OF EARTH TO AIR. 131 



September, 1° to 2°; October, 2° to 3°; and in November and 

 December, between 3° and 4° must be added to the mean 

 temperature of the air, in these months respectively, in order 

 to obtain the approximate mean temperature of the soil. This 

 will apply to aU places having a climate resembling that of 

 Chiswick. But where the temperature of the air falls very 

 low in the end of autumn, and beginning of winter, as at Upsal, 

 where the mean of the air in October is 12° below that in 

 September, 6 or 7° must be added to the mean of the air, for 

 the mean temperature of the earth ; and 8 or 9° in each of the 

 two following months. On the contrary, where the winters are 

 milder than at Chiswick, the difference between the earth and 

 air temperatures will be somewhat less than that which appears 

 in the table, and represented in the diagram for that place. 



"Within and near the tropics the earth, it appears, is, on the 

 average, always warmer than the air by several degrees. In 

 some months, and at some places, both temperatures are nearly 

 alike ; and in other instances they differ as much as 6 or 7°, 

 much depending on the fall of rain, and the nature of the soil. 

 If we add 2° to the temperature of the air, in June, July, August, 

 and September, and 4° in the other months, we shall approach 

 the monthly mean temperature of the soil between 0° and 30° 

 latitude, sufficiently near for all practical purposes ; certainly 

 much nearer than the temperature to which plants from that 

 soil have been subjected by artificial treatment in this country." 



It must, however, be understood that such calculations are 

 necessarily uncertain, and can only be taken as rough approxi- 

 mations to truth, near enough for practical purposes, but 

 nothing more. An infinite multitude of circumstances, 

 reducible within no general rules, modify all such estimates. 



As regards the Indian seasons, the greater part of the west 

 and south coast of the peninsula is so damp that the growing 

 and flowering season lasts all the year round. Even of Eice 

 there are, according to Buchanan Hamilton, two crops in 

 Malabar : — one sown in May, transplanted in June, and reaped 

 in July ; another sown in August, transplanted in September 

 and October, and reaped in November. In Northern India the 

 growing season for tropical crops — Eice, Millet, &c. — generally 



k2 



