geoghaphy. 
■^enly bodies always rife and fet ^rerpehdicularly ; and 
as the horizon cuts all the circles in half, the days arid 
nights will be equal throughout the whole year. Hence 
this pofition is called a right fjjhere: for under the equa¬ 
tor tire liars muft every day appear to rife out of the 
horizon, then to afeend perpendicularly for fix hoursi 
and defeend perpendicularly lor the fix following hours, 
fet and remain below r-he horizon twelve hours fuccef- 
fively. The planets do the fame, there being no dift'e- 
rence between them and the fixed liars, but that the 
latter always deferibe the fame parallel; whereas the 
parallels deferibed by the former differ every day, being 
greater or lefs, according to their dillance iroih the 
equator. 
The parallel fpkere. —This Ihews the fituation of tlte 
heavens to an inliabitant who lives under the poles. 
Bring the pole of the armillary fphere into the zenith, 
as Ihewn in fig. i; in this pofition the axis is at riglit 
angles to the horizon, the equator and horizon coincide, 
and the tropic and polar circles are parallel thereto ; 
here there will be but one day and night throughout 
the whole year : for the fame reafon, the moon, in half 
her monthly revolution, will never rife, and in the other 
half will never fet. The fixed itars will every day de¬ 
feribe circles parallel to the horizon, fome never riling. 
Others never fetting. Hence, alfo, an inhabitant of the 
north pole has the fun above his horizon, and therefore 
perpetual all the lime the fun is on the north fide of the 
equator, that is, for fix months together. But the fun 
is below his horizon, and it is night with him all the 
time the fun is on the fouth fide of the equator, which 
is alfo for fix months : or, in other words, the fun will 
be feen for half a year, and then it will be day ; and it 
will be hidden for half a year, and then it will be night. 
The oblique fphere. —We in England are fituated in an 
oblique fphere. This pofition agrees with all the in¬ 
habitants who live neither under the poles, nor under 
the equator ■ here the equator, and all the circles pa¬ 
rallel to it, make oblique angles with the horizon. It 
is evident, that in this fituation, all the parallels to the 
equator are divided by the horizon into two unequal 
parts, but the equator into two equal parts ; confe- 
quently, the day and night are never equal to an inha¬ 
bitant in an oblique fphere, but when the fun is in the 
equator, that is, twice a-year, on the 20th of March, 
and the zzd of September. All the reft of the year the 
days are either longer or ftiorter than the nights; and 
the fun, which always appears to move in the ecliptic, 
deferibes one of the parallels to the equator, which are 
all Cut by the horizon into two unequal parts. On the 
northern fide of the equator, the days are longer than 
the nights, as long as the fun is on the north fide of the 
.equator ; but the nights are longer than the days, when 
the lull is to the Ibuth of the equator. 
The portion of tlio parallels above the horizon is 
greater in proportion as they are nearer the elevated 
pole; but when the diftance of the parallel from the 
pole becomes lefs than the elevation of the pole, then 
that parallel, and all thofe which are included within 
it, are wholly above the horizon, no part of them fet¬ 
ting or paifing under it. The contrary happens in the 
parallels that are fituated towards the deprelfed pole, a 
fmaller portion of thefe being above the horizon, and 
the greater part lying under it. Thofe parallels which 
are nearer the deprelfed pole, than the elevation of the 
pole, or latitude of the place, remain perpetually, to¬ 
gether with the ftars included within them, under the 
horizon, and are never vifible to us. 
The arc of the equator intercepted between the be¬ 
ginning of Aries, and a ftar, &c. v/hen on the meridian, 
is its right afeenfon. It is fo called, becaufe in a right 
fphere, that point of the equator, which rifes with a 
ftar, &c. comes alfo to the meridian with it. Hence we 
find the right afeenfion of a ftar, &c. by bringing it to 
tlie meridian; for that point of the equator which 
Vor,.VIII. No, 508. 
