Page:Popular Astronomy - Airy - 1881.djvu/69

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Now, you will understand from the description which I have given, that in the first place, by measuring a base by means of a yard measure; in the next, by measuring successive triangles originating with that base; and therefore, in fact, computing the length of every side of these triangles by means of a yard measure; you will understand we have really ascertained, by means of the yard measure, the distance from the Isle of Wight A, to the little Island of Balta B, in the Shetland Isles. Now, we want to measure the corresponding curvature of the earth, that is, to find how much the line drawn from A to B on the surface of the earth is bent. For that purpose we use an instrument called the Zenith Sector, Figure 19—a telescope swinging upon pivots AB, and having attached to it an arc CDE graduated into degrees and minutes. There is a plumb-line CF connected with the upper end of the telescope, or with one of the pivots; it is a very fine silver wire, supporting a weight F, which weight is hanging in water, to keep it steady. It gives us the direction of the