Prof. Thomson on the Dynamical Theory of Heat. 105 



abstraction of at least three or foui" millions of gallons over the 

 area indicated above. As these wells do not aftect one another^ 

 however, in the slightest degi'ee — and severa of them have been 

 flowing uninten-uptedly for many years — we may fairly assume 

 that the reservoir is capable of yielding a very much larger sup- 

 ply without detriment to existing interests. 



There are perhaps few localities in which the subject of Arte- 

 sian wells can be better studied than in this valley between SUsoe 

 and Barton-in-the-Clay ; the latter ^'illage lying under the bold 

 escarpment of the chalk with its projecting spurs and rounded 

 hollow coombes, and the fonner on the opposing range of the 

 lower greensand. The relative heights, the mineral characters, 

 and the dip of the strata are readily observable ; and from various 

 positions the eye can take in at a glance the physical and geolo- 

 gical nature of the whole of the suiTounding district — the marly 

 impernous valley extending between the sandstone hills and the 

 projecting chalk, and resting upon the under-dipping beds of the 

 former strata. 



In these examinations, besides ofber points of minor import- 

 ance, five conditions have to be more particularly considered. 

 First, the general levels of the countiy ; secondly, the relative 

 positions, inclination, and thickness of the strata ; thirdly, their 

 permeable or impermeable nature; foiu-thly, the outcrop area 

 and surface configuration of the water-supplying beds ; and 

 fifthly, the chemical composition, &c. of the same, as likely to 

 affect or not the quality of the water. Mineralogical characters, 

 therefore, although useless, and even hurtful in their attempted 

 interpretations, in questions of pure or abstract geology, become, 

 in these local and practical investigations, of the highest value. 



XIV. On the Dynamical Theory of Heat, with numerical results 

 deduced from Mr. Joule's equivalent of a Thermal Unit, and 

 ]M. llcgnault's obsei-vations on Steam. By William Thomson, 

 M.A., Fellow of St. Peter's College, Cambridge, and Professor 

 of Natural Philosophy in the University of Glasgow. 

 [Continued from p. 21.] 



Part II. — On the Motive Power of Heat through Finite Ranges 

 of Temperature. 



24. TT is required to determine the quantity of work which a 

 J- perfect engine, supplied from a source at any temperature, 

 S, and parting with its waste heat to a refrigerator at any lower 

 temperatvire, T, will produce from a given quantity, H, of heat 

 drawn from the source. 



25. We may suppose the engine to consist of an infinite 



