I therefore think it very proper to acquaint the reader, that there were lately completed, new sets of plates for 18 and 12-inch globes, under the title of the NEW BRITISH GLOBES. It is now more than 40 years since the plates for the 17-inch of Sener's, and the 18-inch of Adams's, were engraved; and the important discoveries, both geographical and astronomical, that have recently been made, have afforded the opportunity of contributing to render the above new globes the most complete of any extant. On the terrestrial are inserted all the latest communications and places, from authentic observations, and engraved from a drawing by Mr. Arrowsmith; and, on the celestial, are placed near 6000 stars, clusters, planetary nebulæ, &c. from the latest observations of Dr. Maskelyne, Dr. Herschel, &c. as computed by me for the year 1801, so that these globes may be considered as properly adapted for the present century, and to supersede all former globes... EDIT. N ESSAY II. PART II. CONTAINING A DESCRIPTION OF THE GLOBES MOUNTED IN THE BEST MANNER; TOGETHER WITH SOME PRELIMINARY DEFINITIONS, DEFINITIONS.. BEFORE we begin to describe the globes, it will be proper to take some notice of the properties of a circle, of which a globe may be said to be constituted. A line is generated by the motion of a point. Let there be supposed two points, the one moveable, the other fixed. If the moveable point be made to move directly towards the fixed point, it will generate in its motion a straight line. If a moveable point be carried round a fixed point, keeping always the same distance from it, it will generate a circle, or some part of a circle, and the fixed point will be the centre of that circle. All straight lines, going from the centre of the circumference of a circle, are equal. Every straight line that passes through the centre of a globe, and is terminated at both ends by its surface, is called a diameter. The extremities of a diameter are its poles. If the circumference of a semicircle be turned round its diameter, as on an axis, it will generate a globe, or sphere. The centre of the semicircle will be the centre of the globe; and as all points of the generating semicircle are at an equal distance from its centre, so all the points of the surface of the generated sphere are at an equal distance from its centre. DESCRIPTION OF THE GLOBES. There are two artificial globes. On the surface of one of them the heavens are delineated; this is called the celestial globe. The other, on which the surface of the earth is described, is called the ter restrial globe. 1 Fig. 2, plate 16, represents the celestial, fig. 1, plate 16, the terrestrial globe, as mounted in my father's manner. In using the celestial globe, we are to consider ourselves as at the centre. In using the terrestrial globe, we are to suppose ourselves on some point of its surface. The motion of the terrestrial globe represents the real motion of the earth. The motion of the celestial globe represents the apparent motion of the heavens. The motion, therefore, of the celestial globe, is a motion from east to west. But the motion of the terrestrial globe is a motion from west to east. On the surface of each globe several circles are described, to every one of which may be applied what has been said of circles in page 194. The centre of some of these circles is the same with the centre of the globe; these are, by way of distinction, called great circles. Of these great circles, some are graduated. The graduated circles are divided into 360°, or equal parts, 90 of which make a quarter of a circle, or a quadrant. Those circles, whose centres do not pass through the centre of the globe, are called lesser circles. The globes are each of them suspended at the poles, in a strong brass circle, NZÆS, and turn therein upon two iron pins, which are the axis of the globe; they have each a thin brass semicircle NHS, moveable about these poles, with a small thin circle H sliding thereon: it is quadrated each way to 90°, from the equator to either pole. On the terrestrial globe, this semicircle is a moveable meridian. Its small sliding circle, which is divided into a few of the points of the mariner's compass, is called a terrestrial, or wisible horizon. On the celestial globe, this semicircle is a moveable circle of declination, and its small brass circle an artificial sun, or planet. Each globe has a brass wire circle, TWY, placed at the limits of the crepusculum, or twilight, which, 1 together with the globe, is mounted in a wooden frame. The upper part, BC, is covered with a broad paper circle, whose plane divides the globe into two hemispheres; and the whole is supported by a neat pillar and claw, with a magnetic needle in a compass-box marked M, to set the axes of the globes duly north and south. A DESCRIPTION OF THE CIRCLES DESCRIBED ON THE BROAD PAPER CIRCLE BC; TOGETHER WITH A GENERAL ACCOUNT OF ITS USES. It contains four concentric circular spaces, the innermost of which is divided into 360', and numbered into four quadrants, beginning at the east and west points, and proceeding each way to 90°, at the north and south points: these are the four cardinal points of the horizon, The second circular space contains, at equal distances, the thirty-two points of the mariner's compass. Another circular space is divided into twelve equal parts, representing the twelve signs of the zodiac; these are again subdivided into 30 degrees each, between which are engraved their names and characters. This space is connected with a fourth, which contains the calendar of the months and days; each day, on the eighteen-inch globes being divided into four parts, expressing the four cardinal points of the day, according to the Julian reckoning; by which means the sun's place is very nearly obtained for the common years after bissextile, and the intercalary day is inserted without confusion. |