EQUATORIAL. 
bright, The apparatus for corred! ng the error in alti¬ 
tude occafioned by refradion, which is applied to the 
eye-end of the telefcope, confifts o.f a Aide G, moving in 
a groove or dove-tail, and carrying the feveral eye-tubes 
ot the telefcope, on which Hide there is an index corre- 
fponding to five frnall divifions engraved on the dove-tail; ■ 
it has a very frnall circle, called the refradion circle H, 
moveable by a finger-ferew at the extremity of the eye- 
end of the telefcope ; which circle is divided into half 
minutes, one whole revolution of it being equal to 3' 18", 
and by its motion it raifes the centre of the crofs-hairs on 
a circle-of altitude ; and alfo a quadrant I, of i£ inch 
radius, with divifions on each fide, one expreffing the 
degree ot altitude of the objed viewed, and the other 
exprefiing the minutes and feconds of error occafioned 
by refradion, corref ponding to that degree of altitude. 
T. o this quadrant is joined a frnall round level, K, which 
is adjutted partly by the pinion that turns the whole of 
this apparatus, and partly by the index of the quadrant; 
for which purpofe the refradion circle is fet to the fame 
minute, &c. which the index points to on the limb of the 
quadrant; and if the minute, See. given by the quadrant, 
exceed the 3' 18" contained in one entire revolution of 
the refradion circle, this mu ft be fet to the excefs above 
one or more of its entire revolutions; then the centre of 
the crofs-hairs will appear to be raifed on a circle of alti¬ 
tude to the additional height which the error of refrac¬ 
tion will occafion at that altitude. 
The principal adjuftment in this inftrument, is that of 
making the line of collimation to deferibe a portion of an 
hour circle in the heavens: in order to which, the azi¬ 
muth circle mull be truly level; the line of collimation, 
or fome corresponding line reprefented by the frnall brafs 
rod M parallel to it, mull be perpendicular to the axis 
of its own proper motion ; and this laft axis muft be per¬ 
pendicular to the polar axis. On the brafs rod M there 
-is occafionally placed a hanging level N, the ufe of which 
will appear in the following adjuftment: The azimuth 
circle may be made level by turning the inftrument till 
one of the levels he parallel to an imaginary line joining 
Swo of the feet ferewsj then adjuft that level with thefe 
879 
two feet ferews; turn the circle 180 0 , or half round; 
and if the bubble be not then right, corred half, the error 
by the ferew belonging to the level, and the other half 
error by the two foot ferews, repeating this operation till 
the bubble come right; then turn the circle 90^ from 
the two former pofitions, and fet the bubble right, if it 
be wrong, by the foot ferew at the end of the level; 
when this is done, adjuft the other level by its own ferew, 
and the azimuth circle will be truly level. The hanging 
level muft then be fixed to the brafs rod by two books 
of equal length, and made truly parallel to it : for this 
purpofe, make the polar axis perpendicular or nearly per¬ 
pendicular to the horizon; then adjuft the level by the 
pinion of the declination femicircle : reverie the level, 
and if it be wrong, corred half the error by a frnall fteel 
ferew that lies under one end of the level, and the other 
half error by the pinion of the declination femicircle, re¬ 
peating the operation till the bubble be right in both 
pofitions. To make the brafs rod, on which the level is 
fufpended, at right angles to the axis of motion of the 
telefcope, or line of collimation, make the polar axis 
horizontal, or nearly fo ; fet the declination femicircle to 
o°, and turn the hour circle till the bubble cotne right; 
then turn the declination circle to 90 0 ; adjuft the bubble 
by raifing or deprefiing the polar axis (firft by hand till 
it be nearly right, afterwards tighten with an ivory key 
the focket which runs on the arch with the polar axis, 
and then apply the fame ivory key to the adjlifting ferew 
at the end of the faid arch till the bubble come quite 
right) ; then turn the declination circle to the oppoftte 
90 0 ; if the level be not then right, corred half the error 
by the aforefaid adjufting ferew at the end of the arch, 
and the other half error by the two ferews that raife or 
deprefs the end of the brafs rod. The polar axis remain¬ 
ing nearly horizontal as before, and the-declination femi¬ 
circle as o°, adjuft the bubble by the hour circle ; then 
turn the declination femicircle to 90 0 , and adjuft tlie bub¬ 
ble by raifing or deprefiing the polar axis ; then turn the 
hour circle 12 hours; and if the bubble be wrong, cor¬ 
red half the error by tlie polar axis, and the other half 
error by the two pair of capftan ferews at the feet of the 
two fupports on one fide of the axis of motion of the tele¬ 
fcope; and thus this axis will be at right angles to the 
polar axis. The next adjuftment, is to make the centre 
of the crofs-hairs remain on the fame object, while the 
eye-tube is turned quite round by the pinion of the re¬ 
fradion apparatus: for this adjuftment, fet the index on 
the Hide to the firft divifion on the dove-tail; and fet the 
divifion marked 18" on the refradion circle to its index; 
thert look through the telefcope, and with the pinion 
turn the eye-tube quite round ; then if the centre of the 
hairs does not remain on the fame fpot during that revo¬ 
lution, it muft be correded by' the four final 1 ferews, 
two and two at a time, which will be found upon un- 
ferewing the neareft end of the eye-tube that contains the 
firft eye-glafs; repeating this correction till the centre of 
the hairs remain on the fpot looked at during a whole re¬ 
volution. To make the line of collimation parallel to 
the brafs rod on which the level hangs, fet the polar axis 
horizontal, and the declination circle to 90 0 , adjuft the 
level by the polar axis; look through the telefcope on 
fome diftant horizontal objed, covered by the centre of 
the crofs hairs : then invert the telefcope, which is done 
by turning the hour circle half round ; and if the centre 
of the crofs-hairs does not cover the fame objed as be¬ 
fore, corred half the error by the uppermoft and lower- 
raoft of the four frnall ferews at the eye-end of the large 
tube of the telefcope; this corredion 1.. give a fecond 
objed now covered by the centre of the hairs, which 
muft be adopted inftead of the firft objed; then invert 
the telefcope as before; and if the fecond objed be not 
covered by the centre of the hairs, corred half the error 
by the fame two ferews as were ufed before: thjs cor¬ 
redion will give a third objed, now covered by the 
centre of the hairs, which muft be adopted inftead of the 
fecond 
