Such is a general view of the artificial sphere usually employed to represent the earth. It is called the terrestrial globe, to distinguish it from that used to represent the hea vens, and which is denominated the celestial globe. The latter, like the terrestrial globe, is suspended by the poles in a brazen or universal meridian, and mounted in the same way in a horizon. On its sum face are described the equi noctial, the ecliptic, the tropics, the polar elides, the equi noctial and solstitial colures, circles of celestial longitude, and parallels of celestial latitude. As the celestial sphere revolves from eascto west, the graduation of the brazen me ridian is towards the east, but, in other respects, it is the same as that of the terrestrial globe. The ecliptic is di vided in the same way as on the other globe, but opposite each degree is a dot, representing the day on which the sun is at that point. The degrees of the equator are rekon ed in one direction only, viz, towards the east, and the equi noctial and solstitial points are determined from actual observation of the heavens. On the surface of the celes tial globe are represented the fixed stars, in their true re lative positions, distinguished according to their magni tudes, and arranged in constellations.
Besides the parts already described, there are other ap pendages common to both globes, which yet remain to be explained. The principal of these are, the horary or hour circle, the quadrant of altitude, and the compass.
The horary is a small brass circle generally fixed on the north pole of the globe, and divided into Z4 equal parts, representing the hours of the day. The divisions are mint bered from 1 to 12 along the first semicircle, and again from 1 to 12 along the next, in ale contrary direction to that in which the globe nattn-PHY revolves. The circle is moveable separately by hand, so that any given hour. brazen meridian ; but when left to may be brought to the itself, it turns with the globe, and thus serves to measure the whole, or any part of a revolution.
The quadrant of altitude is a graduated slip of brass, so thin and flexible, as to be easily applied to the surface of the globe. It is furnished at one extremity with a nut and screw, by which it may be fastened to any part of the me ridian. When this extremity is fixed on the zenith of the globe, the zero, or commencement of the graduation, co incides with the horizon, thus forming the fourth part of a vertical circle. The graduation is also continued to the other extremity of the quadrant, which is generally about 18 or 20 degrees. It takes its name from being applied to measure the altitude of heavenly bodies.
The compass consists of a magnetic needle, suspended over the centre of a circle, on the circumference of which are marked the 32 points or rhombs of the horizon. It is
in fact, the simplest form of the mariner's compass, fixed in the under part of the frame or mounting, and used for placing the brazen meridian in the meridian of the place.
It would be tedious even to enumerate the various im provements and alterations, that have been from time to time recommended, in the construction and use of the globes, as well as of their appendages. Such an enumera tion, however, is we conceive unnecessary, because any body who understands the general principle of the globes, as we have now explained it, will find no difficulty in using any of the instruments with which they may he accom panied ; and because niore information will be acquired, in half an hour, from inspecting the globe itself, than we could communicate in a whole volume of descriptions and draw ings. While, therefore, we have endeavoured, in the pre ceding short sketch, and by help of the representations, Figs. 5 and 6. Plate CCLXV. to convey some idea of the nature of the globes, we would conclude, by recommend ing to such of our readers as wish to become thoroughly acquainted with the subject, to draw their information from the instrument itself.
The most natural, as well as the most correct method of tracing out circles of a globe, may easily be deduced from the preceding description. Suppose, for example, it is required to delineate on the surface of a spherical body, the various lines and figures of the terrestrial globe. From either of the points that represent the poles, with a radius equal to half the distance between them, a circle is des cribed to represent the equator, and divided into degrees. From the 90th degree of the equator with the same radius, another circle is described, passing through the poles, and representing the first meridian. Other meridians are des cribed in a similar manner, by taking for a centre every fifth, tenth, or fifteenth degree of the equator, according to the number required. The first, or any other meridian, being divided into degrees, from the equator towards the poles, the tropics and polar circles may be described from the poles as centres, with radii extending to 23° and 66? respectively. Other parallels of latitude are described in a similar manner. The ecliptic is described, by taking as a centre the point which is in 90° west longitude, and 66? north latitude, and for a radius one-fourth of the circum ference of the globe. Having thus described all the cir cles, it only remains to lay down the different places of the earth, according to their respective longitudes and latitudes as determined by observation, and described in books of geography.