So straight do they move, that their path has been likened to the level of the ocean, on which a ship may sail for thousands of miles, always keeping the same level and even course. On some such ocean as this in space all the planetary systems and solar systems seem to move, ever moving on and on with the same uniformity of level through infinite space. Further, this plane of the ecliptic is to the celestial sphere what the sea-level is to the earth. The height of a mountain on the earth is stated to be so much above the sea-level. In a similar way astronomers say that a star is a certain height above the plane of the ecliptic. What then is the physical explanation of this scientific term? We will lead up to it by first considering the effect that rotation has upon a liquid body. It has been demonstrated that if a mass of oil is placed in a transparent liquid of the same density, so long as the oil is perfectly at rest, its shape will be that of a sphere which will float about in the liquid, but as soon as the oil is made to rotate by means of a piece of wire, then the spherical shape is changed into that of an oblate spheroid. Further, the faster it is made to rotate, the more it will bulge out, so that its equatorial diameter will greatly exceed its polar diameter. The same principle may be illustrated by making a hoop to revolve rapidly on its axis, when a similar effect of bulging out will be produced. Now let us apply this principle to the earth with its electro-magnetic Aether currents circulating round it, and ask what is the effect of the rotation first upon the earth, and then upon the rotating Aether currents? It is a matter of common knowledge that the effect of rotation upon the earth when it was in a fluid state was to make its equatorial parts bulge out as it rotated, with the result that as it solidified the equatorial diameter exceeded the polar diameter by 26 miles. If, therefore, the result of rotation upon the earth when in its fluid state was to make it spread out greater in the equatorial regions than in any other part of its surface, what must be the effect of a similar rotation upon the rotatory Aether currents? It can easily be seen that the rotation of these currents will be to make them spread out into space in a region which corresponds to the equatorial regions of the earth, so that the rotating Aether currents will be congregated more in the equatorial regions of the earth than in any other part of the earth's surface. The further also they extend into space the less depth they will have, gradually tapering off, as shown in the illustration, where _E_ represents the earth and _B_ _C_ the Aether currents (Fig. 29). Any body, therefore, situated within the sphere of their influence would be carried round the earth by the currents, and the currents would be to them their governing and controlling level.