6 DR. DAUBENY ON THE QUANTITY AND QUALITY 



latter, and thus creating a pressure upon the contents of its included cavity, would 

 prove the means of propelling a stream of air, proportionate to the degree of dimi- 

 nution that had taken place in its own temperature, through fissures towards the 

 surface. 



In some such way, perhaps, we may imagine the evolution of gaseous matter to go 

 on for centuries, in a manner nearly as uniform, as the shrinking in the dimensions of 

 the cavity, caused by the yielding of its walls to the pressure from without, may be 

 supposed to proceed*. 



Be that as it may, the above estimate of the quantity discharged will afford a 

 standard, by which a comparison may be made at present between this and other 

 thermal springs in the above respect, and which may be appealed to hereafter, 

 should it be wished to ascertain, from time to time, whether any change has taken 

 place in the nature of this particular spring, or in the causes from which its heat 

 proceeds. 



Whilst engaged in thus determining the aggregate amount of all the gas evolved by 

 the spring, saving the small quantity that finds its way from apertures near the sides 

 of the bath, by means of the apparatus above described, which enabled me to collect 

 whatever was disengaged within an area of twenty feet, I conceived that it might be 

 worth while at the same time to estimate, what proportion of the whole rises up 

 immediately through the stone cylinder, eighteen inches in diameter, which exists in 

 the centre of the bath. 



This was readily ascertained by means of a smaller shield, or funnel-shaped appa- 

 ratus, which exactly fitted that opening, and the results obtained in the space of each 

 minute are accordingly registered in separate columns by the side of the former. 



The mean quantity obtained, taking the average of nineteen observations, was 34*75 

 cubic inches; the maximum ever obtained, 80; the minimum, 5; the average variation 



* The dimensions of such a cavern, or series of caverns, need not be supposed so much more considerable than 

 those of manj'^ which have fallen under our observation, as to give rise to any serious difficulty. That at Speed- 

 w^ell Mine, in Derbyshire, contains a pool of water 320 feet in depth, and rises to a height of more than 450 

 feet above the surface of the water. It is therefore nearly 800 feet in perpendicular height. That at Adelsburg, 

 in Carinthia, one only of a series existing in that limestone formation, is in many places more than 100 feet 

 in height, and extends, it is said, to a distance of nine or ten miles. One lately noticed by a traveller in the 

 Caucasus (Colonel Monteith) is 600 feet high, 1200 feet in span, and 800 feet in depth. It probably com- 

 municates with others by means of fissures. See Geographical Journal, vol. iii., lately published. Now, if we 

 suppose 250 cubic feet of gas to have been expelled daily from a cavern underneath Bath, for a period of about 

 5000 years, the whole quantity given off would amount to 456,250,000 cubic feet. A cavity, therefore, equal 

 to 2000 cubic feet in its entire dimensions, if in the course of that period it had contracted to -rV^h ^^ i^^ original 

 size in consequence of the cooling of its walls, would have expelled a quantity of gas corresponding with that 

 which the Bath waters have disengaged within that period. But a contraction of -rV^h may without difficulty 

 be imagined to have resulted from the cooling down of a mass of rock from 13,000° Fahr. to 400° ; for glass 

 contracts about x-^V^rdth part between 212° and 32°, or by cooling 180° ; consequently a rock which contracted 

 in an equal ratio with glass would diminish in bulk .V^h by an abstraction of heat equal to 12,600°, assuming 

 even that the expansion at elevated temperatures is not more considerable than it is at low ones. 



