which a large quantity of water was constantly 

 drawn for domestic purposes and irrigation, I obtained 27° 6' to 

 27° 8' centigrade. 



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