Sun-Dials.?The simplest form of sun-dial, and a useful one for setting a timekeeper when no standard is available for comparison, is one for showing when the sun is on the meridian. With a timekeeper showing mean time and an equation table, a meridian line may, of course, be at once traced for future reference. In the absence of these, the following, which are practically Ferguson's instructions, may be followed: " Make four or five concentric circles, a quarter of an inch from one another, on a flat stone, and let the outmost circle be but little less than the stone will contain. Fix a pin perpendicularly in the centre, and of such a length that its whole shadow may fall within the innermost circle for at least four hours in the middle of the day. The stone being set exactly level, in a place where the sun shines, suppose from eight in the morning till four in the afternoon, about which hours the end of the shadow should fall without all the circles; watch the times in the forenoon when the extremity of the shortening shadow just touches the several circles, and there make marks. Then, in the afternoon of the same day, watch the lengthening shadow, and where its end touches the several circles, in going over them, make marks also. With a pair of compasses, find exactly the middle points between the two marks on any circle, and draw a straight line from the centre to that point, which line will be covered at noon by the shadow of the pin."

Figure 1. Meridian Dial.

By observation the hours of the morning and afternoon may also be marked on the meridian dial, and it will be noticed that, although the position of the hour immediately preceding corresponds with the one immediately after noon, these divisions will not answer for any of the remaining hours.

Curious Meridian Dial.?The very ingeniously contrived meridian dial shown below and reproduced from "L'Horlogerie " by Joseph Rambal, formed part of St. Peter's Cathedral, Geneva, from 1760 till the renovation of the building in 1894, and has since been restored on the initiative of the Society of Arts. The white spot in the centre of the disc's shadow not only indicates accurately solar noon when it is bisected by the central vertical line, but also approximately mean solar noon when it is centrally over a line of the figure-of-8 loop which allows for the equation of time on each particular day. The full line of the loop serves from June to December, and the dotted line during the complement of the year. As the year is not made up of a complete number of days, and a day is interpolated every fourth year, the exact equation in each year of the four is different; still the approximate equation would be practically sufficient for all but scientific purposes.

Figure 2. Curious Meridian Dial.

The art of dialling is somewhat complex. A glance at the figure below will show why, except for places on the equator, the hour spaces are not all equal. A sun-dial may be regarded as a circle round the earth, or as the edge of a disc which passes through the centre of the earth from the spot where the dial is fixed. a, b, e, d, e, f, g, &c., are longitudinal circles, representing the hours, B the spot where the dial is situated, D the corresponding latitude, P P the poles, and E the centre of the earth.

A dial prepared for any particular place is useless for another place in a different latitude, with the exception that a horizontal dial for a certain latitude will be a vertical dial for a latitude which is the complement of the first, or what it wants of 90·. That is, a horizontal dial for our latitude of 51 1/2 deg., would have to be placed in a vertical position facing the south in latitude 38 1/2 deg.

Figure 3.

Figure 4. Horizontal Sun Dial.

Horizontal Sun-Dial.?To set out a horizontal dial, first draw two lines parallel to each other, at a distance equal to the thickness of the gnomon which is to cast the shadow. Next, draw a line at right angles to these, the extremities of which will indicate respectively the hours of six in the morning and six in the evening Then, with A and B as centres (see Fig. 4), draw quadrants of circles, and divide each into 90 deg. Now, assuming the dial to be for the latitude of London, lay a rule over B. and draw the first line through 11 2/3 deg., the second through 24 1/4 deg., third 38 1/12 deg., fourth 53 1/2 deg., and fifth 71 1/150 deg. Proceed the same with the other side. Extend the afternoon hour lines of four and five across the dial, and these will form the morning hours, while eight and seven of the morning hours prolonged will give the same evening hours. To form the style or gnomon, draw a radial line through that degree of the quadrant which corresponds to the latitude = 51 1/2 deg. This will show the elevation of the style, which is here represented as if lying on the surface of the dial. The thickness of the style must be equal to the distance between A and B. Place the style truly upright on the dial, and it is finished.

A dial, or rather a series of dials of every conceivable description, forming a structure, as shown in Fig. 5, was erected at Whitehall in 1669, by order of Charles II. It was the invention of Francis Hall alias Line, a Jesuit and professor of mathematics at Liège. Vertical dials, inclining dials, and dials for showing time as computed by various nations at different periods were all included.

Figure 5. Dials at Whitehall, 1669.

Of these, the bowls or brackets appear to be the most attractive. One, on the first platform, to show the hour by fire, consisted of a little glass bowl filled with clear water. This bowl was about 3 in. diameter, placed in the middle of another sphere, about 6 in. diameter, consisting of several iron rings or circles, representing the hour circles in the heavens. The hour was known by applying the hand to these circles when the sun shone, and that circle where you felt the hand burnt by the sunbeams passing through the bowl filled with water showed the true hour.

This curious erection had no covering; exposure to the elements and other destroying influences led to its speedy decay and subsequent demolition. The engraving is taken from the Mirror, vol. xiv.

The commonest form of portable dial is shown in Fig. 6. When held to the sun, by means of the small ring at top, a ray of light passed through a tiny hole and impinged on the inner surface of the opposite side of the rim, which was engraved with numerals corresponding to the hours of daylight. The hole was formed in a slide which covered a slit in the rim. The slide could be moved higher or lower, and signs of the zodiac were engraved on the rim as a guide to its position in different months of the year.

Figure 6. Pocket Sun-Dial.

Dials of this sort were in general use during the sixteenth and seventeenth centuries. A small horizontal dial like Fig. 4, but with a hinged style and a compass attached, formed a more costly pocket "horologium."