554 



Dr. W. B. Carpenter on the 



[June 13, 



" that at the Poles is reached. We have in this case a plane with a slope of 

 " 10,000 feet in 6200 miles, forming the upper surface of the water of 

 * ' maximum density. Now this slope exercises no influence in the w r ay of 

 " producing a current, as some seem to suppose ; for this is not a case of 

 " disturbed equilibrium, but the reverse. This slope is the condition of 

 "static equilibrium when there is a difference between the temperature of 

 " the water at the Equator and the Poles." Taken by itself, this propo- 

 sition would seem irreconcilable with the simplest principles of Physics ; 

 but I presume that it is to be understood in its connexion with what Mr. 

 Croll seems to regard as a permanent inequality of level between Equatorial 

 and Polar waters. For he goes on to say : — " The only slope that has any 

 "tendency to produce motion of the water is the slope formed by the sur- 

 " face of the ocean in the Equatorial regions being higher than the surface 

 " at the Poles ; but this is a slope of only 18 feet in 6200 miles." — Now 

 if, as I maintain, there is no such permanent inequality of level — any dif- 

 ference produced by inequality of Temperature always tending towards 

 equalization, — the inclined plane of denser because colder water, of which 

 the slope (as admitted by Mr. Croll) must be nearly two feet per mile, 

 constitutes a serious disturbance of equilibrium, that must be capable of 

 producing very decided effects. 



26. Mr. Croll's whole manner of treating the subject is so different from 

 that which it appears to me to require, and he has so completely misappre- 

 hended my own view of the question, that I feel it requisite to present this 

 in fuller detail, in order that Physicists and Mathematicians, having both 

 sides fully before them, may judge between us : and this must be my apo- 

 logy for dwelling at some length upon considerations so elementary, that I 

 should not otherwise have thought it requisite even to advert to them. — 

 Such, in the first place, is the mode in which Cold applied to the surface 

 of an extensive basin of fresh water operates in reducing the Temperature 

 of the whole mass. Supposing the cold to be applied to the entire area 

 of the basin (as when frost acts on a pond or lake), so that the whole sur- 

 face-film is chilled at the same time, that film will sink through the sub- 

 jacent water, carrying downwards its reduced temperature — it may be to 

 the bottom of the basin ; and anew film from the warmer layer immediately 

 beneath the surface rises into its place. This, being cooled in its turn, 

 sinks until it meets with a layer as cold as itself ; its place on the surface 

 being taken by another film from immediately beneath. Of course, these 

 films do not subside without some intermixture and interchange of tempe- 

 rature with the layers through which they successively descend ; but they 

 retain enough of their integrity to produce a downward convection of Cold, 

 precisely answering to the upward convection of Heat which takes place 

 when Heat is applied to the bottom of a vessel of water. This can be 

 readily shown experimentally, by diffusing either colouring-matter, or solid 

 particles of some substance that will remain in suspension when finely di- 

 vided, through the surface-film. By the successional descent of the surface- 



