162 GEOGRAPHY. 



distance from both the poles, is called the equator. The earth's 

 polar diameter is 7899 miles ; its equatorial diameter 7925 miles ; 

 and its mean circumference 24,856 miles. Its mean distance from 

 the sun is about 95,000,000 miles ; and it revolves around the sun 

 in 365 days, 5 hours, 48 minutes, and 48 seconds ; that is, from the 

 vernal equinox to this same point again. The tropics, are two 

 small circles, each 23 28' from the equator ; that of Cancer being 

 on the north, and that of Capricorn on the south. The polar cir- 

 cles, are two other imaginary circles, at the same distance, or 23 

 28' from the poles; the Arctic lying around the north, and the 

 Antarctic around the south pole. The distance of these circles 

 from the poles, and from the equator, is determined by the inclina- 

 tion of the earth's axis to the plane of its orbit, that is, the path which 

 it describes annually around the sun. 



These and other small circles, running due east and west, each one 

 of them being everywhere equidistant from the equator, are called 

 parallels of latitude ; .while great circles passing through the poles, 

 and crossing the equator at right angles, are called meridians. Lati- 

 tude, is distance measured north or south from the equator; and 

 longitude, is distance east or west, from some selected first meridian ; 

 both being measured in degrees and minutes. The ancients sup- 

 posed the earth to have a greater extent eastward and westward, than 

 to the north and south. Hence, distance eastward or westward, they 

 called longitude, or length ; while distance towards the north or 

 south, they termed latitude or breadth.* The ecliptic, is a great 

 circle, fixed in the heavens, but movable on the earth's surface ; 

 and always crossing the equator at an angle of 23 28' ; it being the 

 intersection of the plane of the earth's orbit with the earth's surface ; 

 or with the celestial sphere, supposed to be at an infinite distance 

 from the earth, on every side. A map, is a representation of either 

 the whole, or a part, of the earth's surface on a plane ; and a chart, 

 is either a nautical map, or a map on a cylindrical projection, which 

 represents the meridians as parallel straight lines, and thus magnifies 

 the parts towards the poles. The data for maps are obtained from 

 Surveying and Practical Astronomy; but the principles of their con- 

 struction, belong to Descriptive Geometry. 



On Physical Geography, we must here be extremely brief; but 

 more will be said on this subject, in the province of Physiconomy. 

 The earth rotates, or turns on its axis, once in 24 hours ; thereby 

 causing the alternation of day and night : and it revolves around the 

 sun once in a year, which period of time it measures by this motion. 

 Its annual path or orbit, is a plane curve, nearly circular, but slightly 

 elliptical ; and its axis is oblique to its orbit, but continues nearly 

 parallel to itself; always pointing towards the north star in the hea- 

 vens, and thus causing the obliquity of the equator to the ecliptic. 

 Hence, in our summer, the north pole inclines towards the sun, 

 causing long and warm days ; although the earth is then farthest 

 from the sun ; but in our winter the case is the reverse. Thus, the 



* This idea is illustrated by Ptolemy's Map of the World, (Plate V.), the copy 

 of which here inserted is taken from the one in Cernotis' translation of Ptole- 

 my's Geography, published at Venice, in 1598. 



