=5. Dip of the Horizon.= Another correction which has to be applied in many observations depends upon the sphericity of the earth. We have described the rational horizon, and pointed out how it differs from the sensible horizon. We have also said that at sea the sensible horizon nearly accords with the rational horizon (see Part I, Sect. 3). But the accord is not complete, owing to what is called the dip of the horizon. In fact, the sea horizon lies below the rational horizon by an amount varying with the elevation of the eye above the surface. Geometry enables us to determine just what the dip of the horizon must be for any given elevation of the eye. A rough and ready rule, which may serve for many purposes, is that the square root of the elevation of the eye in feet equals the dip of the horizon in minutes of arc, or of angular measure. The reader will readily see that the dip of the horizon is a necessary consequence of the rotundity of the earth. It is because of this that, as a ship recedes at sea her hull first disappears below the horizon, and then her lower sails, and finally her top-sails. The use of a telescope does not help the matter, because a telescope only _sees straight_, and cannot bend the line of sight over the rim of the horizon. Atmospheric refraction, however, enables us to see an object which would be hidden by the horizon if there were no air. In navigation, which, as a science, is an outgrowth of astronomy, these things have to be carefully taken into account. [Illustration: _Fig. 8. Dip of the Horizon._ It is to be remembered that it is the _sensible_ horizon which dips, and not the _rational_ horizon. The sensible horizon of the observer at the elevation A dips below the horizontal plane and he sees round the curved surface as far as a; in other words his skyline is at a. The observer at the elevation B has a sensible horizon still more inclined and he sees as far as b. If the observation were made from an immense height the observer would see practically half round the earth just as we see half round the globe of the moon. ] [Illustration: Polar Streamers of the Sun, Eclipse of 1889 ] [Illustration: The Solar Corona at the Eclipse of 1871 From drawings. ]