GEOGRAPHY. 



by means either of a plumb line, or of a 

 spirit level, we may ascer.ain the angular 

 situation of any part of the earth's sur- 

 face with respect to a fixed star passing 

 the meridian : by going a little further 

 north or south, and repeating- the obser- 

 vation on the star, we may find the differ- 

 ence of the inclination of the surfaces at 

 both points; of course, supposing the 

 earth a sphere, this difference in latitude 

 will be the angle, subtended at its centre 

 by the given portion of the surface, 

 whence the whole circumference may be 

 determined, and on these principles the 

 earliest measurements of the earth were 

 conducted. The first of these which can 

 be considered as accurate, was executed 

 by Picart, in France, towards the end of 

 the seventeenth century. 



But the spherical form is only an ap- 

 proximation to the truth. It was calcu- 

 lated by Newton, and ascertained expe- 

 rimentally by the French academicians, 

 sent to the equator and to the polar cir- 

 cle, that, in order to represent the earth, 

 the sphere must be flattened at the poles, 

 and prominent at the equator. We may 

 therefore consider the earth as an oblate 

 elliptic spheroid; the curvature being 

 greater, and consequently e"very degree 

 shorter at the equator, than nearer the 

 poles. If^the density of the earth were 

 uniform throughout, its ellipticity, or the 

 difference of the length of its diameters, 

 would be j^- of the whole ; on the other 

 hand, if it consisted of matter of incon- 

 siderable density, attracted by an infinite 

 force in the centre, the ellipticity would 

 be only -j-1^ ; and whatever may be the 

 internal structure of the earth, its form 

 must be between these limits, since its 

 internal parts must necessarily be denser 

 than those parts which are nearer the 

 surface. If, indeed, the earth consisted 

 of water or ice, equally compressible 

 with common water or ice, and following 

 the same laws of compression with elastic 

 fluids, its density would be several thou- 

 sand times greater at the centre than at 

 the surface; and even steel would be 

 compressed into one-fourth of its bulk, 

 and stone into one-eighth, if it were con- 

 tinued to the earth's centre : so thatthere 

 can be no doubt but that the central parts 

 of the earth must be much more dense 

 than the superficial. 



Whatever this difference may be, it 

 has been demonstrated by Clairaut, that 

 the fractions expressing the ellipticity, 

 and the apparent diminution of gravity 

 at the equator, must always make toge- 



at me equ 



ther _|^ ; and it has been found, by the 

 most accurate observations on the lengths 

 of pendulums in different latitudes, that 

 the force of gravity is less powerful by 

 __i at the equator than at the po\e f 

 whence the ellipticity is found to be _ 

 of the equatorial diameter; the form be- 

 ing the same as would be produced, if 

 about three-eighths of the whole force of 

 gravity were directed towards a central 

 particle, the density of the rest of the 

 earth being uniform. 



This method of determining the gene- 

 ral form of the earth is much less liable 

 to error and irregularity, than the mea- 

 surement of (he lengths of degrees in 

 various parts, since the accidental varia- 

 tions of curvature produced by local dif- 

 ferences of density, and even by superfi- 

 cial elevations, may often produce con- 

 siderable errors in the inferences which 

 might be deduced from these measure- 

 ments. For example, a degree measured 

 at the Cape of Good Hcpe, in latitude 

 33 south, was found to be longer than 

 a degree in France, in latitude 56 north, 

 and the measurements in Austria, in 

 North America, and in England, have all 

 exhibited signs of similar irregularities. 

 There appears also to be some difference 

 in the length of degrees under the same 

 latitude, and in different longifudes. We 

 may, however, imagine a regular elliptic 

 spheroid to coincide very nearly with 

 any small portion of the earth's surface, 

 although its form must be somewhat dif- 

 ferent for different parts : thus for the 

 greater part of Europe, that is, for Eng- 

 land, France, Italy, and Austria, if the 

 measurements have been correct, this 

 oscillating spheroid must have an ellip- 

 tictyof^-. 



The earth is astronomically divided 

 into zones and into climates. The tor- 

 rid zone is limited by the tropics, at the 

 distance of 23 28' on each side of the 

 equator, containing all such places as have 

 the sun sometimes vertical, or imme- 

 diately over them: the frigid zones are 

 within the polar circles, at the same dis- 

 tance from the poles, including all places 

 which remain annually within the limit 

 of light and darkness, for a whole diurnal 

 rotation of the earth, or longer : the tem- 

 perate zones between these, have an 

 uninterrupted alternation of day and 

 night, but are never subjected to the 

 sun's vertical rays. At the equator, there- 

 fore, the sun is vertical at the equinoxes, 

 his least meridian altitude is at the sol- 

 stices, when it is 66 32', that is, more 



