CHAP. XXIX.

ON THE USE OF THE GLOBES.

Art. 766. THERE are two Globes, one called Terrestrial, upon which the places on the earth are delineated, and the other called Celestial, upon which the principal fixed stars are put down in their proper places, and the figures of the constellations drawn. The terrestrial globe is a perfect map of the earth, representing accurately the relative situations of all the places upon its surface. The celestial globe serves to explain all the phænomena arising from the diurnal motion of the earth about its axis, and also the variation of seasons arising from its motion about the sun, only supposing the sun to move in the ecliptic instead of the earth, which will not alter any of the appearances. Fig. 186. represents the construction of each globe. HR is a flat circular frame of wood supported by semicircular pieces coming from the foot, the plane of which passes through the center of the globe. PQPE is a brass circle called the Brazen Meridian; it is supported at its lowest point upon a roller on which it turns in its own plane, and passes through the horizon HR in two groves cut for that purpose. The globe itself is supported within this circle by an axis Pp on which it turns; this axis passes through the brazen meridian at P and carries an index round with it over a circular plate hc, which is divided into hours, &c. EQ represents the equator, and CL the ecliptic; to each of which circles on the celestial globe secondaries are drawn to every 10 or 15 degrees; but on the terrestrial, they are drawn only to the equator. From C and L are drawn the two tropical circles; and on the terrestrial globe are drawn the parallels of latitude. There is also part of another circle Za, called a Quadrant of Altitude, which is occasionally fixed to the brazen meridian; it is a thin plate of brass, having a nut and a screw at one end to fasten it to the meridian in its zenith Z, and then the lower end is put between the globe and horizon, and can be turned round to any point; it is divided into degrees, &c. by which the altitude of any object above the horizon may be measured, and at the same time it refers the object to the horizon, by which its azimuth may be determined. From one point E of the brazen meridian corresponding to the equator, the degrees begin and are continued up to 90° at each pole P, p; but for the other semicircle of the meridian, the degrees begin at the poles and are continued to 90° at the equator. On the horizon, the degrees begin at the east and west points, and are continued both ways to 90°, or to

FIG. 186.

the north and south points. The points of the compass are also generally put upon the horizon; and on two other circles drawn thereon are put the signs of the zodiac, and the months and days corresponding to the sun's place, which serves as a calendar to show the place of the sun on any day; this however cannot be accurate, as the sun is not always in the same point of the ecliptic on the same day. The ecliptic and equator begin their degrees at one of their intersections, called Aries, which are continued the same way all round up to 360°, and the former is divided into and marked with the twelve signs; the equator is also divided from the same point into 24 hours, which is therefore sometimes made use of instead of the hour circle. Upon the foot of the globe there is often put a compass, by which the brazen meridian may be set north and south. In the Phil. Trans. 1789, Mr. SMEATON has given a description of an improved quadrant of altitude. Instead of a strip of thin flexible brass, he makes it of a more solid construction. It is fixed to a brass socket, and made to turn upon an upright steel spindle, fixed in the zenith, by which you measure altitudes and azimuths with as much accuracy as you do any other arcs. He approves of the common hour circle, and says, that one of four inches diameter may be divided into 720 distinguishable divisions, answering to two minutes of time; and if instead of a Pointer, an Index Stroke is used in the same plane with that of the divisions, half minutes may be easily distinguished. He therefore thinks the hour circle should rather be improved than omitted, as it is upon some globes.

ON THE USE OF THE TERRESTRIAL GLOBE.

To find the Latitude of a Place.

767. Bring the place under that semicircle of the brazen meridian where the divisions begin at the equator, and observe what degree it is under, and it is the latitude required.

To rectify the Globe for the Latitude of a Place.

768. Elevate the pole above the horizon till its altitude is equal to the latitude of the place, and it then stands right for the solution of all problems for that

To find the Longitude of a Place from any given Meridian.

769. Bring the place to the brazen meridian, and observe the point of the equator which lies under it, and the distance of that point from the point where the given meridian cuts the equator, is the longitude required.

Given the Latitude and Longitude of a Place, to find that Place.

770. Bring the given degree of longitude to the meridian, and then under the degree of latitude upon the meridian you have the place required.

