Hot and Thermal Springs. 291 



of the shaft h'es 86 — 10.2 feet above the flood-mark of the sea ; 

 the point of Phillips's observations ISSl — 0.8 feet below the 

 opening of the shaft, and consequently 1497 — 0.6 feet below 

 the surface of the sea. At the upper end of the shaft, the mean 

 temperature of the air was 47°.58, and that of the salt-water 

 below 72°.59. Supposing the temperature not to have increa- 

 sed in the last 100 feet (?), so that 72°.59 were the temperature 

 at the depth of 1484 — 0.7 feet. Philips calculates an increase 

 of 2°.25 F. (1° R.) in a depth of 133—6.8 feet. 



These observations point out in what manner the most accu- 

 rate results as to the increase of temperature towards the centre 

 of the earth are to be obtained. In old exhausted mines gal- 

 leries are often to be found in which water might probably be 

 shut off in a similar manner for the sake of observations, as was 

 done in the Freyherg mine above referred to. Although the 

 expense of the observations would be increased by such opera- 

 tions, yet, on the other hand, they would be very much simpli- 

 fied, as it would be sufiicient to make them twice a-year, in 

 spring and autumn. 



Another very favourable opportunity of obtaining accurate 

 thermometrical observations offered itself in the sounding at 

 Pregny near Geneva, already referred to more than once. 



Mr Giroud wished to have an artesian well sunk at his coun- 

 try seat at Pregny. On coming to a depth of 582 — 11.7 feet, 

 without meeting with water, he gave up the undertaking. 

 Messrs de la Rive and Marcet upon this opened a subscription 

 for the continuation of the work, and thus they succeeded in 

 reaching a depth of 726 — 10.2 feet. The failure of the first 

 object of the undertaking, namely, to obtain spring water, was 

 particularly favourable to the observations, as the variati(ms of 

 temperature, caused by the gushing in of Avater into the well, 

 were thereby entirely avoided. The water in the well rose once 

 to within 13—10.2 feet of the surface ; at 106—6.9 to 159—10.3 

 feet deep it was so muddy that no such influx of water could 

 possibly have been supposed to exist, especially near the bot- 

 tom it was rather a very wet earth than water. The pre- 

 judicial effects of currents of air in dry wells were consequently 

 also prevented. Besides which the thermometer was enclosed 

 in a cylinder of 4^ inches in diameter, which exactly fitted into 

 the well, so that, at least, whilst it was sunk to any certain 



