Page:Popular Astronomy - Airy - 1881.djvu/61

Rh eight miles apart ; a third, G, at a considerable distance, perhaps inaccessible, at least in a straight line from E and F. I can measure the distance between E and F, because there is even ground between them. But how do we get the distance of G? In the first place we actually measure the distance between the two nearest, E and F. In the prosecution of surveys of this kind, it is a great object that we should choose ground favourable for taking the measure; it is necessary that the ground should be very level, and, if possible, firm. The line so measured is called the Base Line. Bases have been measured in the British survey on Hounslow Heath, Romney Marsh, Misterton Carr, Salisbury Plain, and other places; but the principal base measured in the United Kingdom for several years past, and on which the measure of almost every part of the kingdom depends, is one in Ireland, traced along the east side of Loch Foyle, near Londonderry. It was measured on the sand; and the smoothness and level of this soil served well for the purpose.

Now this base is measured by a very troublesome operation indeed. You may think it easy to measure a straight line, but, in fact, there is nothing so difficult. In the first place what are you to measure it by? Are you to use bars of metal? They expand by heat. It is to be measured by the yard. If so, what do you mean by a yard? By the measure of a yard we mean a certain distance, not something imaginary or variable, but a distance definite and certain. But we do not mean the length of any piece of metal, because it changes its state by the action of temperature: it becomes longer when hot, and shorter when cold. If I use a piece of metal, I say a yard means the length of this bar of iron or