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



how this would have given a lower mean temperature than 65°. 

 We might, by means of Dr. Carpenter's method, arrive at a 

 pretty accurate estimate of the mean temperature of the cross 

 section of the stream ; and it is quite probable that the mean 

 temperature of the cross section would be found to be under 

 65°. But although the mean temperature of the cross section 

 should be below 65°, it does not follow on this account that the 

 mean temperature of the water flowing through this cross section 

 must be below that temperature. It is perfectly obvious that 

 the mean temperature, of the mass of water flowing through the 

 cross section in a given time must be much higher than the 

 mean temperature of the cross section itself. Perhaps Dr. 

 Carpenter has overlooked this fact, and hence the reason why he 

 has arrived at so low an estimate of the mean temperature of 

 the stream. 



The reason why the temperature of the water must be higher 

 than that of the cross section is this : — It is in the upper half of 

 the section where the high temperature exists ; but as the ve- 

 locity of the stream is far greater in its upper half than in its 

 lower half, the greater portion of the water passing through this 

 cross section is water of high temperature. If Dr. Carpenter 

 would take this fact into consideration and again go over his 

 calculations, I feel persuaded he would arrive at the conclusion 

 that the mean temperature of the Gulf-stream at the moment of 

 leaving the Gulf is not under 65°. 



But, be all this as it may, let us assume that the volume of the 

 stream is but one half what he supposes I took it to be in my 

 calculations, and also that the quantity of heat conveyed per 

 pound of water is but 12^ units instead of 25 units, as in my 

 estimate ; in other words, let us assume the mean temperature 

 of the stream to be 52^° instead of 65°. Surely these are con- 

 cessions that will satisfy not only Dr. Carpenter, but every one 

 else. Let us consider now what are the consequences to which 

 we are still led in regard to his theory of a general interchange 

 of equatorial and polar water independently of the Gulf-stream. 



The area from which Dr. Carpenter derives his heat is the 

 area of the Atlantic, extending from the equator to the tropic 

 of Cancer, including the Caribbean Sea and the Gulf of Mexico. 

 By referring to my last paper (Phil. Mag. for Oct. 1870, pp. 

 255-257) it will be seen that, taking the volume of the Gulf- 

 stream at one-half of what Dr. Carpenter supposes I estimated it 

 to be in my last paper, and assuming that the quantity of heat 

 conveyed into the Atlantic in temperate regions by means of his 

 general circulation is equal to that conveyed by the Gulf-stream 

 (whose heat he assumes does not pass beyond the temperate 

 regions), the amount of heat removed from the Torrid zone and 



