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. ]