S50 GEOGRAPHY. 
be north, fet off w I etjtial to it, and I is the place re¬ 
quired; but if the latitude be fouth, fet oft'mV equal 
to it, and V is the place. If the longitude be. weH:, fet 
oil' a 71 equal to it, and take cG, or aW, equal to the 
latitude, according as it is north or fouth, and G, orW, 
will be the place. Thus, all the places upon the furface 
ol the eartli, whofe latitudes and longitudes are known, 
m y be laid dotvn accurately upon a globe or map; and 
the boundaries of the different countries may be traced 
out, and each exhibited in its proper fituation and figure. 
By meatis of a globe, therefore, we may get a perfedt 
idea of the relative magnitudes, figures, and fituations, 
of all the countries of the earth, and of the fituations 
of all the principal places in them; but a map, being a 
plane furface, cannot fo corredfly reprefcnt the propor¬ 
tions, boundaries, and pofitions, of places. The deter- 
mitiatiOn of the latitude and longitude is therefore elfen- 
tial to geography, and confequently to navigation. 
The arc G v contains the fame number of degrees 
as the ;irc am; the degrees of longitude therefore be¬ 
tween any two places, when meafiircd upon a Imall 
circle parallel to the equator, dintinilh as that circle 
approaches the pole. The arc am contains the fame 
number of degrees as the angle a P m; hence, the angle 
formed by the meridians palling througli any two places, 
is .the meafure of the difference of tlie longitudes of 
thole places. 
II. Tfie altitude of one pole, and the deprejion of the 
other, at any place on the earth’s furface, is equal to 
the latitude of that place.—For let R, in the Geogra¬ 
phical Plate II. fig. I, be a place upon the earth’s fur- 
face; Z N, its zenith and nadir; PS, the poles of the 
heavens, and Fs, the poles of the earth ; EF-, the ce- 
leftial equator, ee, the terrelfrial equator, and IIO, the 
horizon. The latitude of the place is f R, or the equal 
arc E Z ; P O, is the elevation of one pole ; and H S, the 
deprellion of the other. Becaufe Z O, being the dilhince 
of tile zenith from the horizon, is an arc of 90 degrees; 
and becaufe E P, being the diltance of the pole from the 
equator, is alfo an arc of 90 degrees : Z O, and E P, are 
therefore equal. Take from each of thefe the common 
'arc Z P, and the remainders, EZ and PO, are equal. 
But ITS and PO are alio equal, becaufe they fubtend 
the equal angles HTS, PTO : therefore the elevation 
of one pole P O, and the depreflion of the other IT S, are 
equal to the latitude of the place EZ.—Hence tjie cir¬ 
cumference of the earth may be meafured, by mealuring 
the lengtli on the furface of the earth palled over in a 
line which lies north and foiith, while the pole gains one 
degree of elevation, and multiplying this lengtli by 360. 
A degree of latitude contains 6 ^^ Englifli miles, whence 
a.;.930 miles is the meafure of tlie circumference of the 
eartJi, and the radius 3985; the earth being a fpheroid, 
whole polar diameter is to the equatorial as 229 to 230. 
12. The elevation of the equator, at any place, is 
equal to the complement of its latitude.—Becaufe Z O, 
fig. I, is equal to E P, (each being an arc of 90 degrees) 
E Z is equal to P O ; that is, to the latitude of the place. 
But EH, the elevation of the equator, is the comple- 
Kiei'.t of E Z ; it is therefore equal to the complement of 
the latitude of the place. 
13. The etirth revolving daily round its axis from 
weft to eaft, the heavenly bodies will appear to a fpec- 
tator on the earth to revolve in the fame time from eaft 
to weft. Let RGB F, fig. i, reprcfent the earth, T its 
center, HT O, the rational horizon to a fpefilator at R, 
whofe zenith is Z ; let a ftar appear in the horizon at 
IT. The earth revolving from weft to eaft, that is, in 
the order of the letters R, C, B, F, in a fourth part of 
«ne revolution, tlie fpebtator will be carried from R to 
G : confequently, his horizon will become Z H, and the 
ftar which appeared in his horizon at H, when he was at 
R, will now appear nearly in the zenith. When another 
fourth part of the revolution is completed, the fpebtator 
wm be at B, and IS bei’ig now his zenith, and ii O, his 
i 
horizon, the ftar will be fet with refpefl to him, and 
will not rife till he is again in the ftation R, that is, till 
the earth has completed one revolution. Thus whilft 
the earth has turned once round upon its axis from weft 
to eaft, all the heavenly bodies in the concave fphere of 
the heavens will appear to have turned round from eaft 
to weft. 
