ESSAY II. PART II. CONTAINING A DESCRIPTION OF THE GLOBES MOUNTED IN THE BEST MANNER; TOGETHER WITH SOME PRELIMINARY DEFINITIONS. BE DEFINITIONS. EFORE we begin to describe the globes, it will be proper to take fome notice of the properties of a circle, of which a globe may be faid to be constituted. A LINE is generated by the motion of a point. Let there be fuppofed 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 it's motion a strait line. If a moveable point be carried round a fixed point, keeping always the fame diflance from it, it will generate a CIRCLE, or fome part of a circle, and the fixed point will be the CENTER of that circle. All ftrait lines going from the center to the circumference of a circle, are equal. Every ftrait line that paffes through the center of a globe, and is terminated at both ends by it's furface, is called a DIAMETER, The extremities of a diameter are it's poles. If the circumference of a femicircle be turned round it's diameter, as on an axis, it will generate a globe, or sphere. The center of the femicircle will be the center of the globe; and as all points of the generating femicircle are at an equal distance from it's center, fo all the points of the furface of the generated sphere are at an equal distance from it's center. DESCRIP |