Page:Popular Astronomy - Airy - 1881.djvu/136

122. done at two or three. In the English language we have no term for expressing that peculiar act of determining the direction of a side of a triangle, or the direction of a chain of triangles, and therefore we have adopted a word from the French, "orientation"; it is, however, a bad word, used only for the want of a better word in the English language. Where the word "orientation" is used, it is understood to mean the ascertaining the general direction of a chain of triangles.

In regard to the use of the Zenith Sector, of which I have spoken so frequently, I should wish you to charge your memory with this one notion: when that instrument is used for determining the measure of a degree of the earth, by being transported to two different stations, as A and B, Figure 18, and by being employed for observing the same chosen star at both places, the direction of the telescope is really the same at the two places, but the direction of the plumb-line is different at the two places. But, if we consider it only as a matter of observation at each of the places, then we fancy that the direction of the plumb-line is apparently the same in the two places, and that the direction of the telescope is apparently different. Thus the direction of the telescope, when pointed to the same star, is apparently different at Shanklin from what it is at Balta; but, in point of fact, the direction of the telescope is the same at both, and the direction of the plumb-line, or the direction in which a stone would fall, is different at the two places.

In yesterday's lecture I entered upon the subject of the motion of the planets; and I endeavoured to give you a notion of the complexity of the phenomena of the planets, and of the system first used to explain