The Drift of Floating Bottles 



339 



line pursued by the bottle in its journe}^, 

 it is necessary to go back to the motive 

 power which gives rise to the surface 

 currents of the sea, viz., the winds. A 

 perfectly steady wind acting continu- 

 ously on the surface of the sea will, 

 through friction, give rise to a move- 

 ment of the surface waters in the same 

 direction as the wind itself. If the lat- 

 ter continues for a sufficient length of 

 time the impulse, first felt only at the 

 surface, will gradually communicate 

 itself downward, owing to the viscosity 

 of the water, and the lower strata to a 

 successively greater and greater depth 

 will thus partake of the movement until 

 it is finally shared by the whole mass, 

 the velocit}^ of the motion diminishing 

 as the depth increases. The rate, how- 

 ever, at which this motion is communi- 

 cated to the depths of the ocean is ex- 

 ceedingly slow. It has, for instance, 

 been estimated that in a depth of 2,000 

 fathoms a surface current of given ve- 

 locity would require a period of 200,000 

 years to transmit its due proportion of 

 this velocity to a point halfway toward 

 the bottom. Similarlj^, when once es- 

 tablished, these submarine currents 

 exhibit a corresponding reluctance to 

 undergo any variation in direction or 

 intensity.* 



Perfectly steady winds, however, do 

 not exist, even in the region of the 

 trades. The winds are constantly 



* Zopprits, Aiinalen d. Hj'drographie, 187S. 



changing, and the surface currents 

 change with them. The lower strata 

 of the ocean, however, are insensible to 

 these changes, and at a considerable 

 distance below the surface the waters 

 of the ocean have probably a slow but 

 perfectly uniform motion, the direction 

 of the motion probably agreeing closely 

 with that of the resultant surface winds. 



We have, therefore, in the body of the 

 sea two distinct sets of currents ; first, 

 those at the immediate surface, which 

 move practically at the obedience of the 

 surface winds, sometimes in one direc- 

 tion, sometimes in another : second, 

 those of the lower strata, which are 

 constant in direction and velocity and 

 represent the aggregate effect of the 

 winds that have blown for ages past, 

 the sea in this respect furnishing a close 

 analogy to the atmosphere, the motion 

 of the lower strata of which is con- 

 stantly disturbed, while that of the 

 higher strata, as shown by the motion 

 of the cirrus clouds, is comparatively 

 uniform. 



It is the motion of these lower strata, 

 as I take it, that the uniform paths pur- 

 sued by these drifting bottles to some 

 extent represent, and it is the evidence 

 contained in them that should be studied 

 in investigations dealing with the cur- 

 rents of the ocean in their entirety, 

 rather than the evidence obtained from 

 any given set of current measurements 

 made at or near the surface and for 

 some given point. 



THE BRITISH ANTARCTIC EXPEDITION 



THE Discovery, carrying the British 

 National Antarctic expedition, 

 is now well on her way to South 

 Polar regions. The proposed • work of 

 the party has been carefully outlined by 

 the presidents of the Ro3^al Society and 

 of the Ro3^al Geographical Society in 

 their instructions to Captain Scott and 



to Dr. George Murray, the scientific 

 director. The instructions to the com- 

 mander are as follows : 



I. The Royal Society and the Royal 

 Geographical Society, with the assist- 

 ance of His Majesty's Government, have 

 fitted out an expedition for scientific dis- 

 covery and exploration in the Antarctic 



