GEOGRAPHY. 
S5i 
at the meridian of any place which lies to the weft of 
London, and later than at the meridian of any place to 
the eaft of London : that is, fince it is noon at any place 
when the fun is in its meridian, it will be noon at Lon¬ 
don Iboner than at places weft, and later than at places 
eaft, of it. For example, if any place lies 15 degrees 
eft of London, that is, has 15 degrees, of eaftern longi¬ 
tude from London taken as the firft meridian, the fun 
will be one hour fooner at its meridian than at the me¬ 
ridian of London; for, fince the fun every day appears 
to make a complete r-volution from any meridian to the 
fame, in twenty-four hours, it will in every hour deferibe 
a 24th part of the circle, that is, 15°. And fince a mi¬ 
nute of a circle is a 60th part of a degree, and a fecond 
of a circle a 60th part of a minute, and 15' the 60th part 
of ij°, and 15" the 60th part of 15', the fun will move 
at the rate of 15' in every 60th part of an hour, and 15" 
in every 60th part of a minute, that is, in every minute 
or fecond of time. Confequently, it will be noon one 
minute or one fecond fooner at a place which is 15' or 
15" eaft of London, than at London. 
51. The dift'erence of longitude at two places maybe 
foutid by obferving, at the fame time from both places, 
forne inftantaneous appearance in the heavens.—If the 
ecjipfe of Jupiter’s innermoft fatellite, on the inftant of 
its immerlioa into the lhadow of Jupiter, be obferved by 
two perlons at difl'erent places, it will be feen by both at 
the fame inftant. But if this inftant be half an hour, 
for example, fooner at one place than at the other, be- 
caufe the places dift'er half an hour in their reckoning 
of time, their difference of longitude is 7® 30'.—From 
tables of eclipfes corredfly calculated for any place, the 
lo ngitude of any place may be found by one obferver. 
But fuch obfervations can only be made with certainty 
by land, on account of the motion of a fhip at fea. In 
order to determine accurately the longitude at fea, it is 
neceflary to have a clock which ftiall not be fenfibly 
aft'edted by difference of climate, dift'erence of gravity 
at different places, or the motion of the Ihip, Such a 
clock fet for the meridian of London would conftantly 
fliew the hour of the day at London, which it is eafy to 
compare with the hour of the day where the ftiip is, 
found by obfervations on the fun or ftars. 
52 Thofe who live in oppofite femicircles of the fame 
meridian^ but in the fame circle of latitude, have oppo¬ 
fite hours of the day, but the fame feafons.—Being both 
■on the fame fide of the equator and at the fame diftance 
from it, when the fun’s declination makes it funimer or 
winter in one of the places, it will be the fame at the 
other: but becaufe they are diftant from each, j8o de¬ 
grees of longitude, when it is noon at one place it will 
be midnight at the other : thefe are called periaci. 
53. Tliofewho live in oppofite circles of latitude, but 
in the fame fcmicircle of the meridian, have oppofite 
feafons of the year, but the fame hour of the day,— 
"When the fun has declination towards the north pole, 
it will be fummer to thofe who live in the northern cir¬ 
cle of latitude, and winter to thofe who live in the fouth- 
ern circle of latitude. But, having the fame longitude, 
their hours of tlicday will be the fame : thefe are called. 
antaid. 
34. Thofe who live in oppofite circles of latitude and 
oppofite femicircles of tlie meridian, have both oppofite 
feafons of the year, and oppofite hours of the day.—Be¬ 
caufe they are in oppofite latitudes, they will have op- 
yofite feaidns ; and becaufe they are in oppofite femicir¬ 
cles'of the meridian, they will have noon when it is mid¬ 
night at the other ; thefe are called antipodes. 
. 35. Twelve fecondaries to the celeftial equator being 
conceived to be drawn at equal diftances from each 
Other, that is, dividing the equator into twenty-four 
equal parts, and the meridian of any place being made 
one of thefe I'econdaries, they are called kour-circks of 
that place. 
