OCEAN CURRENTS 57 



any experience in its use. An anchor or heavy weight 

 is made fast to the end of the wire ; above this at the 

 requisite distance the meter is fixed, and above this 

 again, but below the surface, is a buoy, the floating 

 power of which is not great enough to lift the anchor 

 from the bottom, though it keeps the line taut. The 

 buoy contains clockwork which drops the two mes- 

 sengers down the line to start and stop the mechanism. 

 The wire above the buoy is slack, and the ship steams 

 up to it. 



An Outline of Oceanic Circulation. 



BY THE EDITOR. 



No account of oceanic water can be deemed com- 

 plete without a brief sketch of its circulation. It 

 must be premised that while the movements of surface 

 water are comparatively simple and fairly well under- 

 stood, the circulation of deeper waters is still obscure. 

 Even as regards the surface waters, not all oceano- 

 graphers are agreed as to the exact extent to which 

 all the various forces concerned are responsible for 

 the driving power. But such controversies are beyond 

 our scope here, and we may content ourselves with the 

 crude statement that the main factors in setting the 

 pace and direction of most ocean currents are the 

 prevalent winds, as determined by the pressure belts 

 (pp. 6, 7), and as modified by the influence of land 

 masses, it being at the same time admitted that such 

 other factors as the rotation of the earth, differences in 

 temperature and salinity, and so forth, also play a part. 



At the outset we must distinguish between stream 

 and drift currents. Of these the stream is due in its 

 simplest form to a head of water running down over 

 water, as a mill-stream runs down over land ; the drift 

 current is caused chiefly by the action of prevalent 



