434 Mr. J. Croll on the P hysical Cause of Ocean-currents. 



Another method of receiving signals has been tried, though 

 not adopted anywhere. The current is passed through the pri- 

 mary wire of an induction-coil, in the secondary circuit of which 



is the receiving instrument. The signal here depends on -*p 



the rate of increase of the current ; and the arrival-curve has its 

 maximum at about S'oa, while with condenser pure and simple 

 it is la. It does not, therefore, appear evident why, as Mr.Var- 

 ley states, he found the condenser more satisfactory. Mr. G. 

 K. Winter reinvented this system, and reports very favourably 

 on its effect (British Association, Brighton, 1872). 



LV. On the Physical Cause of Ocean- currents. 

 To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN a lecture at the Royal Institution, and also in the ' Athe- 

 naeum/ 'Nature/ Philosophical Magazine, and other quar- 

 ters, Dr. Carpenter has been advancing a somewhat plausible- 

 looking objection to my views in reference to under- currents. 

 As this objection bears upon a point whicl^ in my last commu- 

 nication I omitted to consider, perhaps you will permit me, 

 through your columns, briefly to refer to it. The objection in 

 question, as stated in Dr. Carpenter's own words, is as follows :•— 



"According to Mr. CrolPs doctrine, the whole of that vast 

 mass of water in the North Atlantic, averaging, say, 1500 fa- 

 thoms in thickness and 3600 miles in breadth, the temperature 

 of which (from 40° downwards), as ascertained by the ' Chal- 

 lenger ' soundings, clearly shows it to be mainly derived from a 

 polar source, is nothing else than the reflux of the Gulf stream. 

 Now, even if we suppose that the whole of this stream, as it 

 passes Sandy Hook, were to go on into the closed Arctic basin, 

 it would only force out an equivalent body of water. And as, 

 on comparing the sectional areas of the two, I find that of the 

 Gulf-stream to be about one 900th that of the North Atlantic 

 underflow ; and as it is admitted that a large part of the Gulf- 

 stream returns into the Mid-Atlantic circulation, only a branch 

 of it going on to the north-east, the extreme improbability (may 

 I not say impossibility ?) that so vast a mass of water can be 

 put in motion by what is by comparison a mere rivulet (the 

 north-east motion of which, as a distinct current, has not been 

 traced eastward of 30° W. long.) seems still more obvious." 



The objection seems to me to be based upon a series of mis- 

 apprehensions : — (1) that the mass of cold water 1500 fathoms 

 deep and 3600 miles in breadth is in a state of motion towards 



