DIAL AND DIALING. A is an instrument for measuring time by means of the motion of the sun's shadow cast by a stile erected on its surface. It is an instru ment of very great antiquity, the earliest mention of it being in Isaiah xxxviii. 8; and before clocks and watches became common, it was in general use as a time-keeper. The art of constructing dials to suit any place and situation, was then an important branch of mathematical study; now the subject is more an object of curiosity than utility.
A dial consists of two parts—the stile or gnomon, usually the edge of a plate of metal, always made parallel to the earth's axis, and pointing towards the north pole: and the which may be of any hard substance, and on which are marked the directions of the shadow for the several hou•s-of the day, their halves, quarters, etc. Dials receive various names, acco•din7, mostly, to the positions which they are con structed to occupy. When the dial-plane is on the plane of the horizon, the dial Ia called a horizontal dial; when perpendicular to that plane, a vertical dial. An equi noctial dial is one whose platiels pittdilel to the equinoctial plane. Restdes these names, there are others, such as the s. dial, n. dial, e. dial, w. 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 cylindrical dial is a dial drawn on the curved surface of a cylinder. The ring dial is an ingenious small portable dial, but rather a curious toy than a philo• sophical instrument.
A night or nocturnal dial is an instrument for showing the hour of the night by the shadow of the moon or stars. Moon-dials may be constructed relative to the moon's motion; or the hour may be found by the moon's shadow on a sun-dial by the following rule: Observe the hour pointed out by the moon's shadow; find the days of the moon's age in the calendar, and take three fourths of that number for the hours to be added to the time. shown by the.shadow to give the hour of the night.
Dialing. —The stile of a dial being parallel to the earth's axis, those familiar with spherical trigonometry will readily see that the problem of constructing a dial resolves itself into that of ascertaining where the hour-lines cut a given circle, with a view to the graduation of the dial-plane. We do not here presume the reader to be acquainted with spherical trigonometry, and accordingly proceed to illustrate the principles of dialing in a popular manner, taking our illustrations from Ferguson's Lectures (4th ed., 1772, lecture 10), which should be referred to for fuller information. Suppose a hollow and transparent sphere, as of glass, to represent the earth; and suppose its equator divided into 24 equal parts by the meridians, one of them passing through a given place, say London (see Hontzox). If the hour 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-circles of London, because, as the sun appears to move round the earth iu 24 hours, lie will pass from one meridian to another in one hour. Then, if the sphere has an opaque axis, terminating in the poles, the shadow of this axis would fall, in the course of the day, on every particular meridian 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 meridian as London. If the
sphere were cut through the middle by a plane in the rational horizon of London, and if straight lines were drawn from the center of the plane to the points where its circum ference is cat by the hour-circles of the sphere, those lines would be the hour-lines of a horizmtal dial for London; 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 suppose the sphere cut by any other plane facing the meridian, the hour-circles of the sphere will cut the edge of the plane in those points to which the hour-lines must be drawn straight from the center; and the axis of time sphere will cast a shadow ou these lines at the respective hours. The like will hold of,auy plane, whether it face the meridian or not, provided it do not coincide with it, or do not coincide with a plane through the poles, and perpendicular to the plane of the equator. In the latter ease, the axis would have no elevation above the plane of the dial; in the former, the shadow would not move circularly.
The universal dialling cylinder, an invention of Ferguson's, is a glass cylindrical tube, closed at both ends with brass plates, on the centers 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 24 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 cylinder stands for midnight; the XII next the board, for noon. When the axis is adjusted for the lat itude, and the board leveled,}vith both XII, noon and midnight, in the plane of the meridian, and the end toward4rthe n., the axis, when the sun shines, will serve as stile, and cast a shadow on the hour of the (lay 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 center to the extremities of the parallels will be the hour-lines of an equinoctial dial, and the axis will be the stile. A horizontal plate, if put into the tube, with lines drawn from the center to the several parallels cutting its edge, will he a horizontal dial for the given latitude; and similarly a vertical plate fronting the meridian, and touching the tube with its edge, with lines drawn from its center to the parallels, will be a vertical s. 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 being 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.