hour circle upon which this intersection falls is the hour of the day; and as the meridians are drawn within the tropics, at 20 minutes distance from each other, the point cut by the intersection of the string upon the parallel of the day, being so near the equator, may, by a glance of the observer's eye, be referred thereto, and the true time obtained to a minute.

The plumb-line thus moved is the azimuth; which, by cutting the parallel of the day, gives the sun's place, and consequently the hour circle which inter

sects it.

From this last operation results a corollary, that gives a second way of rectifying the globe to the sun's rays.

If the azimuth and shade of the illuminated axis agree in the hour when the globe is rectified, then making them thus to agree, must rectify the globe.

COROLLARY. Another method to rectify the globe to the sun's rays.

Move the globe, till the shadow of the plumb-line, which passes through the zenith, cuts the same hour on the parallel of the day that the shade of the pin, held in the direction of the axis, falls upon amongst the polar parallels, and the globe is rectified.

The reason is, that the shadow of the axis repres sents an hour circle; and by its agreement in the same hour, which the shadow of the azimuth string points out, by its intersection on the parallel of the day, it shews the sun to be in the plane, of the said

parallel; which can never happen in the morning on the eastern side of the globe, nor in the evening on the western side of it, but when the globe is rectified. This rectification of the globe is only placing it in such a manner, that the principal great circles and points may concur and fall in with those of the heavens.

The many advantages arising from these problems, relating to the placing of the globe in the sun's rays, the tutor will easily discern, and readily extend to his ●wn, as well as to the benefit of his pupil.

THE

GENERAL PRINCIPLES

OF

DIALLING,

ILLUSTRATED BY

THE TERRESTRIAL GLOBE.

THE art of dialling is of very ancient origin, and was in former times cultivated by all who had any pretensions to science; and before the invention of clocks and watches it was of the highest importance, and is even now used to correct and regulate them.

It teaches us by means of the sun's rays to divide time into equal parts, and to represent on any given surface the different circles into which, for convenience, we suppose the heavens to be divided, but principally the hour circles.

The hours are marked upon a plane, and pointed out by the interposition of a body, which receiving the light of the sun, casts a shadow upon the plane. This body is called the axis, when it is parallel, to the axis of the world. It is called the style when it is so placed that only the end of it coincides

with the axis of the earth; in this case, it is only this point which marks the hours.

Among the various pleasing and profitable amusements, which arise from the use of globes, that of dialling is not the least. By it the pupil will gain satisfactory ideas of the principles on which this branch of science is founded; and it will reward with abundance of pleasure, those that chuse to exercise themselves in the practise of it.

If we imagine the hour circles of any place, as London, to be drawn upon the globe of the earth, and suppose this globe to be transparent, and to revolve round a real axis, which is opake, and casts a shadow; it is evident, that whenever the plane of any hour semicircle points at the sun, the shadow of the axis will fall upon the opposite semicircle.*

Let aPCp, plate 13, fig. 1, represent a transparent globe; abcdefg the hour semicircles, it is clear, that if the semicircle Pap points at the sun, the shadow of the axis will fall upon the opposite semicircle.

If we imagine any plane to pass through the centre of this transparent globe, the shadow of half the axis will always fall upon one side or the other of this intersecting plane.

Thus, let ABCD be the plane of the horizon of London; so long as the sun is above the horizon,

Long's Astronomy, vol. I. p. 82.

the shadow of the upper half of the axis will fall somewhere upon the upper side of the plane ABCD; when the sun is below the horizon of London, then the shadow of the lower half of the axis E falls upon the lower side of the plane.

When the plane of any hour semicircle points at the sun, the shadow of the axis marks the respective hour-line upon the intersecting plane. The hour-line is therefore a line drawn from the centre of the intersecting plane, to that point where this plane is cut by the semicircle opposite to the hour semicircle.

Thus let ABCD, plate 13, fig. 1, the horizon of London, be the intersecting plane, when the meridian of London points at the sun, as in the present figure, the shadow of the half axis PE falls upon the line EB, which is drawn from E, the centre of the horizon, to the point where the horizon is cut by the opposite semicircle; therefore EB is the line for the hour of twelve at noon.

By the same method the rest of the hour-lines are found, by drawing for every hour a line, from, the centre of the intersecting plane, to that semicircle which is opposite to the hour semicircle.

Thus plate 13, fig. 2, shews the hour-lines drawn upon the plane of the horizon of London, with only so many hours as are necessary; that is, those hours during which the sun is above the horizon of London, on the longest day in summer.

If, when the hour-lines are thus found, the semicircles be taken away, as the scaffolding is when the