Mr. J. Croll on the Physical Cause of Ocean-currents. 277 



some particular place. This is evident ; for the mean velocity 

 entirely depends upon the sectional area of the stream. The 

 place where the mean velocity is 49*4 miles per day must be the 

 place where it is 39^ miles broad and 1200 feet deep ; for he 

 is here endeavouring to show us how small the volume of the 

 stream actually is. Now, unless the mean velocity refers to the 

 place where he gives us the breadth and depth of the stream, his 

 figures have no bearing on the point in question. But a stream 

 39^ miles broad and 1200 feet deep has a sectional area of 8*97 

 square miles, and this, with a mean velocity of 49*4 miles per 

 day, will give 443 cubic miles of water. The amount, according 

 to the estimate taken in my last paper, is 459 cubic miles per 

 day; it therefore exceeds Mr. Findlay's estimate by only 16 

 cubic miles. 



Mr. Findlay, so far as I am aware, does not consider that I 

 have overestimated the mean temperature of the stream. He 

 states (Brit. Assoc. Report, 1869, p. 160) that between Sand 

 Key and Havanna the Gulf-stream is about 1200 feet deep, 

 and that it does not reach the summit of a submarine ridge, 

 which he states has a temperature of 60°. It is evident, then, 

 that the bottom of the stream has a temperature of at least 60°, 

 which is within 5° of what I regard as the mean temperature of 

 the mass. But the surface of the stream is at least 17° above 

 this mean. Now, when we consider that it is at the upper parts 

 of the stream, the place where the temperature is so much above 

 65°, that the motion is greatest, it is evident that the mean tem- 

 perature of the entire moving mass must, according to Mr. Find- 

 lay, be considerably over 65°. It therefore follows, according to 

 his own data, that the Gulf- stream conveys into the Atlantic an 

 amount of heat equal to one fourth of all the heat which the 

 Atlantic, from the latitude of the Straits of Florida up to the 

 Arctic regions, derives from the sun. 



But even supposing we were to halve Mr. Findlay's own esti- 

 mate, and assume that the volume of the stream is equal to only 

 222 cubic miles of water per day instead of 443, still the amount 

 of heat conveyed would be equal to one eighth part of the heat 

 received from the sun by the Atlantic. But would not the 

 withdrawal of an amount of heat equal to one eighth of that re- 

 ceived from the sun greatly affect the climate of the Atlantic ? 

 Supposing we take the mean temperature of the Atlantic at, say, 

 56°; this will make its temperature 295° above that of space. 

 Extinguish the sun and stop the Gulf- stream, and the tempera- 

 ture ought to sink 295°. How far, then, ought the temperature 

 to sink, supposing the sun to remain and the Gulf-stream to 

 stop ? Would not the withdrawal of the stream cause the tem- 

 perature to sink some 30° ? Of course, if the Gulf-stream were 

 withdrawn and every thing else were to remain the same, the tern- 



