SECTION XX. 311 your meridian line on the first plane, instead of setting off the same angle, andobserve also to set it offon the contrary side, that so it might make a right angle with a meridian line if that could have come on the plane. Problem XXX. " How to use a meridian line." The various uses of a meridian line are these. Ist Use. A meridian line is necessary in order to draw an horizontal dial on the same plane, or to fix -an horizontal dial true if be made before. Iid Use. A brass horizontal dial may be removed from one place to another in several rooms of the same house ; and show .the hour wheresoever the sun comes, if either a meridian line or line of east and west be drawn in every window, by which to set an horizontal dial true. IIId Use. By a thread and plummet, or any perpendicu- lar pin, or post casting a shadow precisely along the meridian line, we find the hour of 12, or the point of noon, and may set a watch or clock exactly true any day in the year, if we have no dial at hand. IVth Use. It is necessary also to have some meridian line in orderto find how a.houseor wall stands withregard to the four quarters of the heavens, east, west, north, or south, which iscalled the bearingof a house or wall, that we may determine what sort of upright dials may be fixed there, or what sort of fruit-trees may be planted, or whichpart of a house or garden is most ex- posed to the sun, or to the sharp winds. Vtli Use. By observing the motion of the clouds, or the smoke, or a vane or weather-cock, you cannot determine which way the wind blows, but by comparing it with a meridian line, or with a line of east and west. When once you have got a true meridian line, and know which is the south, then the opposite point must be north ; and when your face is to the north, the east is at your right-hand, and the west at your left. Vith Use. A meridian line will shew the azimuth of the sun at any time by holding up a thread and plummet in the sun, and observing where the line of shadow crosses it. Or the sharp smooth edge of at upright style or post will cast a shadow across a meridian line, and show the sun's azimuth. VIIth Use. If you have a meridian line on a horizontal plane, you may draw a circle on that as a diameter, and di- vide it into360 degrees ; then set up a fixed or moveable per- pendicular style, and it will shew the azimuth of the sunat all hours. Villth Use. A perpendicular style on a meridian line will shew the sun's meridian altitude by the tip of the shadow accord-
RkJQdWJsaXNoZXIy OTcyMjk=