But though thlz method, of delineating globes is in itself simple, and capable of being carried to almost any degree of exactness, those who business it is to construct them for sale, have found it necessary, in order to lurnish them at a moderate price, to adopt al.-)ther method, less accurate perhaps, but much more This method con sists in delineating, on separate pieces paper of the form JENQS, (Plate CCLXV. Fig. 7.) gores, separate portions of the earth or the heavens, as they are intended for a terrestrial or celestial globe, and afterwards fixing them in order on the surface of the sphere, when N and S coincide with the poles, ICES, NMS, and NQS become meridians, and £MQ an arch of the equator. Strictly speaking, indeed, no portion of paper can be ac curately fitted to a spherical surface ; but if £Q be very small compared to the whole circumference, the portion (.1' the sphere, covered by the segment /ENOS, will not sen sibly differ from a plane in the direction ; that is. the arches of the equator if:M, QM, and of tIe parallel of la titude a b, c b, may be considered as straight lines perpen dicular to NS. The number of necessary to co ver the globe, will depend on the length of the arch £Q ; but when the whole have been once carefully designed and accurately fitted to the sphere, it is only necessary to make correct engravings of the originals, in order to con struct, with comparatively little labour, any number of globes of the same dimension. Sometimes the segments are truncated at each extremity AB, CD, so as to leave a small circular space about the poles. These spaces are considered as plane surfaces, and are accordingly covered with one circular piece of paper, on which the portions of the meridians form radii of a circle.. The method of deli neating the gores will be explained when we come to the projection of maps.
Having thus shortly noticed the different methods of constructing globes, we should now proceed to what more properly constitutes the object of this Chapter, the appli cation of these instruments to the solution of problems.
Before concluding this Section, however, we would ob serve, that in perusing the terrestrial globe, the eye of the observer is in its natural position ; but in the case of the celestial globe, he must conceive himself situated in the centre, and looking towards the concave surface. This will perhaps be better understood by referring to the ar millary sphere, as represented Fig. 8. Plate CCLXV. This instrument consists of a number of metallic rings, so con nected as to t epresent the circles of the sphere, and at the same time to exhibit the apparent relative positions of the earth and heavens. As delineated in the figure, N and S represent the poles, and the line NS the axis of the world, with the earth G in the centre ; HR the horizon, iEQ the equinoctial, EL the ecliptic, JENQS the solstitial colure, KM the equinoctial colure, TC the topic of Cancer, Tie the tropic of Capricorn, AP the arctic circle, and A'P' the antarctic circle. The meridian E,NQS is graduated in the same way as the brazen meridian of the celestial globe, viz. from IE towards N and S, and from N and S towards Q ; and the horizon is moveable, so that-by raising or de pressing it at the north and south points, it may be made to intersect the meridian YENQS, in any two opposite points. The whole instrument rests on the southern ex tremity S of the axis, and is generally so constructed, that the earth G, with the meridian tENQS, and horizon RR, may be made to revolve from west to east, while the other circles remain at rest ; or the latter may be made to re volve from east to west, while the former remain fixed. By means of the joint 13, the sphere may be placed so as that the north pole N may have any elevation, the angle of that elevation being measured by the graduated arch F, at tached to BS, and passing through a slit in BD. In using the celestial globe, then, the eye is supposed to be situated on G, and viewing the circles of the armillary sphere, the latter in all cases being placed in the same situation with the globe.