Page:Popular Astronomy - Airy - 1881.djvu/66

52 we have, by the process I have described, got the length of these other sides of the triangle. Then, in like manner, having thus got the length of the side FG, we can use it as a base measure to determine the distance of the signal on another distant point; and thus we go on, step by step, until we get from one end of the kingdom to the other. I have represented, in Figure 17, the triangulation connecting Shanklin Down with Clifton Beacon. This is a part of the great Meridional Arc of England. Some persons, I have no doubt, are present now who have seen a place in the Isle of Wight, called Shanklin. Northeast of the village is a high swelling Down; a point on this Down is the southern extremity of this triangulation. The names of the stations which follow are marked in the diagram. Signals were placed upon the successive stations; at each of these theodolites were placed; from each of these the signals were observed upon the other stations; and so we step on from one point to another, till we arrive at Clifton, a village in the south of Yorkshire; and (by continuing the triangulation) at Balta, in the Shetland Islands. The outlines of the counties through which the survey was made are roughly drawn on the diagram, in order to give a notion of the size of the triangles. The line, for instance, which connects Brill with Stow goes over the widest part of Oxfordshire, from a signal external to it on the eastern side, to another signal external to it on the opposite or western side. In this way we step over a country at few steps; and when the angles of all the triangles are accurately measured, the results may be laid down on paper. I have spoken of beginning from the south end of the triangulation; but in the process of