503 



tervals, from Alexandria to Malacca, or from Sl° 13' of northern 

 latitude to within 2" 14' of the Equator, and between the meridians 

 of 27° and 103° of east longitude. In the columns of the register, 

 the date of the observation, the latitude, longitude, approximate 

 height above the sea, nature of the surrounding geological formation, 

 depth to the surface of the v/ater, depth of the water itself, tempera- 

 ture of the air, and approximate annual mean of the climate in which 

 the wells, &c. occur, are, as far as practicable, specified. A column 

 of remarks is added, containing observations on the chemical nature 

 of the water, and on the size of the wells and springs, and the result 

 obtained by other observers. 



It was found, in general, that in low latitudes the temperature of 

 the deepest wells and springs is a little higher than the mean tempe- 

 rature of the air ; although there occur a few exceptions, especially 

 in the neighbourhood of a high range of hills, whence there probably 

 arise cold springs, having their source at an elevation considerably 

 above that of the plain where the water makes its appearance. 

 Springs which are strongly saline and sulphureous, have, on the 

 average, a higher temperature than those of pure water. Both saline 

 and cold springs are found occurring within a few feet from thermal 

 and freshwater springs : a fact which the author is disposed to ascribe 

 to their rising through different seams of the subjacent strata, often 

 much inclined ; and to the different depths and heights, above and 

 below the crust of the earth, from which the supply of water is de- 

 rived. Wells, and particularly those having a small surface, which 

 are much used for purposes of irrigation, thereby acquire an artificial 

 increase of temperature. The temperature of shallow exposed wells, 

 springs and rivers, especially those which have sandy beds, is subject 

 to diurnal fluctuation from the more powerful influence of the atmo- 

 sphere : and the surface water of deep wells partakes of these vicis- 

 situdes to a depth varying according to the transparency of the water, 

 the extent of surface, degree of exposure and clearness of the sky. 

 In muddy water, the surface is heated to a greater extent ; but at the 

 depth of a foot or two, it is less affected by the heat of the solar rays 

 than clear water. 



With regard to Boussingault's proposal of an expeditious mode of 

 ascertaining the approximate mean temperature of equinoctial re- 

 gions, which consists in sinking a thermometer in the soil, perforated 

 to the depth of about a foot beneath the surface, in a situation shel- 

 tered from the direct rays of the sun, from nocturnal radiation, and 

 from the infiltration of water, the author found that the application 

 of this method gave the following results, namely, that the soil at the 

 depth of a foot is subject to an annual, and, in light soils, to a diurnal 

 variation, regulated in its amount by the relative intensity of the solar 

 rays, and the quantity of radiation, depending, of course, on the state 

 of the atmosphere, and the degree of shelter afforded to the sur- 

 face. 



