Art. XXVII.—To what Extent is Earth-rotation the Cause of the Ocean Currents?
[Read before the Wanganui Philosophical Society, 17th June, 1918; received by Editor, 20th July, 1918; issued separately, 16th July, 1919.]
Those who have given the currents of the ocean their serious consideration must certainly have been struck by the fact that the main currents are circling in a contrary direction to that in which the earth is rotating—that is to say, they are moving in a clockwise direction in the Northern Hemisphere and anti-clockwise in the Southern. This fact seems to me to point to a cause governing the movements of the water in both hemispheres, the cause being earth-rotation.
Given a rotating sphere with its surface interspersed with great land and water areas, in what way will the water behave when acted upon by the attractive power of external bodies in or near the same plane of rotation? It appears to me that currents will undoubtedly be set running in a contrary direction to that in which the sphere is rotating, more especially if the land areas extend in a north-and-south direction to fairly high latitudes, and for this reason: That there exists an affinity, or
clinging tendency, between external bodies and the water is proved by the sun and moon lifting and drawing the tides westward; and, as the tides are always lagging behind these bodies, they must have a strong tendency to draw the water in a circle towards the Equator on all west coasts, westward near the Equator, and from the Equator on all east coasts, thus causing a stream to flow continually in that direction; but in reality the sun and moon (and, I suppose, the planets to a small extent) tend to hold the water, and the earth in turning eastward makes the water appear to lag behind; or, in other words, the water is retarded by these bodies while the earth rotates eastward.
This does not necessarily imply that the water would flow at the same rate as the tide, but that the water will have a tendency to be drawn in that direction as if it were slightly downhill.
This retarding action is continually in operation, and in the same direction. We have the lunar and anti-lunar, solar and anti-solar tides, all tending to induce the water by earth-rotation to move in a circle between the land-masses and the Equator, for the Equator is a boundary when viewed from the centre of rotation, and naturally the water will tend to return eastward in higher latitudes to replace that which is being drawn northward. These latitudes offer less resistance owing to their being nearer to the neutral line between the opposing tides, and also to the fact that the gravitational pull of the sun and moon, acting as it does obliquely to the surface of the water in these latitudes, has a much weaker effect, area for area, than it has on or near the Equator, where the pull, being mostly perpendicular to the surface, must consequently have a greater retarding action on the water while the earth moves on, thus giving the current a westerly trend relative to the land; or, to put it more briefly, the difference in the gravitational pull on these two positions causes the water to circle clockwise in the Northern Hemisphere and anticlockwise in the Southern.
These conclusions were arrived at many years ago, but how to embody the conditions in a working model perplexed me for a long time, for I could not divest myself of the idea that it must be done on a globe; but at last I began to see a way out of the difficulty.
If one takes an imaginary bird's-eye view of a hemisphere—say, for instance, the Southern—from a great height above the South Pole, that hemisphere will appear as a rotating disc: in fact, water will behave in the same way on a rotating disc or tray, providing gravity is acting perpendicularly to its surface, and this is easily accomplished by having the tray to rotate on a vertical axis, and if necessary we can give it a uniform speed by using a controller.
So far, then, things are quite simple. All we have to do to represent the Southern Hemisphere is to provide a shallow circular tray mounted on a vertical shaft, with the different land-masses—South Africa, South America, Australia, Antarctica, and a few of the larger islands—modelled in wood, placed in the tray and fastened in their relative positions and almost submerged in water, and then to rotate the tray at a uniform speed. But by doing this we are confronted by another difficulty, for if we give the tray a uniform speed how are we to give the water the very necessary retardation without in any way interfering with its free movement?
To overcome this difficulty, instead of giving the tray a uniform movement for a long period, I gave it an accelerating movement for a short period by means of a falling weight just sufficient to put it in motion. I wish the point to be thoroughly understood, that accelerating the land
is equivalent to retarding the water. By this means I obtained the necessary retardation of the water in relation to the land without giving it a bias in any direction with the exception of that impressed upon it by the rotation of the land around it. The currents can be observed from the moment the tray begins to move until the cord is unwound, which is about two minutes and a quarter, by which time the tray is beginning to move too fast to observe them, and the water is by centrifugal action beginning to bank up against the wall representing the Equator; but there is ample time to observe that a system of currents is set up generally similar to the existing currents of the Southern Hemisphere.
There is one exception, which I think can be satisfactorily accounted for—the one marked on the maps as the “Cape Horn current,” flowing eastward, due, it seems to me, to the fact of the South Atlantic equatorial current impinging on that portion of the coast north of Cape San Roque. This coast-line, running obliquely across the Equator, diverts a great portion of this current towards the Gulf of Mexico, thereby strengthening the Gulf Stream at the expense of the southerly current along the east side of South America, and so allowing part of the current in the South Pacific Ocean to overpower it and flow eastward by Cape Horn. The water in the model, however, being necessarily confined by a wall representing the Equator, does not allow any of the current generated in the South Atlantic to escape northward, and so, flowing with its full strength down the east coast of South America, overpowers the one flowing eastward by Cape Horn, thereby causing this current to flow westward.
The wall representing the Equator, and the shallowness of the water, is a hindrance to the model, for the undue friction gives the tray a tendency to drag the water with it. It nevertheless shows the currents quite strongly. The diminutiveness of the model prevents it showing the smaller currents, which no doubt are mostly in-draughts caused by the larger.
On finding the Southern Hemisphere so satisfactory I modelled the principal land-masses of the Northern Hemisphere in the same way, and on giving the tray an anti-clockwise motion the results were equally satisfactory, with the exception of the Gulf Stream, which shows slightly weaker through not receiving so much of the South Atlantic equatorial current as it does in nature.
Specification of the Working Model. (Plate XXII.)
The model consists of a galvanized-iron tray 5 ft. in diameter and 5 m. deep. The perpendicular wall representing the Equator has a wire edge to stiffen it. In the centre of the tray a hole 2 in. in diameter is cut and a tube 5 in. long is soldered in; this is done so as to allow of a 2 in. shaft passing through without danger of leakage.
The tray is supported by a wooden table of the same diameter firmly bolted to the shaft at about 40 in. from the floor, so as to be at a comfortable height for observation. The shaft is 4 ft. long, and runs on ball bearings top and bottom. About half-way from the table to the floor a stud is screwed in the shaft, to which is fastened the cord; the cord is then carried over a pulley, and a weight of about 3½ lb. attached thereto, the weight having a fall of about 7 ft.
The land-masses are built up of three thicknesses of 7/8 in. board, the bottom layer being splayed off, so as to be more in keeping with the shores. These are fastened to a false bottom in their relative positions with regard to latitude and longitude, placed in the tray, and almost submerged by
water, on which is sprinkled some fine sawdust in order to show up the currents more distinctly.
The cord is wound around the shaft so as to give a clockwise motion to the tray, this being the direction of the earth's rotation when viewed from the South Pole. This is, of course, for a model of the Southern Hemisphere, for the Northern the cord being wound the opposite way so as to give an anti-clockwise rotation. The weight is then attached to the cord, whereupon the tray slowly begins to move, and, while it is slowly accelerating, all the phenomena of the main oceanic currents in the Southern Hemisphere can be observed in miniature.