14 Fishery Board for Scotland. 



north and south of the Equator the surface waters would be a little 

 warmer than the stagnant waters of a shallow and currentless ocean. 

 The same phenomenon occurs at present, with certain modifications. 



At the same time, also, the whole mass of water would tend to follow 

 a curious spiral course, in consequence of the rotation of the earth. 

 For as the earth rotates, every particle at its surface is travelling 

 through space from west to east, with a velocity which is greatest 

 at the Equator, and falls to nothing at the pole ; and so long as a 

 water particle has no other motion impressed upon it, it shares, by 

 virtue of its inertia, in this rotatory motion of the earth's surface, 

 and does not change its place relatively to the solid earth below. 

 But let us suppose that our water particle begins to move from the 

 Equator northward, under the influence of the convection current 

 described in the last paragraph ; then it embodies two velocities, (1) 

 its own proper velocity northward, and (2) that velocity in an east- 

 ward direction which it had acquired at the Equator from the rotation 

 of the earth. As it proceeds northward, it comes to a part of the 

 earth's surface which is travelling eastward with a less velocity than 

 its own — a less velocity, that is to say, than the normal Equatorial 

 one ; and the obvious consequence is that the particle will have a 

 component of velocity eastwards greater than that of the solid ground 

 over which it has come to lie, and will, in short, be found travelling, 

 relatively to tlie earth's surface, eastward as well as northward. 

 The moving! particles, whetlier they be going northward or southward, 

 will always tend to swerve to the right in the northern, and to the 

 left in the southern hemisphere. There will, that is to say, be a 

 constant tendency for the whole surface waters of the ocean, as they 

 move poleward, to swirl in a north-easterly drift in the northern 

 hemisphere, and in a south-easterly drift in the southern. And 

 the cold currents from the pole, travelling in the opposite direction 

 (whether at the surface or the bottom), will, in contrary fashion, be 

 swayed towards the west as they travel towards the Equator. But 

 under the conditions which we have imagined of an uninterrupted 

 ocean, the conditions would be symmetrical all round ; and though 

 the great drift would be sweeping oh its spiral course, the temperature 

 phenomena would soon settle down into equilibrium, and no tem- 

 perature fluctuations would be apparent, save for the steady fall 

 from the equatorial to the polar regions. 



But the case is very different within the definite boundaries of an 

 actual ocean. Here the currents are on the one hand confined to 

 the bed of a channel, which in the case of the Atlantic is like that 

 of a vast and winding river ; on the other hand, they eddy here and 

 there in great pools, such as the North Sea and the much greater 

 basin of the Caribbean Sea ; and tliey stream or rush through 

 gateways, such as Davis Straits or the English Channel, or the 

 broader lanes to east and west of Iceland. 



Moreover, while it was customary at one time to attribute the 

 main systems of ocean currents almost entirely to the two phenomena 

 which we have just described, namely, to the convection currents set 

 up by equatorial heat and polar cold, and to the side- way motion 

 impressed upon them by the rotation of the earth, nowadays it is 

 clearly understood that the direct action of the winds is also a 

 factor of very great and even of more manifest importance. But this 



