110 SPRINGS, RIVERS, CANALS, LAKES, 



is immaterial, whether the heat of the sun be excited more in the 

 earth or in the air ; for whichever has the larger proportion will in 

 the end communicate a part to the other, and so restore the balance. 

 The same observation applies to such causes of cold as may operate 

 at the surface of the earth, as evaporation, &c. The air therefore, 

 near the surface of the earth, will show by a thermometer in the 

 shade nearly, if not exactly, the same degree of heat that the sun 

 communicates to our terrestrial globe ; and if a mean of the heats 

 thus shown be taken for the year round, and we penetrate into the 

 earth to that depth, that it is no longer affected either by the daily, 

 monthly, or annual variations of heat, the temperature at such depth 

 should be equal to the annual mean above mentioned. To ascertain 

 this with the utmost precision, it must be obvious that numerous 

 observations should be made every day, corresponding to the fre- 

 quent changes of temperature, which are known to happen in the 

 course of 24 hours in all climates ; and on these a daily mean should 

 be taken, and the annual mean deduced from them. This has not 

 yet been done, but where we have observations from which a mean 

 temperature can be deduced with any degree of certainty, it will be 

 found not to differ greatly from the heat of deep caves, or wells in 

 the same climate. 



For obtaining the temperature of the earth, the best observations 

 are probably to be collected from wells of a considerable depth, 

 and in which there is not much water. Springs issuing from the 

 earth, though indicating the temperature of the ground from which 

 they proceed, are not so much to be depended on as wells; for the 

 course of the spring may be derived from high grounds in the neigh, 

 bourhood, and it will thence be colder ; it may run so near the sur- 

 face as to be liable to variations of heat and cold from summer and 

 winter ; or it may be exposed to local causes of heat in the bowels 

 of the earth. Wells seem also better than deep caverns, for the 

 apertures to such are often large, and may admit enough of the ex- 

 ternal air to occasion some change in their temperature. Wells are 

 not however to be met with in all places, and in that case we must 

 remain satisfied with the temperature of the springs. 



The following observations were made in the island of Jamaica, 

 where there are flat lands in many parts towards the coast, but all 

 the interior part of the country is mountainous. The heat is greatest 

 m the low lauds, and decreases as you ascend the mountains. The 



