Page:Popular Astronomy - Airy - 1881.djvu/70

56 vertical there, or the direction of the perpendicular to the horizon, (see page 15.)

Now, assuming that we are observing a star very nearly overhead—it is plain if the telescope be directed to the star, then by observing the point of this divided arc CDE, which is crossed by the plumb-line, I have got a measure in degrees, minutes, and seconds, of how far the star is from the vertical. And the peculiar advantages of using this instrument instead of the Mural Circle, are these: first, it is easier to carry about from one situation to another; and next, the observations made by it are confined to that part of the heavens where the refraction is scarcely sensible. Refraction is a thing which, (from the uncertainty attending the calculation of it,) baulks us perpetually, and which it is very desirable to get rid of as much as possible.

Now then, the way in which this instrument is used, in order to ascertain the form of the earth, is as follows: we take our Zenith Sector to Shanklin Down and to the Shetland Islands. Now, consider for a moment. What do I mean by the earth and water being curved? The direction of the vertical is perpendicular to the surface of water; and therefore, if the water be curved, it is connected essentially with the circumstance that the direction of the vertical is varied, or that the direction in which the plumb-line hangs is not the same at different places. Therefore, if the earth, Figure 18, be curved, as we suppose, and as previous rough considerations have given us reason to think, the plumb-line at A would