DIAL ANo DIALING. _1 ,un-dial is an in strument for measuring time by means of the motion of the sun'- shadow. it is an instrument very great antiquity, the earliest mention of it being in Isa. xxxtiii, ti : and before and watches became common it was in general use as a timekeeper. The art of constructing dials to suit any place and situation tors I hell an im portant branch of mathematical studv: now the subject is more air object of curiosity than utility. • A dial of t WO parts—the :ail/, or gno mon, usually the edge of a plate of metal. made parallel to the earth'- axis. and pointing toward the pole: and the dial plane, which may he of any hard substance. and on which are marked the direetion. of the shadow for the several hours of the day. their halves, quarters, etc. Dials re voice various names. according. mostly, to the positions which they are constructed to oeenpy. When the dial plane is on the plane of the horizon. the dial is (-ailed a horizontal dial: when perpendicular to that plane, a vertical dial. An equinoetial dial is one whose plane is parallel to the equator. 110,ides these names, there are others. such as the south dial, north dial, east dull. west dial, polar dial, declining dial, of which it is useless to write at length. These names all depend on the position of the dial plane. The eylindrieal dial is a dial drawn on the curved surface of a cylinder. The ring dial is an ingenious snmll portable dial, but rather a curious toy than a scientifie instrument.
A night or nocturnal dial is nn instrument for showing the hour of the night by the shadow of the moon Or planets. Moon-dials con structed relative to the moon's motion: or the hour may be found by the moon's shadow on a sun-dial by emnputaf ioutl)t.tt.iNc. The stile of a dial being parallel to the earth's axis. those familiar with spherieal trigonometry will readily -ye that the problem of constructing a dial resolves itself into that of nseertainin" where the hour-line- cut a given circle, with a view to the graduation of the dial plane. Rut even without a knowledge of trigo nometry the principle may he readily tinder-tood front the following illustrations. Suppose a hol low and transparent sphere. as of glass. to repre sent the earth: and suppose its equator divided into twenty-four equal parts by the meridians. one of them passing through a given plaee—say London. (See lloRE/ON.) If the IIMIr of twelve be marked at the equator. both on the latter meridian and that opposite it. and all the rest of the hours in order on the other meridians, those meridians will be the hour-ei•eles of Lon don, because, a• the sun appears to move round the earth in twenty-four hours, he will pass from one meridian to another in one hour. Them if the sphere ha- ant opaque axis. terminating in the poles. the of this nxis would fall, in OW course of the day. on every merid hut and hour. as the sun Came to the plane of the opposite meridian. and would thus show the
time at London and at all other places on the same meridians as London. If the sphere were cut through the middle by a plane in the horizon of London, and if straight lines were drawn from the centre of the plane to the points where its circumference is cut by the hour-circles of the sphere. those lines would be the hour-lines of a horizontal dial for T.ondon; for the shadow of the axis would fall upon each particular hour line of the dial when it fell upon the like hour circle of the sphere. Similarly, if we sup pose the sphere cut by any other plane facing the meridian. the hour-cireles of the sphere will cut the edge of the plane in those points to which the honr-lines must be drawn straight from the centre, and the axis of the sphere will pass a shadow on these lines at the respective hours. The like will hold in general of any plane, wheth er it face the meridian or not. The positions on the dial plane of the several hour-lines can be calculated if we know the latitude of the place where the dial is to lie used and the position of its plane relatively to the horizon and meridian.
The vizi rersal 'dialing cylinder, an invention of Ferguson's, is a glass cylindrical tube closed at Loth ends with brass plates, on the centres of which a wire axis is fixed. The tube is either fixed to a horizontal board at an angle equal to the latitude of the place, or moves on a joint, so that it may be elevated till its axis is parallel to the earth's at any latitude. The twenty-four hour-lines are drawn on the outside of the glass, equidistant from one another and parallel to the axis. The XII on the upper side of the cyl inder stands for midnight; the XII next the board for noon. When the axis is adjusted for the latitude and the board leveled, with both XII noon and midnight in the plane of the meridian, and the end toward the north, the axis, when the sun shines, will serve as stile, and cast a shadow on the hour of the day among the parallel hour-lines. As the plate at the upper extremity of the cylinder is perpendicular to its axis and parallel to the equator. right lines drawn from the centre to the extremities of the parallels will be the honr-lines of an equinoctial aml the axis will be the A horizontal plate, if put into the tube, with lines drawn from the centre to the several parallels cutting its edge, will be a horizontal dial for the given latitude; and similarly, a vertical plate front ing the meridian and touching the tube with its edge, with lines drawn from its centre to the parallels, will be a vertical south dial. the axis of the instrument in both cases serving for the stile, and similarly for any other plate placed in the cylinder. If, instead of lying of glass, the cylinder were of wood, any of these dials might be obtained from it by simply cutting it in the planes of the plates and drawing the lines on the surface of the section.