AND RIVERS OF INDIA AND EGYPT, ETC. 129 



posited in an insoluble condition, while the alkali, probably by the agency of the 

 carbonic acid of the atmosphere, is set free, and remains dissolved in the water in 

 considerable quantity. In Southern India many thermal springs, hitherto entirely 

 unnoticed, are suspected to occur ; Colonel Sykes states that he has been informed 

 of their existence in Canara : I have heard of one among the Raidrtig hills in the 

 Ceded districts, — in the Koondahs on the west coast, — and discovered another at the 

 base of the hills south of Cuddapah having a temperature of 88°, as noted in the re- 

 gister. A spring near Salem in South India is probably thermal, having a tempera- 

 ture of 84°, ascertained for me by Mr. G. Fischer. 



Temperature of Rivers. — The supposition that the temperature of rivers is lower, 

 from the influence of evaporation, radiation, and the elevation at which they rise, 

 than that of the country through which they flow, appears subject to some modifica- 

 tion as regards great streams whose course lies chiefly through equinoctial regions. 

 Many, like the Nile, derive the great bulk of their water from the rains that fall pe- 

 riodically near the equator when the sun is nearly vertical, and evaporation reduced 

 to its minimum from the saturated state of the atmosphere. The fallen waters de- 

 rive additional heat in overspreading the wide extent of sand and alluvium that form 

 and skirt the channels through which they roll on towards the ocean ; and which, 

 duiing great part of the year, have been left dry and freely exposed to the rays of a 

 scorching sun. The beds of the most considerable rivers of South India present in 

 many parts of their course, during the dry season, dreary wastes of arid sands, 

 through which the river, reduced to a slender thread, barely finds its way to the 

 sea. The mean of more than 200 observations, taken day and night, on the tempe- 

 rature of the Nile, in July, between Cairo and Thebes, I found to exceed the mean 

 annual temperature of the air at Cairo (72°4) by 7°"1- The temperature of the river 

 was increased, at the commencement of the inundation in June, by the freshes from 

 Abyssinia from 79° to 80°*5. The observations were taken at Thebes, immediately 

 preceding, and immediately after, the appearance of the turbid milky hue that an- 

 nounces the periodical arrival of Egypt's great benefactor. 



The Ganges, though having its source amid the snows of the Himalaya, and pur- 

 suing an opposite course to the Nile, that is, a course from northerly latitudes towards 

 the equator, has a mean temperature, as it approaches the ocean, higher than that of 

 the country on its banks. Its mean, between Calcutta and the sea, obtained from a 

 great number of observations by Mr. G. Prinsep, is stated not to be less than 81° 

 Fahr. ! while that of Calcutta does not exceed 78°. The Ganges, it is well known, 

 is little indebted to the melting of the snows near its sources, but derives its waters 

 chiefly from the periodical rains that fall near the borders of, and within, the tropics, 

 between 30° and 22° N. lat. During the inundation, its waters in the lower parts of 

 Bengal are spread over a superficies of alluvial soil and sand, more than 100 miles in 

 breadth, the greater part of which has been parched by the droughts prevalent be- 

 tween the monsoons. 



MDCCCXLV. s 



