42 Alethod to dcterniiiie the 



tclliffent beings ; thus anfwcring the double purpofe of an 

 axlditional light tou?, aud a convenient habitation for others; 

 while flie, in return, receives a like benefit from the earthy 

 which anfwers the purpofe of a luminary to her. She is feea 

 to have broad feas, bounded with rocky (hores ; extended 

 continents, diverfificd with hill and dale ; deep valleys and 

 towering mountains ; mountains, whofe extended lliadows 

 can be traced, and as didinctly feen, as the ihadow of a 

 fun-dial ; and, lately difcovered (owing to the great improve- 

 ments made in telefcopes), volcanic or burning mountains. 



An eafy method to determine the magnifying power of 

 thofe inllmments, whofe excellence it is to have made u* 

 intimately acquainted with the various phenomena above 

 tiKntioned, and which give us fourccs of enjoyment Irom the 

 contemplation of diftant tcrrellrial obje&s, of which our 

 forefathers had no conception, camiot but be acceptable to 

 many, efpecially to thofe who have not the opportunity of 

 employing the more expenfive procelles that are reforted to 

 for the fame end. 



The fin^pleft method is, to mcafure the emergent pencil 

 or cone of light, as it appears^ in the eye-hole of the tele- 

 fcope^ and to divide the diameter of the aperture of the ob- 

 jecl-glafs, if it is an achromatic, or of the great milla, if a 

 refleilor, by the diameter of this emergent pencil, and 

 it w ill give the magnifying power. For this purpofe, I take 

 a piece of thin writmg paper, a little more than two inchco 

 in length, and about half an inch in breadth ; I draw with 

 a good pen, or other proper inftrumcnt, the line a, b, fig. 28, 

 (IMate I.) and then, with a pair of good dividers, I take 

 from a fcale one tenth of an inch, whicb I fet off from h to c, 

 and then, with the fame extent, divide the line a, b, into 

 20 equal parts, making vifible marks or punftures at each 

 divifion. I then draw the line a, c, and make the divifions 

 J, 2, 3, 4, &c. to 10, at every other diviiion or point made 

 •with the dividers, and put a little fwect oil upon the paper 



I 



