No. VII. — Islands of Procida and Ischia. S49 



these eases is 'precisely eighteen degrees. Certainly the phe- 

 nomena of the ice caves of the Alps and the cool grottoes of 

 Italy require much elucidation, and in the still imperfect state 

 of hygrometric science it is'difficult to maintain or disprove de* 

 cisively any hypothesis. My own opinion is, that evaporation 

 in some form or other will be found to be almost the sole 

 cause, — an opinion which I have long entertained, and which 

 some years ago I endeavoured to support * even in the very 

 extreme case of Saussure's experiment at the Monte Testaccio 

 at Rome. Since writing that paper, I have indeed seen more 

 clearly the great discrepancies between authors on the subject 

 of hygrometry ; and it is not till I shall have found leisure to 

 investigate it more completely that I shall be able to offer any 

 decisive opinion. Meanwhile, however, no other theory but 

 that of evaporation seems feasible ; the hypothesis of Dr 

 Anderson, that cold air is conveyed by crevices from moun- 

 tains of such a height that the mean temperature does not ex- 

 ceed that of the cold stream observed, is not merely inappli- 

 cable to the Monte Testaccio but also to the present instance. 

 I have elsewhere shown that the mean temperature of the 

 summit of Epomeo cannot be below 56°, yet in the month of 

 March, when the atmosphere had rather more than its mean 

 temperature, Saussure observed the grotto to be so cold as 

 45°. Yet after all, the phenomena of the cavern at Lacco if 

 taken singly, might be sufficiently well explained by the known 

 facts of evaporation. It is somewhat remarkable that the 

 coolness observed by Breislak near the extreme summer heat 

 was almost precisely that of the mean temperature of the place. 

 On the theory of Saussure, therefore, that there are spacious 

 caverns in the earth which never exceed in temperature the 

 mean of the place, we need have no recourse to the theory of 

 evaporation, though, in my mind, a far more natural supposi- 

 tion would be the free passage of ventilation through the 

 loose materials of an extensive mass, and therefore exposed to 

 the influence of evaporation, which, on the supposition of con- 

 fined and of course damp caverns, could have no place. This 

 is also confirmed by the apparent constancy of the difference 



* In a paper published in this Journal, vol. viii. p. 205-216. 

 NEW SERIES. VOL. II. NO. II. APRIL 1830. Z 



