Page:Popular Astronomy - Airy - 1881.djvu/42

28 and so many miles make a degree. But the publication in a book of measures of such an expression as "69 miles make a degree" is in the highest degree reprehensible, as giving false ideas on one of the most important expressions in science. No schoolmaster ought to introduce books into his school, teaching that 69 miles make a degree. What do we mean by a degree? The use of the word degree is to define inclination, and it ought to be looked upon as defining a measure of inclination only, and not as defining a measure of length. If I had to describe the position of two arms of a pair of compasses, I should say they were inclined; but the notion of their inclination is entirely different from the notion of a measure of length. But we want some means for describing how much these two arms are inclined. Now the method of describing how much these two arms are inclined, is got at in this way: we use the word degree for a certain small inclination, such that if we first give one arm an inclination of one degree to the other, then incline it one degree further, then one degree in addition, and so on to 360 degrees, the arm will have gone through the whole circle of inclination, and will have returned back again to its first position. But these degrees, as you will perceive, have nothing to do with lineal measures; they are inclinations, and nothing else; they have nothing more to do with lineal measures than they have to do with pounds weight, or pounds sterling. We do, however, find it necessary to use the word degree in determining what might at first sight appear to be linear measures. For instance, if a star be seen at the point A, Figure 10, and if another star be seen at the point B, and if I want to measure the distance between them, I say they are so many degrees apart; but yet I do not