56. If the celeftial fphere had an opake axis, the 
fliadow of the axis would always be oppofite to th.e fun; 
and when the fun was on one fide of any hour-circle, the 
fliadow of the axis would fall upon the oppofite fide 
of the fame hour-circle.—For all the hour-circles being 
fecondaries to the equator, pafs through the poles, and 
the celeftial axis is in the plane of every hour-circlf. 
And the fliadow of any opake body, being oppofite to 
the fun, is in the fame plane with the fun. Therefore 
in whatever hour-circle the fun is, the fliadow of the 
fuppofed opake axis would be in the plane of that circle 
and oppofite to the fun, that is, while the fun is in one 
femicircle of any hour-circle, the fliadow of the axis 
would fall upon the oppofite femicircle.—Hence, as the 
fun performs its apparent courfe from eaft to weft, the 
fliadow of the fuppofed axis would move from weft to 
eaft.—The gnomon of a fun-dial reprefents this fuppofed 
axis, and therefore its fliadow is a meafure of time. 
In every fun-dial the gnomon, when fixed, is parallel 
to the earth’s axis. Now when the fun is in the meri¬ 
dian of any place, the twelve o’clock hour-circle is per¬ 
pendicular to the plane of the horizon, and the arc from 
the pole to this plane is equal to the latitude of the 
place ; and the one o’clock hour-circle makes an angle 
at the pole with it of 15®, and forms the hypothenufc 
of a right-angled triangle to the above perpendicular, 
and the bafe is the arc meafuring the angle between 
twelve and one o’clock ; therefore we have, by Spheri¬ 
cal Trigonometry, Rad : Sin. L : : tan. 13° : tan. of the 
hour-angle between twelve and one o’clock. If inftead 
of 13, we fubftitute 30, 45, See. we get the angles be¬ 
tween twelve and 2, 3, Sec. o’clock; the-fame may be 
done for the half-hours or other divifions.—The rational 
andy^?(y?iJ/e horizons are, in this cafe, fuppofed coyici. 
debt, which, on account of the fun’s great dillance, will 
not occafion any fenfible error. 
In a vertical fouth-dial, wemuft conceive a plane paf- 
fmg through the centre of the earth perpendicular both 
to the horizon and meridian; and on the fouth fide, lines 
arc drawn from the centre to the points where the hour- 
circles cut that plane. In finding thefe points, we fay, 
as Rad : co. f. Lat : : tan. i3®;'taii. of the hour angle 
between twelve and one o'clock. For the arc of the 
meridian, from the pole to the plane, is equal to the 
complement of latitude. The other hour-angles. Sec, 
muft be obtained in the fame way as in the laft.—Sec 
Ho RO.LOG Y. 
37. The orbit inwliich the earth revolves about the 
fun, is elliptical. It is known from obfervation, that 
the apparent motion of the fun, that is, the real mo¬ 
tion of the eartli, in the ecliptic, is not uniform. BiU 
by the univerfal lav/ of bodies revolving about a.centre, 
if its orbit were circular, its velocity muft be uniform ; 
fince it muft deferibe equal areas in equal times. Where¬ 
as if its orbit be an ellipfe, and the fun be placed in one 
of the foci, the fame law will require that its velocity 
Ihoutd not be uniform, but that in palling through its 
greateft diftance, to its leaft diftance, it fliould be acce¬ 
lerated, and in paffuig from the leaft diftance to the 
greateft, it fliould be retarded. Since then the motion 
of tire earth is in fact thus retarded and accelerated in 
different parts of its orbit, it is manifeft that its orbit is 
elliptical. 
58. A part of the earth’s furface, bounded by two 
leffercircles parallel to the equator, and of fuch a breadth 
a.s that the iongeft day in the parallel nearer tlie pole 
exceeds the Iongeft day in that next the equator, by 
fome certain fpace, as iialf an hour, or an hour, or a 
month, is called a climate. The beginning of a climate, 
is a parallel circle in wiiich tlte day is the fliorteft; and 
the end of the climate, is that in which the day is the 
Iongeft. Tlie climates therefore are reckoned from the 
equator to the pole ; and are fo many zones or bands, 
teriuuiated by lines parallel to the equator : though, in 
llrittnefa. 
