Mr. J. Croll on the Physical Cause of Ocean- currents. 183 



the surface of the ocean is retarded both by the wind and by the 

 surface-water moving in opposition to its course ; but being con- 

 tinually pressed forward by the impulse of the northerly winds 

 acting along its whole course back almost to the pole perhaps, 

 or as far north at least as the sea is not wholly covered with ice, 

 the polar current cannot stop when it enters the region of op- 

 posing winds and currents ; it must move forward. But the 

 water thus pressed from behind will naturally take the path of 

 least resistance. Now in the present case this path will neces- 

 sarily lie at a considerable distance below the surface. Had the 

 polar stream simply to contend with the Gulf-stream flowing in 

 the opposite direction, it would probably keep the surface and 

 continue its course along the side of that stream ; but it is op- 

 posed by the winds, from which it cannot escape except by dip- 

 ping down under the surface ; and the depth to which it will 

 descend will depend upon the depth of the surface-current flow- 

 ing in the opposite direction. There is no necessity for suppo- 

 sing a heaping up of the water in order to produce by pressure 

 a force sufficient to impel the under current. The pressure of 

 the water from behind is of itself enough. The same explana- 

 tion, of course, applies to the case of the Gulf-stream passing 

 under the polar stream. And if we reflect that these under 

 currents are but parts of the general system of circulation, and 

 that in most cases they are currents compensating for water 

 drained off at some other quarter, we need not wonder at the 

 distance which they may in some cases flow, as, for example, 

 from the banks of Newfoundland to the Gulf of Mexico. The 

 under currents of the Gulf-stream are necessary to compensate 

 for the water impelled southwards by the northerly winds ; and 

 again, the polar under currents are necessary to compensate for 

 the water impelled northward by the south and south-westerly 

 winds. 



No accurate observations, as far as I know, have been made 

 regarding the amount of work performed by the wind in impel- 

 ling the water forward ; but when we consider the great retard- 

 ing effect of objects on the earth's surface, it is quite apparent 

 that the amount of work performed on the surface of the ocean 

 must be far greater than is generally supposed. For example, 

 Mr. Buchan, Secretary to the Scottish Meteorological Society, 

 has shown* that a fence made of slabs of wood 3 inches in width 

 and 3 inches apart from each other is a protection even during 

 high winds to objects on the lee side of it, and that a wire screen 

 with meshes about an inch apart affords protection during a gale 

 to flower-pots. The same writer was informed by Mr. Addie 

 that such a screen put up at Rockville was torn to pieces by a 

 * Paper read to the Edinburgh Botanical Society on January 8, 1874. 



