ASTRONOMY. $7 



and though less correct, by being less expensive, 

 and more portable than the preceding, it is of 

 much more general use. 



It is impossible to delineate on a plane any figure 

 that can accurately resemble one which is extend- 

 ed in three dimensions. A certain degree of re- 

 semblance may, however, be obtained, and, in the 

 construction of maps, this has been sought for in 

 two ways ; by the projection of the spherical sur- 

 face on a plane, such as it would be seen to the eye 

 situated in a particular point; or by the dffoelope- 

 ment, that is, the spreading out of a spherical on a 

 plane surface. 



71- The Stereographic Projection, is a repre- 

 sentation of the surface of a sphere on the plane 

 of a great circle, such as it would appear to an 

 eye situated in the pole of that circle, or in a 

 point 90 distant from every part of its cirum- 

 ference. 



a. It is usual to suppose the eye placed in the equa- 

 tor, 90 distant from the equinoctial points, so that 

 the plane of projection is the equinoctial colure. 

 If the eye is placed in either of the equinoctial 

 points, the plane of projection is a circle at right 

 angles to the former, called (for a reason that will 

 afterwards appear) the Solstitial Colure. 



The hemisphere concave to the eye, or on the side of 

 the plane opposite to the eye, is first delineated. 



The 



