Hydrography^ and the Art of Navigation. 401. 



sary to penetrate to a much greater depth in the ground, in or- 

 der to reach the bed possessing an equal temperature. 



It is well known that the temperature of the water in wells oi. 

 moderate depth, also affords an easy and exact mode of ascer- 

 taining the mean temperature of the surface. This method, 

 therefore, must not be omitted among those recommended by 

 the Academy. 



Observations to he made on TJiermul Springs, — If it be the 

 case, as every thing leads us to believe, that the high tempera- 

 tures of the springs called thermal, are solely the consequences 

 of the depth from which they rise, it is natural to suppose that 

 the warmest springs should be the least numerous. At the 

 same time, is it not extraordinary, that none have hitherto been 

 observed whose temperature has approached the boiling point 

 within 36° Fahr. ?*. If we are not deceived by some vague 

 reports, the Philippine Islands, that of Lu9on in particular, are 

 likely to afford the means of elucidating this subject. There 

 especially, as in many other places where thermal springs exist, 

 the most interesting data that can be collected are such as tend 

 to prove that the temperature of a very abundant spring varies, 

 or does not vary, with the lapse of ages ; and in particular local 

 observations, with a view to shew the necessity of the fluid 

 having a passage across the very deep-lying strata of the earth. 



The springs of Aix in Provence, regarded in this point of 

 view, have suggested to me a plan of experiment, of which I 

 think it proper here to insert a notice, as it is very probable 

 that the physical conditions on which it is founded will be met 

 with in other places. 



• We do not include in this category of thermal springs the geysers of Ice-, 

 land, and other analogous phenomena, which evidently depend on volcanoes 

 at present in a state of activity. The warmest thermal spring, properly so 

 called, with which we are acquainted, Chaudes Aigues in Auvergne, is 176' 

 Fahrenheit, (4- 80° centigrade). Since this article was written for the expe- 

 dition of M. Bonite^ M. de Humboldt and Boussingault have given me, as the 

 temperature of the spring las Trincheras (Venezuela) in 1800, 195* Fahr. 

 (+ 90°, 4 cent.) ; and in 1823, 200° Fahr. (96° 6 cent.) This spring, accord- 

 ing to them, has no direct connexion with any active volcano. On the other 

 hand, the Duke of Ragusa writes me, that, at Broussa, at the foot of the Mount 

 Olympus, he found the thermal bath, called by the Turks Chirurchiest, to be 

 183°.2 Fahr. (+ 84° cent.) It seems, therefore, tliat 176° Fahr. (80° ceiU.) is 

 the maximum temperature of European springs onh'i ^^^^ 



