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



affirm, as he does, that the winds can only produce a mere sur- 

 face-drift. If the viscosity and molecular resistance of water be 

 such that, when the lower strata of the ocean are impelled for- 

 ward by gravity or by any other cause, the superincumbent 

 strata extending to the surface are perforce dragged after them, 

 then, for the same reason, when the upper strata are impelled 

 forward by the wind or any other cause, the underlying strata 

 must also be dragged along after them. 



If the condition of the ocean between Greenland and the 

 north-western shore of Europe is irreconcilable with the gravi- 

 tation theory, we find the case even worse for that theory when 

 we direct our attention to the condition of the ocean on the 

 southern hemisphere; for according to the researches of Cap- 

 tain Duperrey and others on the currents of the Southern 

 Ocean, a very large portion of the area of that ocean is occupied 

 by water moving on the surface more in a northward than a 

 poleward direction. Referring to the deep trough between the 

 Shetland and the Earoe Islands, called by him the " Lightning 

 Channel," Dr. Carpenter says, " If my view be correct, a cur- 

 rent-drag suspended in the upper stratum ought to have a per- 

 ceptible movement in the N.E. direction ; whilst another, sus- 

 pended in the lower stratum, should move S.W." (§ 40). 



Any one believing in the north-eastern extension of the Gulf- 

 stream and in the Spitzbergen polar under current, to which I 

 have already referred, would not feel surprised to learn that the 

 surface- strata have a perceptible north-eastward motion, and 

 the bottom strata a perceptible south-westward motion. North- 

 cast and east of Iceland there is a general fiWof cold polar water 

 in a south-east direction towards the left edge of the Gulf- 

 stream. This water, as Professor Mohn concludes, iC descends 

 beneath the Gulf-stream and partially finds an outlet in the 

 lower half of the Earoe- Shetland channel"*. 



The Mechanics of the Theory. 



" I now proceed," says Dr. Carpenter, " to the second head 

 of the discussion, viz. the demonstration which Mr. Croll con- 

 siders himself to have given, that the difference of temperature 

 between polar and equatorial water cannot possibly produce the 

 effect I attribute to it" (§ 21). 



" Mr. CrolFs whole manner of treating the subject is so dif- 

 ferent from that which it appears to me to require, and he has 

 so completely misapprehended my own view of the question, 

 that I feel it requisite to present this in fuller detail, in order 

 that physicists and mathematicians, having both sides fully before 

 them, may judge between us" (§ 26). 



* Dr. Petermann's Mittheilwigen for 1872, p. 315. 



