216 



Messrs. Carpenter and Jeffreys on 



[Dec. 8, 



to the application of heat at the bottom, the motor power of which is 

 universally admitted, — being practically utilized in keeping up the circula- 

 tion through the hot- water Warming- Apparatus now in general use *. The 

 movement thus maintained would not, on the hypothesis, be a rapid one, 

 but a gradual creeping flow ; since the absence of limit would prevent the 

 power which sustains it from acting as an accelerating force, as it would 

 do if the Equatorial and Polar areas were connected only by a narrow 

 channel, like the Atlantic Ocean and the Mediterranean Sea (§ 120, 

 Princ. VIL). 



129. That the Vertical Circulation here advocated on h priori grounds 

 actually takes place in any mass of Salt water of which one part is exposed 

 to surface-Cold and another to surface-Heat, is capable of ready experi- 

 mental proof :-— Let a long narrow trough with glass sides be filled with 

 salt water ; and let heat be applied at one end (the Equatorial) by means 

 of a thick bar of metal laid along the surface, with a prolongation carried 

 over the end of the trough into the flame of a spirit-lamp ; whilst cold is 

 applied at the other (the Polar) by means of a freezing-mixture contained 

 in a metallic box made to lie upon the surface, or (more simply) by means 

 of a piece of ice wedged in between the sides of the trough. A circulation 

 will immediately commence in the direction indicated by the theory ; as 

 may be readily shown by introducing some blue colouring liquid at the 

 Polar surface, and some red liquid at the Equatorial surface. The blue 

 liquid, as it is cooled, at once descends to the bottom, then travels 

 slowly along it until it reaches the Equatorial end of the trough, then 

 gradually rises towards the heated bar, and thence creeps along the surface 

 back to the Polar end; the red liquid first creeps along the surface 

 towards the Polar end, and then travels through exactly the same course 

 as the blue had previously done f. 



130. That such a Vertical Circulation really takes place in Oceanic Water, 

 and that its influence in moderating the excessive Cold of the Polar Areas 

 and the excessive Heat of the Equatorial region is far more important than 

 that of any surface-currents, seems to us a legitimate deduction from the 

 facts stated in the Report of the 'Porcupine' Expedition for 1869. For, 

 on the one hand, it was shown (§§1 16-118) that there is a general diffusion 

 of an almost glacial temperature on the bottom of the deep Ocean-basins, 



* The only scientific writer who has even approached what appears to us the truth 

 on this point is Captain Maury, who has put forward the doctrine of a general inter- 

 change of water between the Equator and the Poles, resulting from a difference of 

 Specific Gravity caused inter alia by difference of Temperature. But, as Mr. Croll 

 remarks, " although Capt. Maury has expounded his views on the cause of Ocean- 

 currents at great length in the various editions of his work, yet it is somewhat difficult 

 to discover what they really are. This arises from the generally confused and sometimes 

 contradictory nature of his hydrodynamical conceptions." See Mr. Croll's Paper " On 

 the Physical Cause of Ocean-currents " in the Philosophical Magazine for October, 1870. 



t This experiment has been exhibited, by the kindness of Prof. Odling, at the Royal 

 Institution and at the Royal Geographical Society. 



