I 



ON THE RATE OF INCREASE OF UNDERGROUND TEMPERATURE. 75 



of two kinds — slow-action thermometers and maximum thermometers. 

 The present pattern of slow-action thermometer consists of a thermometer 

 having its biilb surrounded by stearine or tallow, the whole instrument 

 being hermetically sealed within a glass jacket, and had its origin in a 

 conference between the Secretaiy and Dr. Stapff in the St. Gothard 

 tunnel (VIII., IX., XI.) Other slow-action methods described in the 

 reports are — Angstrom's thermometer in bottle of water (I.), large spirit 

 thermometer (I.), Symons' thermometer in a thick casing of felt (II.). 



Our present patterns of maximum thermometer are two — the Phillips, 

 and the Inverted Negretti — both being hermetically sealed in strong glass 

 jackets to prevent the bulbs from receiving pressure when lowered to a 

 great depth in water. 



Both instruments are used in a vertical position, and it is necessary 

 that they register truly in spite of jolts in hauling up. The Phillips pat- 

 tern was used fii'st (I., II., III.), and there were continual complaints of 

 the detached column shaking down, till it was pointed out by Professor 

 Phillips himself, that the fault arose from the bore not being small enough. 

 This defect was remedied (VI.), and the instilment has since worked 

 perfectly, but it requires good light and sharp eyes to read it. 



The Inverted Negretti (IV.) was contrived by the Secretary with the 

 view of overcoming the difficulty as to jolts, but the contrivance had been 

 anticipated many years before by Messrs. Negretti & Zambra themselves. 

 It is easily read and managed, but it has a theoretical defect in requiring 

 a slight correction for the difference between the tem^Dcrature at the time 

 of taking the reading and the maximum temperature recorded. 



References to some other kinds of maximum thermometer will be found 

 in some of the reports, namely, to Walferdin's (IV.), Lubimofif's (IV., 

 v.), and Magnus' (IX.), all these being of the class of overflow ther- 

 mometers. 



References to Becquerel's thermo-electric method of observing under- 

 ground temperature were made in three of the reports (1., XI., XII.), and 

 some laboratory experiments were subsequently carried out by the 

 Secretary which led to the conclusion that the method could not he re- 

 lied on to yield sufl&ciently accurate results. It may be mentioned that 

 Becquerel's observations are only carried to the depth of 100 feet, whereas 

 we require observations at the -depth of 1,000 or 2,000 feet. 



2. As regards subsidiary (that is non-thermo metric) apparatus, Mr. 

 Symons' apparatus for lowering and raising thermometers to and from 

 any required depth in a deep well (1,000 feet deep in this case), is de- 

 scribed with an illustration in the second report. 



Plugs for preventing convection-currents in a bore or well are suggested 

 in the first report, Herr Bunker's two forms of plug successfully employed 

 by him at Sperenberg, are described in the ninth, and Professor Lebour's 

 umbrella-like plug in the ninth, tenth, and twelfth. In its final form 

 (XII.) it appears to be very convenient, as it requires only one wire. It 

 remains collapsed so long as the wire is taut, but opens out and plugs the 

 hole when ib becomes slack. 



B. Methods of Observation. These have chiefly been of two kinds. 



1. Observations in holes bored to the depth of a few feet in newly 

 opened rock, either in the workings of a mine or a tunnel, or in a shaft 

 during the sinking. The rock should not have been exposed for more 

 than a week when the hole is bored, and a day may be allowed to elapse 

 for the heat generated by boring to escape before the thermometer is 



