INTRODUCTION. 



63 



USE OF GLOBES. 



1 Terrestrial Globes. Though the earth be not exactly 

 a sphere, it deviates very little from tlie spherical form. 

 The polar diameter is less than the equatorial by about 

 ] 334th of the latter, while tlie height of the highest moun- 

 tain is not equal to the 2000th part of it. Upon the lar- 

 gest globe that is ever constructed, these differences of the 

 earth from an exact sphere could not be perceived ; and 

 the artificial globe, therefore, is always exactly spherical. 



Through the centre of tlie globe let a straight wire pass, 

 this will represent the axis, and the points wliere it cuts 

 the surface, the north and south poles. A circle drawn at 

 the distance of 90 degrees from either pole is the equator, 

 and another circle drawn from any point of the equator, 

 and at right angles to it, will be the first meridian. 



The equator and the first meridian are divided into de- 

 grees and minutes, which are numbered, beginning at the 

 point where the circles intersect each other. The degrees 

 upon the first meridian are numbered on both sides of the 

 equator, and do not exceed !)0. They point out the lati- 

 tude. The degrees upon the equator are numbered com- 

 pletely round the circle, and extend therefore to 360. Tliey 

 enable us to find out the longitude. 



The equator and first meridian are distinguished from 

 parallels of latitude and other meridian lines, by their 

 being graduated. They are also sometimes denoted by 

 double lines. 



We shall now suppose, that the artificial globe exactly 

 represents the surface of the earth, and proceed to explain 

 the lines which are commonly drawn upon the globe, be- 

 sides tlie equator and first meridian, and to describe the 

 apparatus usually attached to it. 



In order that we might be able to find out from the globe 

 itself, the latitude and longitude of any place, a parallel to 

 the equator and a meridian line would require to be drawn 

 through that place. It is impossible that such lines could 

 De drawn through every point on the globe, and it is un- 

 necessaiy, for the brass circle placed around it, enables us 

 to find out the latitude and longitude. In this circle, 

 which is placed at right angles to the equator, and is there- 

 fore a meridian, the globe is suspended by the axis. One 

 of the sides of the meridian is graduated, or divided into 

 degrees, minutes, and seconds. The globe can be turned 

 round its axis, while the general meridian remains station- 

 ary, so that every point of the surface of the globe must 

 pass under some point of the meridian. To find out the 

 latitude and longitude of any place, therefore, wc have 

 only to turn the globe round till the given place he brought 

 to the meridian. The number of degrees, minutes, &c. 

 under which the place lies will be its latitude, and the 

 number intercepted upon the equator its longitude. 



In addition to the general mex\d\a.n, meridians and paral- 

 lels of latitude are usually drawn upon the globe, through 



every 5th or 10th degree of latitude and longitude, accord- 

 ing to the size of the globe. These lines point out accu- 

 rately the latitude or longitude of those places which are 

 situated upon them, and give us a general idea of the situ- 

 ation of other places. 



Besides meridians and parallels of latitude, the ecliptic 

 is usually drawn upon globes, and also the tropics and 

 polar circles. All these last are commonly drawn with 

 double lines to distinguish them from other meridians and 

 parallels of latitude. 



The globe suspended in the general meridian, is placed 

 upon a wooden frame. The upj)er surface of this frame 

 divides the globe into two hemispheres, one superior, and 

 the other inferior, and represents, therefore, the rational 

 horizon of any place which is brought to the zenith point 

 of the meridian. There are two notches for the meridian 

 to slide in, by which different elevations of the pole may 

 be exhibited. The horizon has commonly drawn upon it 

 the points of the compass, the twelve signs of the zodiac, 

 the months of the year, &c. 



There is attached to the general meridian a quadrant, 

 composed of a thin pliable plate of brass, answering exact- 

 ly to a quadrant of the meridian. It is graduated, and has 

 a notch, nut, and screw, by which it may be fi.\ed to the 

 brazen meridian in the zenith of any place. When so fix- 

 ed, it turns round a pivot, and supplies the place of verti- 

 cal circles. It is hence denominated a qvadra?it of alti- 

 tude. 



A small circle of brass is placed on the north pole. It 

 is divided into 24 equal parts, and is termed an hour-circle. 

 On the pole of the globe is fixed an index, which turna 

 round the axis, and poiiits out the hours upon the hour- 

 circle. 



There is also often attached to the globe a compass, 

 which is placed upon tlit pediment of the frame, parallel 

 to the horizon. 



2. Problems solved Inj the Globe. Having thus described 

 the globe and its apparatus, we shall now explain some of 

 the problems that may be resolved by it. 



I. To find, the latltvdc and longitude of any place. We 

 have already seen, that this is done by bringing the place 

 to the graduated side of the general meridian ; the degree 

 of the meridian cut by the place being equal to the lati- 

 tude, and the degree of the equator then under the meri- 

 dian being the longitude. 



II. To find a place upon the Globe, its latitude and lon- 

 gitude being given. Find the degree of longitude on the 

 equator, and bring it to the brass meridian; then find the 

 degree of latitude on the meridian, either north or soutii, 

 and the point of the globe under that degree of latitude is 

 the place required. 



III. To find all the places on the Globe thai have the savie 



