' 



82 Mr. J. Croll on Ocean-currents in relation to 



In the application of the method to an ocean-current, the two 

 most important elements which we require as data are the volume 

 of the stream and its mean temperature. But although we know 

 something of the temperature of most of the great currents of 

 the ocean, yet, with the exception of the Gulf-stream, little is 

 known regarding the volume of any of them. 



Extensive and accurate observations have been made on the 

 breadth, depth, and temperature of the Gulf-stream by the 

 United-States Coast Survey. In the memoirs and charts of the 

 Survey, cross sections of the stream at various places are given, 

 showing its breadth and depth at those places, and also the tem- 

 perature of the water from the surface downwards to the bottom. 

 We are thus enabled to determine pretty correctly the mean 

 temperature of the stream. And knowing its mean velocity at 

 any given section, we have likewise a means of determining the 

 number of cubic feet of w r ater passing through the section in a 

 given time. But although we can obtain with tolerable accuracy 

 the mean temperature, unfortunately observations regarding the 

 velocity of the water at all depths have not been made at any par- 

 ticular section. Consequently w T e have no means of estimating 

 so accurately as we should have wished the volume of the cur- 

 rent. However, as we know the surface-velocity of the water at 

 places where some of the sections were taken, we are thus enabled 

 to make at least a rough estimate of the volume of the stream. 



From an examination of the published sections some years 

 ago*, I came to the conclusion that the total quantity of water 

 conveyed by the stream is probably equal to that of a stream 50 

 miles broad and 1000 feet deepf, flowing at the rate of four 

 miles an hour, and that the mean temperature of the entire mass 

 of moving water is not under 65° at the moment of leaving the 

 Gulf. I think we are warranted to conclude that the stream, 

 before it returns from its northern journey, is on an average 

 cooled down to at least 40°; consequently it loses 25° of heat. 

 Each cubic foot of water, therefore, in this case carries from the 

 tropics for distribution upwards of 1500 units of heat, or 1,158,000 

 foot-pounds. According to the above estimate of the size and 

 velocity of the stream, 5,575,680,000,000 cubic feet of water 

 are conveyed from the Gulf per hour, or 133,816,320,000,000 

 cubic feet daily. Consequently the total quantity of heat trans- 



* Philosophical Magazine for February 1867, p- 127. 



t The Gulf-stream at the narrowest place examined by the Coast Sur- 

 vey, and the place where its velocity was greatest, was found to be over 

 30 statute miles broad and 1950 feet deep. But we must not suppose this 

 represents all the warm water which is receive<t*'by the Atlantic from the 

 equator ; a great mass of the water flows into the Atlantic without passing 

 through the Straits of Florida. 



