SECTION XX. 309 !do it half an hour before or after ten o'clock in the morning, and so much before and after two in the afternoon ; for otherwise the sun will be either too near noen, or too near the horizon. But in general, it may be advised that thé summer half- year is far the best for observation of shadows in order to any operations of this kind. ProblemXXV. " To draw a meridian line on an equinoc- tial day." On an equinoctial day or very near it, as the 8th, 9th, or 10th of March ; or the 11th 12th, or 13th of September, you may make a pretty true meridian line very easily thus by figure xxix Mark any two points of shadow as A a from a needle c n set up at random, (no matter whether it be either upright or straight.) Let those two shadows be at least at the distance of three or four hours from each other, and it is best they should be observed one in the morning and the other about the same distance from I2 in the afternoon ; and then draw the line A B which represents the equinoctial line, and is the path of the sun that day : Crossit any where at right angles, and m N, or o r, are meridian lines. Note, It is best to mark several shadows that day, as S, S, S, and draw a right line ASS B by those which lie nearest in a right line, that you may be the more exact. Problem XXVI. " To draw a meridian line by a point of a shadow at noon. If you have an exact dial to whose truth you can trust, or a good watchor clock set exactly true by the sun that morning, then watch the moment of 12 o'clock or noon, and hold up a thread and plummet against the sun, and mark the line of sha- dow on a horizontal plane, and that will be a true meridian line. Or you maymark the point or edge of shadow, by any thing that stands truly perpendicular, at the moment of 12 o'clock, and draw a meridian fine by it. Problem XXVII. " To draw a meridian line by a hori- zontal dial." If you have a horizontal dial which is not fastened, and if it be made very true, then find the exact hour and minute by a quadrant, or any other dial, &c. at any time of the day, morn- ing or afternoon ; set the horizontal dial in the place you design, to the true hour and minute; and the hour line of 12 will direct you to draw a meridian. Or if your dial be square, or have any side exactly parallel to the hour line of 12, you may draw your meridian line by that side or edge of tite dial. u 3
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