The plane of every dial represents the plane of some great circle on the earth, and the _Gnomon_ the earth's axis; the vertex of a right _Gnomon_, the centre of the earth or visible heavens. The earth itself, compared with its distance from the sun, is considered as a point, and therefore if a small sphere of glass be placed upon any part of the earth's surface so that its axis be parallel to the axis of the earth, and the sphere have such lines upon it, and such plans within it, as above described, it will show the hour of the day as truly as if it were placed at the earth's centre, and the shell of the earth were as transparent as glass. The diversity of the titles of sun-dials arises from the different situation of the planes, and the different figure of the surfaces whereon they are described, whence they are denominated equinoctial, horizontal, vertical, polar, erect, direct, declining, inclining, reclining, cylindrical, &c. +--------------------------------------------------------------+ | [Illustration: The Pocket Ring Dial.] | +--------------------------------------------------------------+ All the before-mentioned time-measurers were up to a certain period non-portable, and in addition to the drawback of being unserviceable excepting when the weather was clear and the days bright were as useless for private purposes, as they were unadapted for the winter-time or for night. The next step was therefore a portable dial, but this was probably not invented until after a very long interval. The Dial of which the above is an illustration, was probably one of the earliest of portable time-keepers, the time being shown by means of a hole through which the light fell on the inside, which had an inner ring adaptable to the day and the month. Ring-dials of this description were in common use within the last century in this country, and were manufactured in large numbers at Sheffield when watches were too expensive to be generally attainable. Some of these Ring-dials were of superior construction, and were made by means of more than one ring to serve for different latitudes. As an example of a still greater advance in the manufacture of pocket dials, see the illustration on the next page. The Dial consists of a thin silver plate properly divided and marked, and having a compass with glass cover sunk at one end of it. The _Gnomon_ or style moves upon a hinge so as to allow of its lying flat upon the Dial while in the pocket, and thus rendering the instrument conveniently portable. The _Gnomon_ itself is also susceptible of elevation or depression and the beak of the bird carved on a thin slip of silver at its side marks the exact extent of the _Gnomon's_ elevation. This Dial is indubitably of French manufacture.