AND RIVERS OF INDIA AND EGYPT, ETC. 133 



The highest known mean temperature of any place in India is that of Pondicherry, 

 which, though this city stands only a little more than a degree to the south of Madras, 

 is stated to reach 85°-28. That of Madras, in lat. 13° 5' N., is 80''-42, and of Co- 

 lumbo, more than 5° nearer the equator than Pondicherry, only 80°75. I am not 

 aware that any reason has been assigned for this extraordinarily high mean tem- 

 perature; the lower temperature of some wells in the vicinity of Pondicherry leads 

 me to doubt its correctness. 



Boussingault's Mode of ascei'taining the Mean Temperature of Tropical Countries. — 

 An expeditious mode for ascertaining the approximate mean temperature of equinoc- 

 tial regions has been proposed by M. Boussingault, and recommended to travellers, 

 on occasions where time and opportunity do not admit of the usual means. I hardly 

 need remark, that this method is grounded on the hypothesis, that between the 

 tropics the temperature of the earth's crust is constant at the depth of about a foot 

 (one-third of a metre) beneath its surface, and consists in sinking a thermometer in 

 the soil perforated to this depth, under sheds, huts of natives, or other spots sheltered 

 from direct warmth produced by absorption of the solar heat, from nocturnal radia- 

 tion, and from the infiltration of rain water. The result of my own experiments in 

 India indicates that the soil at the depth of a foot is subject to an annual, and, in 

 light soils, taa diurnal fluctuation, varying according to the intensity of the sun's 

 rays on the soil surrounding the sheltered spots where the experiments were con- 

 ducted ; and radiation modified by the dry and open, moist and close nature of the 

 soil. During cloudy weather these fluctuations were consequently found at their 

 minimum. The maximum of diurnal fluctuation observed was at Bellary, on the 

 centre of the table-land of peninsular India, in lat. 15° 5' N., and 1600 feet above 

 the sea's level; mean temperature about 80°-5. The experiments were made in the 

 hot month of May, sky unclouded ; the soil was reddish and light in texture, and 

 completely sheltered by a thatched roof. Every precaution enjoined by M. Boussin- 

 gault was carefully attended to, and fresh holes bored every day. 



Experiment. — First Day. 



