rises to the north of east, and consequently throws the shadow rather towards the south-west, while in Winter he rises southward of east, thereby throwing the shadow northward of west. Again, at twelve o'clock at noon, (in a northern latitude above the tropic of Cancer,) the Sun is always due south, and therefore casts a shadow due north.

Now, if all these facts were carefully noted, and marks made in the ground by which the direction and length of the shadow might be compared at different times, much information as to the progress of time and the seasons might be acquired; indeed a rude sort of Sun-dial would be constructed. All Sun-dials are merely contrivances, by which the shadow of the straight edge of a projecting piece of metal, or wood, becomes indicative of the number of hours that the Sun has been above the horizon, or, rather, the number of hours that he is distant from his meridian position. A stick, thrust upright in the ground, would even render information as to the season of the year in the following manner :—suppose an inhabitant of London (or of any other place in the same latitude, 51°) fix a stick or rod upright in the ground, to the height of exactly two feet. Let him on the 1st of January note the shortest shadow thrown on that day, which will be at twelve o'clock at noon, as before stated. On measuring it he will find its length to be 7 feet 33 inches. Let him repeat the process on the 1st of each following month, and he will find that the shortest (or twelve o'clock) shadow will measure as follows:

We

At any time afterwards, therefore, he can, by measuring the noon-day shadow of a vertical rod two feet high, form a rough idea of the season of the year. must not be understood to attribute any practical value to the foregoing mode of determining times and seasons; but only to observe, that on principles similar to these Sun-dials are constructed.

Our readers have probably seen a Sun-dial. It has been in some measure thrown into oblivion by those more convenient instruments, clocks and watches. The dial still remains, however, as an ornament on the open lawns of rural dwellings, where, fixed on a pedestal, it is exposed on all sides to the Sun. In some countries a Sun-dial is of more service than in others. In southern climates, where the atmosphere is purer and clearer than with us in the north, the dial is very necessary to adjust the clock by; the shadow projected by the style being more marked and constant than with us, owing to the more uninterrupted serenity of the heavens. An elaborate dial may be seen in Kensington Gardens, near the palace; and many old buildings, in and near the metropolis, contain these relics of the ingenuity of our forefathers.

Dials are chiefly horizontal and vertical. But, whatever be their position, there is always a project

ing piece of metal or wood, the shadow of which, thrown on the dial, traverses a certain range of distance from one side of the dial to the other. The position of this index, or shadow-producing body, is a very important part of the construction of a dial; but, whatever be its shape, the edge which casts the shadow ist called the style; the whole contrivance is termed the gnomon; and the part of the dial-plate on which the gnomon rests is the sub-style.

We will now briefly describe the construction of a horizontal and a vertical dial; these being the most practically useful, and therefore oftenest met with. It should be observed, however, that the style of every dial must coincide, or be parallel, with the axis of the Earth. It must, therefore, be elevated as many degrees above the dial-plane, as will accord with the latitude of the place. The equability of the shadow projected by the style, can only be preserved at all seasons and all hours by the parallelism of the style with the Earth's axis; because the apparent course of the Sun is thus at right angles with the style of the dial, as it is with. the axis of the Earth; whence, it will easily be seen, results the uniformity of the shadow.

We construct a horizontal dial for the latitude of London, thus:-having described two concentric circles, within which the hour-numbers are marked, as in the annexed figure, we draw two diameters at right angles to each other, as N s and w E. The east and west line, w E, need not pass through the centre. If drawn lower down, as in the figure, it gives greater scope for the shadow of the gnomon during the middle of the day, when the dial is most available. We next divide each of the two upper quadrants into six parts,

which divisions, however, must not be equal, but the following order must be observed. From the twelve o'clock to the one o'clock line must be an angle of 11° 51'; from one to two o'clock 12° 28'; from two to three = 13° 44'; from three to four = 15° 32'; from four to five = 17° 31'; and from five to six = 18° 54′: which together make up the quadrant of 90°. The same process must be repeated on the other side of the twelve o'clock line, and the dial will be properly

divided. The style must then be inserted where the six o'clock line crosses the meridian line, and it may be either a straight wire, or a thin piece of wood or metal formed into a right-angled triangle, as c A B, which is the gnomon. In our figure it appears to lie down, and

b

overlap some of the hour-lines beneath; but when used it must be turned up by a hinge at the side, 6 A, which is the sub-style. Thus, the gnomon is fixed upon the meridian or twelve o'clock line, N s, which is made double, in order to suit the thickness of the gnomon. When perfectly upright, one end of the sloping edge coincides with the point bc. This sloping edge, b B, thus becomes parallel to the Earth's axis; for, in adjusting the style, or constructing the gnomon, we made the style or line, 6 в, have an inclination equal to the latitude of the place = 51° at London. When the dial is thus constructed, the shadow of the oblique edge of the gnomon, B, coincides successively with all the hour-lines, from eight in the morning till four in the afternoon at Midwinter, and from four in the morning till eight in the evening at Midsummer: these being about the extreme hours of sunrise and sunset in the latitude of London. To mark these hours on the lower quadrants of the dial, the hour-lines twelve hours off are prolonged to the opposite part of the circle, as shown in the figure. The lines, a f, de, are used when the angular distances of the hour-lines are taken from a scale. A horizontal dial for any other latitude must have the style arranged according to that latitude; and the hour-lines separated by angles which may be obtained from tables found in works on Dialling.

In a vertical dial the style is still parallel to the Earth's axis, but the noon hour-line is directed downwards. The style, in consequence, points downwards, and makes with the plane of the dial an angle equal to the complement of the latitude; that is, what the latitude wants of 90°. For London this is 384°; which is the angle made by the style with the dial-plate.