308 GEOGRAPHY AND ASTRONOMY. If you use the prism for a style, you may mark a line or an- gle at the foot of it where you first fix it, and place it right again, though you move it never so often. It is very convenient to mark three or four points of shadow in the morning, and accordingly draw three or four arches or circles, lest the sun should not happen to shine, or you should not happen to attend just at that moment in the afternoon when the shadow touches that circle on which you marked your first point of shadow in themorning. If you would be very exact in this operation, you should tarry till the sun be gone one minute further westward in the afternoon, i. e. till one minute after the shadow touches the same circle, and then mark the shadow; because the sun in six hours time (which is one quarter of a day) is gone eastward on the ecliptic in his annual course one minute of t'íme, which is 15 minutes (or one quarter) of a degree. Problem XXIV. "To draw ameridian line on a horizon- tal plane by astyle or needle set up at random." Another method near a-kin to the former is this. Set up a needle or sharp-pointed style at random, as N n, in figure xxviti. Fix it very fast in the board, and observe a point of shadow in the morning as A. 'l'heu with a pin stuck on the tip of the style N. (without moving the style) drawthe arch A s o: Mark the point of shadow o, in the afternoon when it touches that arch (or rather when it is one minute past it.) Then draw the line A o and dissect it, or cut it in halves by a perpendicular line M E, which is a true meridian. Note, Ln this method you have no trouble of fixing a. style perpendicular, nor finding the point directly under it for a. centre. But in this method as well as in the former it is good to mark three or four points of shadow in the morning, and draw arches or circles at them all, for the same reason as before. Observe here, That in these methods of drawing a meridian line by the shadow of a tip of a.style,. I= think it is best generally to make your observations between: eight.and ten o'clock in the morning, and between two and four in the afternoon. Indeed' in the three summer months May, June, and July, you may perhaps make pretty good observations an hour earlier in the morning, and later in the afternoon ; but at no time of the year should you do it within an hour of noon, nor when the sun is near the horizon; for near noon the altitude of the sun or the length of shadow varies exceeding little,; and when the sun is near the horizon, the point and,bounds of the shadow are not full and strong and distinct, nor can it bemarked exactly. . Therefore if.in the three Winter months, Nodember. Decem- ber, or January, you make your observation, you should then,
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