ON UNDERGROUND TEMPERATURE. 25 



173 metres. This gives, by comparison with the Observatory caves, au in- 

 crease at the rate of 1° C. in 30-85 metres, or 1° F. in 56-25 feet. 



These three determinations are in wonderfully close agreement with each 

 other. All three wells are sunk in the chalk of the Paris basin. In the 

 case of the St. Andre well the thicknesses of the different strata were : — 



metres. 



Plastic clay 13-52 



White chalk 122-46 



Marly chalk 29-24 



Glauconie 13-64 



Greensand 84-36 



263-22 



The thermometer which the Committee have been employing for the last 

 three years is a Phillips's maximum, having so fine a bore that the detached 

 column of mercury which serves as the index is sustained in the vertical 

 position by capillary action, and will bear a moderate amount of shaking 

 without slipping down. Numerous instances, however, have occurred in 

 which the index has slipped in consequence of jerks or concussions sustained 

 by the thermometer in hauling it up from a depth. During the past six 

 months the Secretary has been in correspondence with Messrs. Negretti and 

 Zambra respecting a proposed modification of the maximum thermometer 

 known by their name, which occurred to him more than a year ago, and was 

 described by him privately to some meteorological friends at the last Meeting 

 of the Association. It was then supposd to be new, but it now appears that 

 Messrs. Negretti and Zambra have made something of the kind for the last 

 fourteen or fifteen years. Several changes, however, were necessary before 

 the thermometer was adapted to the uses of the Committee, and the first 

 complete instruments were received in June last. They are enclosed, like the 

 thermometers previously used, in hermetically sealed tubes, for protection 

 against pressure, and they have the advantages (1) of being able to bear 

 more severe jolts without derangement of their indications, and (2) of pre- 

 senting to view a much broader column of mercury, so as to be more easily 

 read in a dim light. 



The instrument is to be used in a vertical position, with the bulb uppermost. 

 Between the bulb and the stem there is a contraction, through which the 

 mercury will not pass except under pressure. It is set by holding it with 

 the bulb end lowest, and tapping this end on the palm of the hand, tiU the 

 part between the contraction and the bulb is fuU of mercury. It can then 

 be held with the bulb up, and the mercury in the stem wiU run down to the 

 lower end, from which the graduations begin. In this position, the top of 

 the column indicates the temperature of setting, which must be lower than 

 the temperature intended to be observed. 



The instrument is then to be lowered into the bore to any required depth, 

 and allowed to remain there for about half an hour, to ensure its taking the 

 temperature of the surrounding water. The expansion of the mercury in 

 the bulb with heat will force a portion of the liquid through the contraction, 

 and subsequent cooling in hauling up will not cause any of it to return. 

 The portion which has thus escaped from the bulb into the stem will usually 

 be found rem^iining close to the contraction, when the thermometer has been 

 hauled up. The instrument must then be gently inclined, so as to make the 

 bulb end slightly the lowest, when the mercury in the stem wiU all unite 

 into one column, which will run down to its place on again raising the bulb. 

 The head of the column will then indicate the required temperature. 



