380 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



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, corres- 

 ponding to the frequent changes of temperature, which are known to happen in 

 the course of the 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 

 neighbourhood, and it will thence be colder; it may run so near the surface 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 external 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 re- 

 main 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 in the low lands, and decreases a 

 you ascend the mountains. The town of Kingston is supplied with water from 

 wells. The ground on which it stands rises with a gentle ascent as you recede 

 from the sea. In the low part of the town the wells are but a few feet deep, and 

 many of them brackish. The heat of the water in some of them I have found 

 as high as 82°; but they were evidently too near the surface not to be affected 

 by the heat of the seasons. As you ascend, the wells are deeper, and the tem- 

 perature is nearly 80^ in all of them. What variations there are, come within 

 1°, that is, half a degree less than 80°, or half a degree more. They are of 

 different depths, and some not less than 100 feet; though, after they are of half 

 that depth, the temperature is nearly uniform. At the Governor's Pen, which is 

 also in the low part of the country, a well, which is above 6o feet deep, is 79^°. 

 There is a well at Half-way-tree, 243 feet deep, which is 79°. Half-way-tree 



