SECTION XX. 307 At the same time while one person takes the altitude of the sun in order to find the azimuth from noon by it, let another hold up a thread and plummet in the sun-beams and mark any two distant points in the shadow as A s, figure xxvi. and then draw the line A B : Suppose the azimuth at that moment be found to be 35 degrees, draw the line A E at the angle of 35 degrees from A B and that will be a true meridian line. You must observe to set off the angle on the proper side of the line of shadow eastward or westward, according as you make your observation in the morning or in theafternoon. Note, Where you use a thread and plummet, remember that the larger and heavier your plummet is, the steadier will your shadow be, and you will draw it withgreater ease and ex- actness. In this and the following operations to draw a meridian line, you must be sure that your plane be truly level and horizontal, or else your performances will not be true. Problem XXIII. " Todraw a meridian line on a horizon- tal plane by aperpendicular style." Note, That when I speak of a perpendicular style, I mean either of those three sorts of styles before-mentioned in prob- lem I. viz. A straight needle stuck into theboard perpendicu- larly, as figure xv. A straight or crooked wire stuck in slop- ing at randomwith theperpendicular point found under the tip of it, as figure xvt ; or the brass prism, as figurexvu. For what I call a perpendicular style may be applied and ascribed to either of these. Make several parallel circles or arches, as figure xxvtt. In the centre of them fix your perpendicular style N c. Mark in the morning what point in any circle the end of the shadow touches, as A. In the afternoon mark where the end of the sha- dow touches the same circle, as o. Divide the arch A o just in halves by a line drawn from the centre, and that line c al will be a true meridian line. The reason of this practice is derived hence, viz. that the sun's altitude in the afternoon is equal to the sun's altitude in the morning, when it casts a shadow of the same length : And at those two moments it is equally distant from the point of noon or the south, which is its highest altitude; therefore a -line drawn ex- actly in the middle between these two points of shadow, must be a meridian line or point to the north and south. This problem may be performed by fixing your perpendicu- lar style first, and observing the shadow A before you make the circles, (especially if you use the brass prism, or the sloping style with the perpendicular point under it) then set one foot of your compasses in the perpendicular point c, extend the other to A, and so make the circle. v2
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