Page:Popular Astronomy - Airy - 1881.djvu/67

Rh making the computation, we must (in this instance) begin from the north end, because it happens that the base EF is there.

The next thing is to get the direction of one of these lines. This is got by a transit instrument, or something equivalent to it, adjusted in the same manner which I described yesterday, by the Polar Star. The transit instrument suppose at K, Figure 17, is adjusted to the north, so that the telescope passes through the Pole: the telescope is turned down to the horizon, and a mark L is fixed by means of it in the true north direction of the horizon. Then the theodolite is used to measure the angle LKM between this mark and some other signal: and knowing the northern direction in that way, we are enabled to lay down the whole of the triangulation on paper, and to see how many yards the point D is north of the point A. That is the first result of the meridional triangulation through a country; it is a point which it is most important for us to understand by way of beginning.

Now let us see how this is to be used. What do we want to ascertain? We want to ascertain something about the size and form of the earth. I remember a man in my youth who used to say he should like to go to the edge of the earth and look over. I don't think that many people, who have ever considered carefully the state of things around them are impressed with notions of that kind; but my friend was, in his inquiries, an ingenious man, a sort of philosopher in his way. Still, if he had looked about him, he would have seen that the earth did not present such a condition as would enable him to go to the edge of it and look over. I dare say that there are many persons here who have come by