36 



We must empliaBlze that this concern extends to every type of circu- 

 lation takino; place, and to every fores, external or internal, that 

 is able to set the water in niotion, because every type of circula- 

 tion that exists has far-reaching effects in all the fields just 

 mentioned, while because of the almost perfect fluidity of water, a 

 variety of forces produce motion within it. Furthermore, every 

 circulatory problem involves both the observable events and their 

 causes. 



Oirculatory phenomena in the sea may be divided into (a) tidal, 

 set in notion by the gravitational attraction of the sun, of the 

 moon, and of other heavenly bodies; and (b) non-tidal, including all 

 other currents or disturbances of whatever sort. 



The study of the tides is now so admirably taken care of by the 

 tidal surveys of all the more important maritime nations that the 

 tides will be omitted from this discussion. 



For convenience, the non-tidal currents may, in turn, be 

 divided into (a) the progressive horizontal, (b) the vertical, and 

 (c) the non- progressive oscillations which do more work in the sea 

 than is generally appreciated. The first of these groups includes 

 all the more apparent ocean currents, also the slower mass-drifts, 

 whether at the surface or in the deeps. The second group refars to 

 the mass sinkings and up~wellings (equally important if less obvious 

 elements in the closed system), likewise to the violent churnings 

 taking place along certain sectors of coastline, as for example, at 

 the mouth of the Bay of Fundy, and over so/ne of the most productive 

 fishing grounds; it also refers to the turbulent effects of tides 

 and vifaves in general. The third refers to wave notion, including 

 the so-called "tidal waves" v/hich are really set up by volcanic 

 action, or by earthquakes beneath the sea. 



Every student of the sea has fully realized the importance of 

 the horizontal ocean currents in the scheme of things; so has every 

 intelligent seaman. The problems of this phase of oceanic circula- 

 tion noT/ unfold along three lines; (1) What is the normal current- 

 system of the ocean in all its parts, in all its depths, and at all 

 seasons of the year? (2) ?/hat is the magnitude of the variations 

 from this nornal state, and how often do they hanpen? (3) '."hat are 

 the motive forces for th.e continuing system of currents in the sea, 

 and for the deviations therefrom? 



No ono who stops to consider the vast areas covered by the 

 oceans, the great expense of special expeditions, and the difficulty 

 of making direct measurements of the current anywhere except close 

 to land and in vsr:,'- shoal water, will be surprised that even the 

 mere pictorial representa.tion of the circulation of the surface of 

 the ocean is still far from complete. 



The first important application of currents to be appreciated 

 was the navigational. From the days of the Phoenecians, or earlier, 

 ship captains have realized that knowledge of the currents was 

 necessary for the safety and expedition of their voyages. By the 

 middle of the last century the gradual collection and digestion of 