14. The alternate fucceftion of day and night is the 
effedt of the revolution of the earth round its axis. For, 
all the heavenly bodies appearing to move from eaft to 
weft, while the earth revolves from weft to eaft, the fun 
will appear, in each revolution, to rife above the hori¬ 
zon in the eaft, and after deferibing a portion of a circle, 
to fet in the weft, and will continue below the horizon, 
till by the revolution of the earth it again appears in the 
eaft; and thus day and night will be alternately pro¬ 
duced. The time of noon may be conftantly found, by 
obferving the inftant when the center of the fun is cut 
by the perpendicular hair in a meridian telefcope, or by 
a corredl fun-dial. 
15. The earth revolving round the fun in 365 days, 
6 hours, 56 minutes, 4 feconds, the fun appears to re-- 
volve round the earth in the fame time, but in the con¬ 
trary direclion.—It is manifeft that the circle in which 
the i'un thus appears to move, is the fame in which the 
earth would appear to move to a fpedtator in the fun- 
Hence the apparent place of tiie fun being found, the 
true place of the earth in its orbit is known. But the 
orbit in which the earth revolves round the fun is not a 
circle, but an ellipfe, having the fun in one of its foci. 
This circle, which the I'un appears to deferibe annually 
in its progrefs through the concave fphere of the heavens, 
is called the ecliptic. 
16. A portion of the heavens, about fixteen degrees 
in breadth, througli the middle of which pafles tite 
ecliptic, is called the zodiac; in which lie the orbits of 
all the planets.—Tlie (tars in the zodiac, or ecliptic, 
are divided into twelve Jigns., Aries, Taurus, Gemini, 
Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Ca¬ 
pricorn, Aquarius, Pifees. The figures, reprefenting 
thefe (igns, are drawn upon the celeftial globe, in that 
portion of its fpherical (iirface, which correfponds to 
the portion of the concave Iphere of the heavens, in 
which the ftars belonging to each fign are rerpetfively 
placed.—See the article Astronom r, vol.ii. p. 326, 
and 414. 
17. Tl’.e axis of the earth in every part of the earth’s 
revolution about the fun, makes, with tlte plane of its 
orbit, tluit is, of the ecliptic, an angle of 66 a degrees. 
This is illuftrated in the Geography Plate II. fig. 2 : 
Let B A reprefent the plane of the ecliptic or earth’s 
orbit, feen edgeways; S, the lun ; and Pp produced, 
the axis of the equator. If the earth be at S, its axis 
is not perpendicular to the plane of the ecliptic, but 
makes an angle with it, PSA, about 66° 30'. In any 
otlier part of its orbit, as at M, or X, the axis of the 
earth is ftill'inclined to the plane of the ecliptic in the 
fame angle.—It muft be noted that th^ planes of the 
equator and ecliptic, make witli each otner an angle of 
23^ degrees nearly. The obliquity of the ecliptic is not 
permanent, but is continually diminilhing, by the eclip¬ 
tic approaching nearer to a parallelifm with the equa- 
tor, at the rate of about half a fecond in a year, or from 
50" to 55" in one hundred years. The inclination, at the 
commencement of the nineteenth century, was 23° 28' 3" 
nearly. The diminution of the obliquity of the ecliptic 
to the equator is owing to the action of the other planers 
upon the earth. The whole variation, it is laid, can. 
never exceed one degree, when it v/ill again increafe.— 
The obliquity'of the ecliptic may-be thus found : Ob- 
ferve with a good inftrument, very accurately divided, 
the meridian altitude of the (tin’s centre on the days of 
the fummer folftice ; then the difference of the two will 
be the diftance betw'een the tropics; the half of which 
will be the obliquity fought,—By the fame method, the 
declination 
