113. =Surface of Liquids not Truly Level.=--Strictly speaking, the surface of a liquid is not level, but rounding. But it is so little so that it can not be perceived unless we take into view a very large surface, as the ocean. Here it is very manifest, for whenever a ship comes into port the first thing seen from the shore is the topmost sail, the rest of the ship being concealed by the water rounded up between it and the observer. This is illustrated in Fig. 66. At _a_ the ship is just in sight, while at _b_ it is so near shore that the eye sees the whole of it. If the earth had no elevations of land, or if there was water enough to cover them, the water would make a perfectly globular covering for the earth, being held to it by the force of attraction. The reason for this is precisely the same as was given in § 58 for the disposition of a drop of liquid to take the globular form. As in that case, so in this, it can be demonstrated that each particle is attracted toward a common centre, and that this will produce in the freely-moving particles a uniformly rounded surface. What could thus be shown to be true if the earth were wholly covered with water, is true of the portions of water which now fill up the depressions in the earth's crust; and it can be perceived, as shown in the first part of this paragraph, in the case of any extended portion of it. [Illustration: Fig 67.] 114. =Spirit-Level.=--What we call a perfectly level surface is, then, one all parts of which are equally distant from the centre of the earth, and is therefore really a spherical surface. But the sphere is so large that any very small portion of it may be considered for all practical purposes a perfect plane. A hoop surrounding the earth would bend eight inches in every mile. In cutting a canal, therefore, there is a variation in this proportion from a straight level line. As the variation is but an inch in an eighth of a mile, it is of no account in taking the level for buildings. Levels are ascertained by what is called a spirit-level. This consists of a closed glass tube, Fig 67, nearly filled with alcohol. The space not occupied by alcohol is occupied by air. The tube is placed in a wooden box for convenience and security, there being an opening in the box at _a_. Now when the box with its glass tube is perfectly level, the bubble of air will be seen in the middle at _a_; but if one end be higher than the other, the bubble will be at or toward that end.