DIAL. Dials are of various construc tions, some being horizontal, others ver tical, and others moveable, so as to apply to any particular latitude at pleasure. The use of a dial is to indicate the hour, which is done by means of a wire, or by a triangular board, &c. placed at right angles to the face or index. This triangular piece is called the stile, or gnomon, and is made to point due north : it should be perfectly vertical, and the dial's face, on which the hours are mark. ed, should'be equally divided thereby ; the line of 12 being in a true direction with the stile. This line of direction iF called the substile : the angle contained between the summit of the stile and the face of the dial is called the elevation. All whieb have their planes, or faces, parallel with the horizon, are called ho rizontal dials ; those which have perpen dicular planes or faces, are called verti cal dials and such as are neither vertical nor horizontal, are called reclining dials. When erect dials do not face either the north, or the south, they are called de clining dials. An universal dial is one that answers for all latitudes.
The line passing under the centre of the stile longitudinally, and marking the hour of 12, is called the meridian : in de clining dials, the substile makes an angle t with the meridian, in proportion to the .deviation from a northerly direction :this angle is the difference of longitude. With respect to the manner of construct ing, and of placing these useful instru ments, we shall now proceed to give some account.
The following is the most simple dial that can be made. (Fig. 1. Plate Dialing.) Divide a circle into twenty-four equal parts, and draw, through the several points of division; rays from the centre. That point which is to be the north is to be marked XII, the next on the right XI, and thus as far as V. or IIII : those on the left of the southern point 12, are to be I, II, III, kc. in regular order, down to VIII. In the centre, whence the cir cle was drawn, fix a pin, equal in length to about a diameter of the circle, and be very careful that it be perfectly upright. Now, placing the dial at such an elevation as may equal the latitude of the place where it is to be used, see that the XIIth hour be on the meridional line. Thus, for the latitude of 50, the northerly part XII, would require to be raised 50 de grees from the horizon, so that the face of the dial would stand in the plane of the equator, and cause the shadow of the pin to fall on the index, thus to point out the time of day. This is called the equi
noctial dial.
The following is the best mechanical method known for making common ho rizontal dials. (Fig. 2.) Draw two con centric circles, between which the hours are to be marked, and assuming any point for the hour of XII., let the thickness of your gnomon, or stile, be set off by two lines a b, c d, passing near the centre e of the circle, perfectly parallel and equidis tant from the meridional line which pass es exactly through the centre, and through the mid-day or XII. point. Now cross the meridional line at right angles, by the VI. o'clock line fg, and from the points b and d, as centres, describe the quadrants f h, taking a good extent, so as to approach nearly to the hour cir cle, for the sake of minute division ; if such be required, divide the quadrants respectively into 90 equal parts ; then from d draw a line at 11°1 from the line e d, which will give the place of I. o'clock ; draw another line at 24°1, which will give the place of II. o'clock ; through will give the place of III. through 53°i for IIII. o'clock ; through for V. o'clock ; and the line cross ing your meridian will give VI. o'clock. The base of your gnomon is to be equal to a radius of the inner hour circle, hav ing the altitude of the place you are at for its angle : thus the gnomon in this fi gure would have b k, its upper line, standing at an angle of 51°,1 from its base line a b ; the lowest part being set on to the centre of the VI. o'clock line, even with the meridional line, and the broad est (i e. highest) part of the gnomon a k, being towards the hour of XII. The left or morning side of the hour table is made by drawing lines, at the angles already described, from the centrel, through its appropriate quadrant. The hours before VI. in the morning, and after VI. in the evening, are adjusted by continu ing through the centres b and d, re spectively, those lines which indicate their ascertained numbers : thus the line of V, P. M. continued through the cery tre d, will give the place of V., A. M.