aati- of the places. When the places are on different sides 
m- of the first meridian, the sum of their longitudes gives 
their difference of longitude. : 
As the sun in his apparent diurnal revolution round 
the earth moves over 360° in 24 hours, or 15° in one 
hour, he is on the meridian of any given place, or it is 
ude, noon at that one hour sooner than at any other 
“9 place 15° farther west. Hence longitude, and diffe- 
rence of longitude, may be expressed in time, allow- 
ing 15° to one hour, or 1° tofour minutes. As the de- 
and hour are similarly subdivided, any number of 
egrees, minutes, and seconds, divided by 15, will give 
the corresponding number of hours, minutes, and se-~ 
conds; and, on the contrary, hours, minutes, and se- 
conds of time. multiplied by 15, give the correspond- 
ing number of degrees, minutes, and seconds. 
. The latitude of a place on the-earth is its distance 
from the equator measured on a meridian, or it is the 
arch of a meridian intercepted between the equator, and 
a parallel of latitude passing through the p It is 
“<1 north or south latitude, according as the place 
lies to the north or south of the equator. _The distance 
of the place from the nearest pole measured on the me- 
de. ridian is called the co-latitude of the place, or the com- 
plement of the latitude... Latitude on the earth, corre- 
sponds to declination on the celestial sphere. 
nce.“ The difference of latitude between two places, is the 
uce. distance between them measured on a meridian; or 
it is the arch of a meridian intercepted between the pa- 
rallels of latitude passing through the places. If they 
lie on opposite sides of the tor, the sum of their 
latitudes gives their difference of latitude. 
- A zone is any portion of the earth’s surface, included 
between two parallels of latitude. There are, how- 
ever, usually reckoned five zones; the Torrid, the two 
Ti te, and the two Frigid. 
The torrid zone is the space included between the two 
_ tropics: the temperate zones extend from the tropics to 
the polar circles ; and the frigid from the polar circles to 
the poles. To explain this division of the globe mathe- 
matically, we may observe, that whatever be the po- 
sition of, the sun, he always illuminates one half of the 
' terrestrial sphere at once, and that the t circle 
» > which tes the light from the dark hemisphere, 
j]. and which is called the circle of illumination, has for its 
ion. pole that point on.the surface of the earth, to which the 
sun is vertical. Now, as the sun appears to be always 
in some point or other of the ecliptic, moving over EL, 
ation. (Fig. 1.), between the middle of winter and the middle 
of summer, and from L to E on the opposite side of 
the sphere in the next half year, the phenomenon is 
> precisely the same as if he vibrated along the arch DL, 
viz. from D to L during the first of these periods, and 
from L to D during the next; the earth, in the mean 
time, revolving daily on its axis. When the sun is at 
- Q, he is vertical to g; that is, the circle of illumina- 
tion ds with a meridian ps; and every point 
of the ’s surface is successively 12 hours above and 
12 hours below that circle. When the sun advances 1° 
towards L, he becomes vertical to a point 1° to the 
north of g, or the circle of illumination falls 1° below 
p towards p’, and rises 1° above s towards r; that is, 
while the sun’s declination is 1° north, the circular 
space about the north pole to the distance of 1° from 
it, never sinks below the circle of illumination, and a 
similar space around the south pole never rises above it. 
As the sun approaches L, a space about P conti- 
nues to be permanently within the circle of illumination, 
and a corresponding space about s to be permanently 
GEOGRAPHY. 
149 
without it, When the sun reaches L, or is vertical to /, the Mathemati- 
circle of illumination falls 23° 28’ below P, and rises ag ¢! Geogra- 
far above s, and may be represented by p’g ; that is, on __ PHY: 
the day of the summer solstice, the whole of the north fri- 
gid zone is within the circle of illumination, and the 
whole of the south frigid zone is without it for 24 hours, 
From this period the sun again returns towards the 
south, when the circle of illumination also begins to 
move backwards, till at the autumnal equinox, three 
months after the summer solstice, it again occupies the 
position ps, so that for six months the north pole is 
above, and the south pole is below the circle of illu- 
mination. The sun still continuing his motion south- 
ward, reaches D by the middle of winter, when the 
circle of illumination occupies the position bf, and the 
circumstances of the frigid zones are just reversed. 
Hence, to every place within the torrid zone, the sun 
is vertical twice a year, which it never is to any other 
part of the earth. _ In the temperate zones, no place is 
either above or below the circle of illumination for 24 
hours er. And in the frigid zones, a place may 
be in the dark or light hemisphere for any period, be- 
tween 24 hours and six months. The zones might also 
be distinguished from each other physically, by the dif- 
ference in the intensity of the sun’s rays, as indicated 
in the difference of mean temperature in different lati- 
tudes. But for the investigation of this subject, we re- 
fer to Paysican Geography. . 
A climate is also a portion of the earth’s surface, in« A Climate. 
cluded between two parallels of latitude, and of such a 
breadth, that the longest day under the parallel nearest 
the pole, is half soi Habe longer than under the other. 
There are twenty-four such climates between the equa- 
tor and either of the polar circles. Between the polar Each he- 
circle and the pole, ince are six climates of such a ™misphere 
breadth, that the longest day under the two parallels oa” 
varies by a month, There are thus thirty climates in peal _ 
all on each side of the equator. 
The principle that has already been employed, in ex- Illustration. 
plaining the division of the earth into zones, may serve PLaTE 
also to illustrate the nature of climates. When the sun tc ee 
is vertical to Q, (Fig. 2.), the circle of illumination co- _— 
inciding with the meridian NS, divides the equator 
7£Q, and every parallel of latitude into two equal » 
or the day and night are equal all over the globe. When 
the sun passes to either side of Q, as northward to- 
wards L, the equator is still divided into two equal 
parts by the circle of illumination, as it must always 
be by a great circle; but the parallels of latitude are 
divided unequally, the greater arch of the northern 
parallels being above, and of the southern below, the 
circle of illumination ; that is, the day and night are 
still equal at the equator, but in northern latitudes the 
y is longer than the night, and in southern the night 
is longer than the day. This inequality. continues to 
increase in all latitudes, as the sun approaches L; and 
of two given parallels, as cd, el at any time, the ine- 
quality is greatest at that which is nearest the pole. 
When the sun reaches L, or at the summer solstice, 
the day is longest in all northern, and shortest in all 
southern, latitudes. At the polar circle it is 24 hours, 
the whole of the parallel a4 being above the circle of 
illumination. At the parallel cd, the day is to the 
night as twice p d to twice pe, or as pd to pc; at eL 
it isas gL to ge, &c.; and at the equator they are equal. 
It is obvious, therefore, that between the equator and 
the polar circle, the length of the longest day varies 
from 12 to 24 hours, and that consequently 24 parallels 
may be found at such distances from each other, that 
