Page:Popular Astronomy - Airy - 1881.djvu/72

58 If you take numbers, you will see how we assume this to be effected. Suppose the measure of AB is 830 miles. Suppose I find that the directions of the two vertical lines AH and BH, in the two places A and B, make an angle of 12 degrees. You will remember what a degree means. It is not a measure of length; it is a measure of inclination of these two lines. I have to pass over a distance of 830 miles, in order to get from one place to another, where the direction of the vertical changes 12 degrees. From that I infer that the curvature of the earth is such, that I have to pass over 69 miles to find the distance of two places whose verticals are inclined one degree. Having got that, it is easy to find what is the semi-diameter of the circle which you must sweep, in order that that distance of 69 miles may give one degree of inclination of the two lines, drawn from the centre to the ends of the 69-mile arc. Making the calculation, you find the semi-diameter is about 4000 miles. And this is the way in which the measure of the earth was ascertained in the first instance. The first accurate measure was made in Holland, by a man named Snell; the next by a celebrated man, Picard, in France.

Shortly after this, Sir Isaac Newton's theory of gravitation was broached. He predicted, as a result of theory, that the earth would be ascertained not to be round, not spherical, but spheroidal, or flattened, turnip-shaped. It was a matter of importance to verify this. The first expedition for this purpose was made by the French Government, under the Kings of France; and all honour be to the French for the part they took in this matter! Many of you are aware that Guizot, the late Prime Minister of France, before he was appointed Minister of the