104 Captain Newbold on the Temperature of 



Salem, in South India, is probably thermal, having a temperature 

 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 modification 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 periodically 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 derive 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, during great part of the year, have been left dry and freely 

 exposed to the rays of a scorching sun. The beds of the most con- 

 siderable 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 two hundred observations, taken day 

 and night, on the temperature 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 in- 

 creased 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 announces the periodical 

 arrival of Egypt's great benefactor. 



The Ganges, though having its source amid the snows of the 

 Himalaya, and pursuing an opposite course to the Nile, that is, a 

 course from northerly latitudes towards the equator, has a mean tem- 

 perature, 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 

 70°. 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. latitude. 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 between the 

 monsoons. 



In order to obtain a better idea of the degree of heat absorbed, 

 and given out by the alluvium of the Nile, the sands and rocks in the 

 beds of the rivers of India, I made the following observations : — 



In July 1840, a thermometer placed on the dark alluvium, then 

 quite dry, of the Nile, opposite the pyramid of Meydun at 12| p. m., 



