Читать книгу Star-land: Being Talks With Young People About the Wonders of the Heavens онлайн

21 страница из 55

Fig.4.—Two Eyes are better than One.

Fig.5.—How we measured the Height of the Ball.

I shall illustrate our method of measuring the actual distance of a body in the heavens by showing you how we can find the height of that large india-rubber ball which is hanging from the ceiling. Of course, I do not intend to have a measuring tape from the ball itself, because I want to solve the problem on the same principle as that by which we measure the distance of the sun or of any other celestial body which we cannot reach. I will ask the aid of a boy and a girl, who will please stand one at each end of the lecture table. The apparatus we shall want is very simple; it consists of two cards and a pair of scissors. The boy will kindly shape his card to such an angle that when he holds it to his eye one side of the angle shall point straight at the little girl, and the other side shall point straight at the ball, just as you see in the picture (Fig.5). The girl will also please do the same with her card, so that along one side she just sees the little boy’s face, while the other side points up to the ball. It will be necessary to cut these angles properly. If the angle be too big, then when one side points to the boy’s face, the other will be directed above the ball. If the angle on the card be too small, then one side will be directed below the ball, while the other is pointed to the boy. The whole accuracy of our little observations depends upon cutting the card angles properly. When they have been truly shaped it will be easy to find the distance of the ball. We first take a foot rule and measure the length of our table from one of our young friends to the other. That length is twelve feet, and to discover the distance of the ball we must make a drawing. We get a sheet of paper, and first rule a line twelve inches long. That will represent the length of the table, it being understood that each inch of the drawing is to correspond to a foot of the actual table. Let the end where the girl stood be marked B, and that of the boy, A, and now bring the cards and place them on the line just as shown in the figure. The card the girl has shaped is to be put so that the corner of it lies at B, and one edge along B A. Then the boy’s card is to be so put that its corner is at A and one edge along A B. Next with a pencil we rule lines on the other edges of the cards, taking care that they are kept all the time in their proper positions. These two lines carried on will meet at C; and this must be the position of the ball on the scale of our little sketch. It only now remains to take the foot rule and measure on the drawing the length from A to C. I find it to be twenty inches, and I have so arranged it that the distance from B to C is the same.