. 3.57 
comes to the meridian with it, is its right afeenfion. 
Tlie right afeenfion of a fixed ftar is always the fame; 
but that of a planet varies. The arc of the equator 
intercepted between the beginning of Aries and the 
point of the equator, rifing or felling along with any 
heavenly body, is the oblique afeenfon or defeerffwn. The 
difference between the right and oblique defeenfion, is 
the afcenfonal difference. Thus many problems, folved 
by the globes, may be performed equally corredl with 
the armillary fphere. 
In the centre of the fphere is placed a fmall globe of 
the earth, fupported by an axis, the ends of which go 
through the fphere in the places of the celeftial poles. 
It is fo contrived, that we can either turn the globe 
round within the fphere, or turn the fphere round tiie 
globe; thus this iuftrument exhibits the real motion of 
the earth round its axis within the fphere of tlie hea¬ 
ven, or the apparent motion of the heaven round the 
earth ; and hence it ftiews the correfpondence between 
the terreftrial and celeftial fphercs; and that the ap¬ 
pearances of the heavenly bodies would be the fame td 
us, whether they moved round the earth as they appear 
to do, and the earth ftood ftill; or they ftand ftill, and 
the earth is carried round the contrary way. 
By the armillary fphere it is allb manifefted, that the 
poles of the earth, when extended, reach the poles of 
the heavens. By placing a fmall patch on the different 
circles of the fphere to reprefent ftars, we ftiall per¬ 
ceive, that thofe wliich are fiirtheft from the poles will 
deferibe the greateft circles ; and thofe will deferibe tJic 
largeft polfible circles that are fituated in the equator, 
which is equi-diftant from both poles. We Hkewile 
perceive, tliat a ftar has acquired its greateft elevation 
when it comes to the upper I'emicircle of the meridian, 
and its greateft depreflion when it is at the low'er circle 
of the meridian ; and that the arc of its apparition is 
bifeCfted at the meridian. 
Of the globes. 
Though the celeftial and terreftrial globes are juftly 
allowed to be the beft artificial inftruments for combin¬ 
ing a general knowledge of aftronomy and geography* 
yet tliere are imperfeeftions in the common globes, which 
often tend to confufe a learner; but which it has been 
recently propol'ed to remedy by an improved ftrufture. 
It is our province to confider the merits and demerits of 
both. A pair of globes, on the improved principle, 
are reprel’ented in the Geography Plate I. fig. 2 and 3 ; 
and their advantages might be ealily pointed out, by'' 
comparing them with thofe of the common conftruction. 
To reftify tlie commonglobes to any particular latitude, 
the axis of the earth muft be fhifted from one falfe po¬ 
fition to another, by which the mind of the pupil is li¬ 
able to be confitfed, fince he with difficulty conceives 
that the axis of the earth never varies its pofition, but 
always preferves the fame inclination to the plane of its 
orbit. Again, the broad circle of tlie common globes 
is defigned to reprefent the ecliptic and the horizon; 
but on exajiiination it reprefents neither the one nor the 
other correttly. The ecliptic is the apparent path of the 
fun, to wliich the etirtli’s axis always makes an angle of 
66| degrees ; yet by fliifting the axis of the globe to 
rectify it for the latitude, this circle can never be in its 
true pofition as ecliptic, except when the axis is at 66^ 
degrees, and, confequently, can only then be ufed qs the 
ecliptic. Next let us confider it as the horizon. Now, 
as we have Ihewn above, every place is always in lift 
zenith of the horizon, and tlie place and horizon always' 
move together; but in the common globes, the broad 
circle is only the horizon in one particular fituation, 
that is, when the place is in the zenith ; for after having 
rectified the globe to the latitude, the moment you 
move the globe, the broad circle is no longer tli.e ho¬ 
rizon. Thus it i,s plain, that this circle,' fn many in- 
llances, cannot with propriety be confidered either as an 
4 Y liqiizoa 