When it is Noon at any Place A, to find the Hour at any other Place B.

771. Bring A to the meridian, and set the index to XII.; then turn the globe till B comes under the meridian, and the index will show the hour at B. If it be not noon at A, set the index to the hour, and proceed as before, and you get the corresponding hour at B.

To find the Distance of A from B.

772. Bring A to the meridian, and screw the quadrant of altitude over it, and carry it over B, and you get the number of degrees between A and B, which multiply by 69,2, the miles in one degree, and you have the distance.

To find the Bearing of B from A.

773. Rectify the globe for the latitude of A, and bring it to the meridian, and fix the quadrant of altitude to it; then direct the quadrant to B, and the point where it cuts the horizon shows the bearing required.

To find the Place A to which the Sun is vertical at any Hour of the Day, at a

given Place B.

774. Find the sun's place in the ecliptic, and bring it to the meridian, and mark the declination; then bring B to the meridian, set the index to the given

the north and south points. The points of the compass are also generally put upon the horizon; and on two other circles drawn thereon are put the signs of the zodiac, and the months and days corresponding to the sun's place, which serves as a calendar to show the place of the sun on any day; this however cannot be accurate, as the sun is not always in the same point of the ecliptic on the same day. The ecliptic and equator begin their degrees at one of their intersections, called Aries, which are continued the same way all round up to 360°, and the former is divided into and marked with the twelve signs; the equator is also divided from the same point into 24 hours, which is therefore sometimes made use of instead of the hour circle. Upon the foot of the globe there is often put a compass, by which the brazen meridian may be set north and south. In the Phil. Trans. 1789, Mr. SMEATON has given a description of an improved quadrant of altitude. Instead of a strip of thin flexible brass, he makes it of a more solid construction. It is fixed to a brass socket, and made to turn upon an upright steel spindle, fixed in the zenith, by which you measure altitudes and azimuths with as much accuracy as you do any other arcs. H approves of the common hour circle, and says, that one of four inches diamete may be divided into 720 distinguishable divisions, answering to two minutes time; and if instead of a Pointer, an Index Stroke is used in the same pla with that of the divisions, half minutes may be easily distinguished. He the fore thinks the hour circle should rather be improved than omitted, as it is up some globes.

ON THE USE OF THE TERRESTRIAL GLOBE.

To find the Latitude of a Place.

767. Bring the place under that semicircle of the brazen meridian wh divisions begin at the equator, and observe what degree it is under, a the latitude required.

To rectify the Globe for the Latitude of a Place.

768. Elevate the pole above the horizon till its altitude is equal t tude of the place, and it then stands right for the solution of all pr that latitude.

only they differ 12 hours in time, it being midnight to one when it is noon to the other. They who live in the same meridian, but on opposite sides of the equator and equidistant from it, are called Antœci. These have day and night at the same time, the hours being the same, but they have different seasons, it being summer with one when it is winter with the other. They who live in opposite parallels to the equator, and in opposite meridians, or who live on opposite points of the globe, are called Antipodes. With these, it is day to one when it is night to the other, and summer to one when it is winter to the other.

ON THE USE OF THE CELESTIAL GLOBE.

To find the Sun's Right Ascension and Declination.

780. Bring the sun's place in the ecliptic to the meridian, and it points out upon the meridian, the declination; and the degree of the equator which is cut by the meridian is the right ascension.

Given the Right Ascension and Declination of an heavenly Body, to find its Place.

781. Bring the degree of right ascension on the equator to the meridian, and the point corresponding to the declination, is the place required.

Given the Latitude of the Place, the Day and Hour, to find the Altitude and Amplitude of the heavenly Bodies.

782. Rectify the globe (768) to the latitude of the place, and bring the sun's place in the ecliptic to the meridian, and set the index to XII.; then turn the globe till the index points to the given hour, and in that position the globe represents the proper situation of all the heavenly bodies upon it, in respect to the meridian and horizon. Then fix the quadrant of altitude to the zenith, and direct its graduated edge to the place of the body, and it shows the altitude of the body, and the degree where it cuts the horizon shows its amplitude. If the body be the moon or a planet, after having found its place, put a very small patch upon it to denote its place.