Mr. J. Croll on the Physical Cause of Ocean-currents. 97 



Sea and Indian Ocean, where a temperature as low as 32° Fahr. 

 was found at a depth of 2656 fathoms. In short the North 

 Atlantic, and probably the intertropical seas also, may be re- 

 garded, Dr. Carpenter considers, as divided horizontally into 

 two great layers or strata — an upper warm, and a lower cold 

 stratum. All these facts I, of course, freely admit ; nor am I 

 aware that their truth has been called in question by any one, 

 no matter what his views may have been as to the mode in which 

 they are to be explained. 



The Explanation of the Facts. — -We have next the explanation 

 of the facts, which is simply this : — -The cold water occupying 

 the bottom of the Atlantic and of intertropical seas is to be ac- 

 counted for by the supposition that it came from the polar regions. 

 This is obvious, because the cold possessed by the water could 

 not have been derived from the crust of the earth beneath : 

 neither could it have come from the surface ; for the temperature 

 of the bottom water is far below the normal temperature of the 

 latitude in which it is found. Consequently "the inference 

 seems irresistible that this depression must be produced and 

 maintained by the convection of cold from the polar towards the 

 equatorial area." Of course, if we suppose a flow of water from 

 the poles towards the equator, we must necessarily infer a counter 

 flow from the equator towards the poles ; and while the water 

 flowing from equatorial to polar regions will be warm } that 

 flowing from polar to equatorial regions will be cold. The doc- 

 trine cf a mutual interchange of equatorial and polar water is 

 therefore a necessary consequence from the admission of the fore- 

 going facts. With this explanation of the facts I need hardly say 

 that I fully agree ; nor am I aware that its correctness has ever 

 been disputed. Dr. Carpenter surely cannot charge me with over- 

 looking the fact of a mutual interchange of equatorial and polar 

 water, seeing that my estimate of the thermal power of the 

 Gulf-stream, from which it is proved that the amount of heat 

 conveyed from equatorial to temperate and polar regions is enor- 

 mously greater than had ever been anticipated, was made a con- 

 siderable time before he began to write on the subject of oceanic 

 circulation*. And in my paper " On Ocean-currents in relation 

 to the Distribution of Heat over the Globe" f, I have endeavoured 

 to show that, were it not for the raising of the temperature of 

 polar and high temperate regions and the lowering of the tem- 

 perature of intertropical regions by means of this interchange of 

 water, these portions of the globe would not be habitable by the 

 present existing orders of beings. 



* Trans, of Glasgow Geol. Soc. for April 1867. Phil. Mag. for Feb. 

 186/ and June 1867 (Supplement). 

 t Phil. Mag. for February 1870. 



Phil. Mag. S. 4. Vol. 47. No. 310.. Feb, 1874. H 



