GLOBE, in practical mathematics, an artificial spherical body, on the convex surface of which are represented the countries, seas, &c. of our earth ; or the face of the heavens, the circles of the sphere, &c. That with the parts of the earth delineated upon its surface is called the terrestrial globe ; and that with the constellations, &c. the celestial globe. These globes are placed in frames, with other appurtenances. Their principal use, besides serving as maps to distinguish the outward parts of the earth, and the situation of the fixed stars, is, to illustrate and explain the phenomena arising from the diurnal motion of the earth.
The globes commonly used are com posed of plaister and paper in the follow ing manner : A wooden axis is provided, somewhat less than the intended diame ter of the lobe, and into the extremes two iron wires are driven for poles : this axis is to be the beam or basis of the whole structure. On the axis are applied two spherical or rather hemispherical caps, formed on a kind of wooden mould or block. These caps consist of' paste board or paper, laid one lay after another on the mould, to the thickness of a crown piece ; after which, having stood to dry and embody, making an incision along the middle, the two caps thus parted are slip ped off the mould. They remain now to be applied on the poles of the axis, as be fore they were on those of the mould; and to fix them in their new place, the two edges are sowed together with pack thread, &c. The rudiments of the globe thus laid, they proceed to strengthen and make it smooth and regular. In order to this, the two poles are hasped in a metal line semi-circle of the size intended, and a kind of plaster made of whiting, water, and glue; heated, melted, and incorpo rated together, is daubed all over the pa per surface. In proportion as the plaster is applied, the ball is turned round in the semi-circle, the edge of which pares off whatever is superfluous, and beyond the due dimension, leaving the rest adhering in places that are short of it. After such application of plaster, the ball stands to dry ; which done, it is put again in the semi-circle, and fresh matter applied : thus they continue alternately to apply the composition, and dry it, till the ball every where accurately touches the semi circle ; in which state it is perfectly smooth, regular, and complete. The ball thus finished, it remains to paste the map or descriRtion on it. In order to this, the map is projected in several gores or gussets, all which join accurately on the spherical surface, and cover the whole ball. To direct the application of these gores, lines are drawn by a semi-circle on the surface of the ball, dividing it into a number of equal parts, corresponding to those of the gores, and subdividing those again answerably to the lines and divisions of the gores.
The papers thus pasted on, there re mains nothing but to colour and illumi nate the globe, and to it, the bet ter to resist dust, moisture, &c. The globe itself thus finished, they hang it in a brass-meridian, with an hour-circle and a quadrant of altitude, and thus fit it into a wooden horizon.
There are ten principal circles repre sented upon globes, viz. six greater and
four lesser ones. The greater circles are the horizon, meridian, and equinoctial, as it is called on the celestial, and equator on the terrestrial globe, the ecliptic drawn along the middle of the zodiac, and the two co]ures.
The lesser circles, of principal use, are the two tropics and two polar circles.
Of these circles some are fixed, and al ways obtain the same position ; others moveable, according to the position of the observer. The fixed circles are the equa tor and the ecliptic, with their parallels and secondaries ;. which are usually delin eated upon the surfaces of the globes. The moveable circles are the horizon, with its parallels and secondaries.
The horizon is that broad wooden cir cle surrounding the globe, and dividing it into two equal parts called the upper and lower hemispheres. It has two notch es, to let the brazen meridian slip up and down, according to the different heights of the pole. On the flat side of this cir cle are described the twelve signs, the months of the year, the points of the compass, &c. The brazen meridian is an annulus or ring of brass, divided into de grees, viz, each quadrant in 90 degrees. It divides the globe into two equal parts, called the eastern and western hemi spheres. The quadrant of altitude is a thin pliable plate of brass, answering ex actly to a quadrant of the meridian. It is divided into and has a notch, nut, and screw, to fix to the brazen meridian in the zenith of any place ; where it turns round a pivot, and supplies the room of vertical circles. The hour-circle is a flat ring of brass, divided into twenty-four equal parts, or hour-disteces ; and on the pole of the globe is fixed an index, that turns round with the globe, and points out the hours upon the hour-ci• cle. Lastly, there is generally added a compass and needle upon the pediment of the frame.
The surface of the celestial globe may be esteemed a just representation of the concave expanse of the heavens, not withstanding its convexity ; for it is easy to conceive the eye placed in the centre of the globe, and viewing the stars on its surface ; supposing it made of glass, as some globes are : also that if holes were made in the centre of each star, the eye in the centre of the globe, properly placed, would view through each of the holes the very stars in the heavens repre sented by them.
As it would be impossible to have any distinct notion of the stars, in respect to their number, order, and distances, with out arranging them in certain forms, call ed constellations, this the first observers of the heavens took care to do ; and these, like kingdoms and countries, upon the terrestrial globe, serve to distinguish the different parts of the superfices of the celestial globe.
The stars, therefore, are all disposed in constellations, under the forms of vari ous animals, whose names and figures are represented on the celestial globe; which were first invented by the ancient astro nomers and poets, and are still retained, for the better distinction of these lumina ries. We shall now give some problems on both the globes, beginning with the terrestrial globe.