ON UNDER6R0UND TEMPERATURE. 157 



0-2 of a degree Centigrade, and read by estimation to 0-05, all necessary cor- 

 rections being applied, including a correction for temperature of stem. The 

 index-errors were known from compai'ison with standards, attention being 

 paid to the difference between the reading in a vertical and in a horizontal 

 position, which, on account of the great length of the column of mercury 

 and consequent pressure on the interior of the bulb, amounted to about half 

 a degree. As the thermometers were costly, and- were very liable to be 

 broken in the process of extraction, these rock- observations Avere compara- 

 tively few. 



One of the Committee's protected Negretti maximum thermometers was 



left by the Secretary with Dr. Stapff, and has been used by him for verifying 



some of his previous observations, the thermometer being pushed to the 



bottom of the hole, with a cord attached, and the hole being then tightly 



" plugged with rags and tallow. 



A minimum thermometer would have been more appropriate, as the rock 

 was colder than the air ; but the Eutherford's minimum which the Secretary 

 had provided was too large for the holes. Besides, it is doubtful whether 

 the index could be trusted to retain its place during the extraction of the 

 thermometer. The extreme slowness of action of the protected Negretti 

 maximum was the one quality which rendered its use possible, and a non- 

 registering thermometer possessing the same characteristic would be more 

 appropriate. It was accordingly agreed between Dr. Stapff and the Secretary 

 that a new pattern of non-registering thermometer should be constructed 

 with a special view to slowness of action. This end is to be attained by sur- 

 rounding the bulb with tallow or some other non-conducting solid, the whole 

 being inclosed in a sealed glass tube. [Six thermometers on this plan have 

 since been constructed by Negretti and Zambra, and two of them, after satis- 

 factory trials, were forwarded by the Secretary to Dr. Stapff on the 1st of 

 November.] 



II. Observations of air-temperature in the tunnel. The air is artificially 

 warmed by the presence of the workmen, by their lamps, and by blasting ; 

 and, on the other hand, is cooled by the escape of the compressed air from 

 the boring-engines. Dr. Stapff states that, notwithstanding these disturbing 

 influences, he " gradually fell into a uniform system of observing air-tempe- 

 rature, so that the mean results obtained were useful." He further found 

 that the mean air-temperature thus determined at any point wlien first laid 

 open by the driving forward of the narrow gallery (to be afterwards widened) 

 was identical (to a fraction of a degree) with the rock-temperature afterwards 

 observed at the same point at the depth of a metre in the walls. For ex- 

 ample, at the distance of 800 metres from the Swiss portal the rock-tempera- 

 ture, 1 metre deep, was 17°"85, the mean air-temperature when first observed 

 having been 17°-80. Again, at the distance of 1443 metres from the Swiss 

 portal the rock-temperature, 1 metre deep, was 18°-16 ; the mean air-tem- 

 perature when the heading had just advanced to this point was 17°'29, the 

 temperature at the same time from 20 to 40 metres further back being 18°"35. 

 The last observations made at the Italian end on the occasion of the verifica- 

 tion of the axis of the tunnel confirm this conclusion as to the approximate 

 identity of the air-temperature in the extreme end of the heading with the 

 temperature of the surrounding rock. 



III. Observations of the temperature of springs. In the Swiss portion, 

 up to the date of the Secretary's visit, there were no springs of any account ; 

 but in the Italian portion they are mimerous. Wlicn water-filled crevices 

 (in the Italian portion) are first bored through, the water issues with the 



