CANADIAN FISHh'h'IES EXPEDITION, Wl'rlo 257 



induced by the action of anticyclonic winds, such as the current round the Sargasso 

 Sea, and the analogous currents in the South Atlantic, the Indian ocean and the 

 Pacific, will be far more markedly apparent. Moreover, the anticyclonic currents pro- 

 ceeding from a rising centre outward, will be comparatively insignificant, whereas the 

 cyclonic movement of water in towards a sinking centre, e.g., towards the Arctic region, 

 or the point where the Labrador current disappears, etc., will be strongly marked. We 

 liave, then, the following rule: Cyclonic currents in the sea are occasioned by physical 

 causes, the anticyclonic currents by the action of the wind. 



As the wind has no immediate effect upon the specific gravity of water, it follows 

 that a current occasioned by the wind will be restricted to a single water layer, and 

 is, in consequence, limited in extent, and simple in character. The Sargasso current, 

 {vide fig. 24) may, as a matter of fact, be taken as the type of all great wind currents 

 in the sea. 



Despite their simplicity of character, however, these anticyclonic wind currents 

 nevertheless present many features of interest, and are well worthy of further study. 

 Such a vortex, with its powerful pressure in towards its centre, will be highly coherent 

 and, in spite of the enormous extent involved, of distinctly individual character, with 

 marked isolation from the surrounding water. One result of the pressure on the centre 

 is that the vortex there attains a considerable depth. The Sargasso current, for in- 

 stance, extends down to 600 metres. And, further, the strong vertical movement of 

 the water in such a vortex enables it to carry down the heat of the surface water to a 

 great depth ; there is a continual wearing upon and heating of the subjacent colder 

 water, so that the vortex is constantly intaking and assimilating water from below. 

 The Sargasso current discharges unto the Gulf Stream 25,000,000 tons of water per 

 second, which is as much as to say that it draws from its substratum just that quantity 

 of water every second. 



Currents arising from physical changes in the sea-water, on the other hand, are 

 otherwise constituted, and behave in a very different way. They are not restricted 

 to a single layer, but may traverse several such, and have thus far greater freedom 

 of movement than the wind currents. Consequently, it is upon the former that the 

 tiisk of bringing about interchange between the waters of differ 3nt regions and different 

 depths devolves. The Gulf Stream is the type of this category. 



The motive power in ' these currents is supplied by the Archimedean forces in 

 the sea. When the specific gravity of the water in a certain layer has been sufficiently 

 increased by physical change, it sinks down thence to a subjacent layer answering to 

 its own specific gravity ; where a decrease in the specific gravity takes place, the move- 

 ment is of course reversed. In the Gulf Stream layer, water is introduced from the 

 subjacent layer owing to rise of temperature taking place in the tropics; in the Arctic 

 regions, on the other hand, water from the Gulf Stream passes, on cooling, from that 

 current to the layer beneath. 



Where water is thus introduced into a layer, the layer in question becomes 

 thicker than its surroundings, and Archimedean forces then arise which drive the 

 water of that laj^er from its place. Similarly, a layer losing some of its water becomes 

 thiinier, and the same forces tend to lead water thither from without. Owing to the 

 earth's rotation, this movement of the water proceeds, not in a straight line, but in 

 V- spiral, as shown in figs. 41 a and c. If, in one and the same layer, water is intro- 

 duced at one point and water carried off at another, then the layer in question will 

 become thicker at the former than at the latter, giving rise to Archimedean forces 

 which drive the water from the point of inflow to the i)oint where water i>asses out. 

 The Gulf Stream, for instance, is 600 metres deep in the tropics, but only 200 metres 

 deep at Spitzbergen. We see, then, that the Archimedean forces in the Gulf Stream 

 drive the water from the tropics towards Spitzbergen. Owing to the earth's rotation, 

 the movement in question becomes S-shaped, vide fig. 42. If the water at the i)oint 



