Of a similar character is the following observation:--If a theodolite or spirit-level be placed upon the sea-shore, and “levelled,” and directed towards the sea, the line of the horizon will be observed to be a given amount below the cross-hair of the instrument, to which a certain dip, or inclination from the level will have to be given to bring the cross-hair and the sea horizon together. It is concluded that as the sea horizon is always observed to be below the cross-hair of the “levelled” theodolite, the line of sight is a tangent, the surface of the water convex, and therefore the Earth is a globe. [Illustration: FIG. 21.] The conclusion derived from the last three observations is exceedingly plausible, and would completely satisfy the minds of scientific men as to the Earth’s sphericity if a perfect explanation could not be given. The whole matter has been specially and carefully examined; and one very simple experiment will show that the effects observed do not arise from rotundity in the Earth’s surface, but from a certain peculiarity in the instruments employed. Take a convex lens or a magnifying glass and hold it over a straight line drawn across a sheet of paper. If the glass be so held that a part of the straight line can be seen _through_ it, and another part seen _outside_ it, a difference in the _direction_ of the line will be observed, as shown in the diagram Figure 21. Let A B C represent a straight line. If a lens is now held an inch, or more, according to its focal length, over the part of the line A B, and the slightest amount out of its centre, that part of the line A B which passes under the lens will be seen in the direction of the figures 1.2; but if the lens be now moved a little out of its central position in the opposite direction, the line B C will be observed at 3.4, or below B C. A lens is a magnifying glass because it _dilates_ or spreads out from its centre the objects observed through it Therefore whatever is magnified by it is seen a little out of its axis or centre. This is again necessitated by the fact that the axis or actual centre is always occupied by the cross-hair. Thus the line-of-sight in the theodolite or spirit-level not being axial or absolutely central, reads upon a graduated staff a position which is necessarily slightly divergent from the axis of vision; and this is the source of that “spherical excess” which has so long been considered by surveyors as an important proof of the Earth’s rotundity. In this instance, as, indeed, in all the others given as evidence that the Earth is a globe, the premises do not fully warrant the conclusion--which is premature,--drawn before the whole subject is fairly examined; and when other causes are amply sufficient to explain the effects observed. SECTION 2. THE EARTH NO AXIAL OR ORBITAL MOTION.