Page:Popular Astronomy - Airy - 1881.djvu/60

46 the stars from the earth by a yard measure. When I say that we really measure these distances by a yard measure, I do not wish to weary you by the word, and I do not wish to introduce anything inelegant; but I do wish to produce distinct and definite ideas in your minds to urge this, that we really do make use of a yard measure, or something equivalent to it, as our fundamental measure for these purposes. I will now proceed to explain the first step towards taking these measures.

The object we have in view is to measure a great distance upon the earth; a distance, for instance, extending the length of a kingdom. Figure 14 (see Frontispiece) represents nearly the whole of the British Islands. I wish to point out how the distance is measured from the Isle of Wight at A, to the Shetland Isles at B. In the first place I must tell you, that the distance has been measured with such accuracy that I think it likely that the distance is known with no greater error than perhaps the length of this room. Now, measures of this kind are effected by a system of triangulation. This is in some degree or other well known to every surveyor, but still I esteem it so important to the whole subject before me, that I shall point out to you the way in which it is done. Suppose then, that we have three places, EFG, Figure 15; the two nearest, E and F, on a plain



with even ground between them, and perhaps six or