Hot and Thermal Springs. 137 



nication with each other, it is well known that an uninterrupted 

 change of air takes place, the direction of which depends on the 

 external temperature being higher or lower than that of the air 

 in the mine. In the former case the warm air sinks down into 

 the shaft, and the cold air escapes from the adit ; in the latter 

 the cold air enters by the adit, whilst the warm air rises out of 

 the shaft. This change of air is not only dependent on the 

 seasons of the year, but on the time of day, and even on the 

 wind, which agitates the air without*. 



In the mines of Cornwall, a difference of \°.\9,5 was found 

 at a depth of 1071 feet between the summer and winter obser- 

 vations -f-. In the auriferous mines of Pestarena di Macugnana, 

 in the valley of Jnzasca, the March and August observations 

 shewed differences of 1°.575 in a depth of 770 feet, which did 

 not entirely disappear until a depth of 1077 feet +. Among 

 the above-mentioned observations, which have been carried on 

 since 1 828 in many of the Prussian mines, holes were bored into 

 the rock in places free from wet and draught, into which the ther- 

 mometers, surrounded with a non-conductor of heat, were sunk, 

 so that the scales projected sufficiently to enable the observer to 

 read off the degrees without the necessity of withdrawing them. 

 Notwithstanding these precautions, fluctuations were caused in 

 the height of the mercury not only in the upper but also in the 

 lower stations, by the imperfect exclusion of the air from the 

 thermometers. From the table given in Chap. VIII. may be 

 seen how irregular these effects shew themselves at depths of 

 27 to 63 feet ; but even from 145 to 738 feet below the sur- 

 face differences of 0.562 to 2.25 occur, the latter of Avhich were 

 observed in the greatest depths. The variations of the ther- 

 mometers in the upper stations corresponded in some cases very 

 regularly with those of the atmosphere, but in others they were 

 totally different. In the latter cases the influence of the air 

 tan only have acted very indirectly. The fluctuations in the 



• Fox communicates some interesting experiments on the effects of change 

 of air in mines in the Transactions of the Koyal Geological Sodctv of Corn- 

 wall. 



t Tliomas Lean in the Annal. de Chim. et de Phys., vol, xiii. p. 200. 



$ Schwcigger's Jafarbuch, voL lii.'p. 2G8, in note. 



