This dial must be placed exactly hori zontal, and its XII. o'clock point, i e. the meridional line, must be precisely north ward. The most certain mode of laying the meridian truly north is by observation of the polar star ; or by taking an obser vation at XII. at noon,' by means of a quadrant, when the exact moment of the sun's utmost altitude for the day being ascertained, the shadow thrown upon a plane, by means of a thin cord sustain ing a heavy plumb, will give a correct northern line, either on the dial plate, already so far prepared on its base, or on any other level surface, from which a true parallel may be taken. With respect to adjusting by aid of a compass, it can not be recommended : those instruments are often faulty, and when they are not so, the variation is not always exactly laid down.
To construct a horizontal dial by means of a terrestrial globe, elevate the pole to the latitude of the place, and bringing the nearest meridional line to the brazen meridian, set the index to XII. upper most. Now turn the globe until the in dex comes exactly to I. which will be effected by a movement of 15° measured on the equator; each hour making a change of 15° on the sun's place thereon; (for 24 hours multiplied by 15 make a total of 360°, equal to a whole circle.) The place where the meridional line moves to should be marked, for that will give the place of I. o'clock; move on 15° more, to find where the meridional line cuts the equator, for the place of H. o'clock; and thus in succession will all the hours' places be indicated by the meridian at each change of 15° on the horizon ; always measuring close to the meridian, or following the hour band on the index. If the latter be marked with halves and quarters of hours, the corres ponding divisions may be made on the equator, as the meridian by which you are governed passes on, taking 7i° for a half hour, and 3i° for a quarter ; but such is seldom required, except in very large dials; the eye usually judging, with tolerable precision, of the quantity cover ed by the shade of the gnomon.
To set this off on paper, measure the number of degrees between the several parts of the meridian, noted down as stations of the governing meridional line, when passed on 15° at each movement. This being done from XII. to VI. will establish a measurement that may be imitated on any scale, observing to draw the VI. o'clock line correctly, and to give a proper length of base, as well as a due latitudinal angle to the gnomon.
The method suggested by Mr. Ferguson, of filling up the interior of the horizon after drawiug forth the globe, is prolix, and suits but few persons: whereas, the knowledge of how many degrees are in cluded in each interval, respectively, prover a sufficient guide. Those inter vals will be found to correspond with what have been directed in describing the second figure.
To make an erect dial directly south.
Fig. 3. On this the sun can shine only from VI. A. M. to VI. P M.; therefore, only the intermediate hours need be noted. Elevate the pole as before di rected, but, in lieu of placing XII. to the north, place it to the south of your dial plate, which in this kind is most conveni ently made an oblong, projected verti cally below the VI. o'clock line. Pro ceed to find the places of the boors as before shewn, trace them through their centres to the opposite parts of the hour circle, for on that they are to stand. The gnomon is to make an angle equal onlr to the co-latitude of the place: thus, of you are in 51i°, the co-latitude (being the amount required to complete to would be 38°.
Or you may proceed as follows : on the meridional line a b, as it points down wards from the foot, or lowest part • the gnomon, measure off at pleasure any distance, as a c, for the size of your dial ; at c erect the perpendicular C D, and make the angle C A D eqtA to the eleva tion of the equator; then make a second' triangle Cl) E, the angle at D being equal to that at A. Through E draw G H right angles with A E. Carry on E B equal to E D, and with that distance as a radius, describe the quadrant E F from B as a centre. Measure off the proper angles from the point B, through the se veral parts of the quadrant, which is di vided off into six equal parts ; these wil fall upon the prolonged line G H, and give points thereon, through which line being drawn from the centre A of the VI o'clock line to the hone frame, the places of the several hours will be given. The gnomon is fixed at A, equal to A D E being conformable to the co-latitude; o it may be simply a wire fixed at C, equa in length to C D, but perpendicular t,, the face of the dial ; some use large gular iron rods. This kind of dial is ofte seen on the sides of country churc steeples facing the south.