DEGREE. 
angle; y ie the angle between the left-hand obje& and the 
centre of the flation s ris the diftance between the centre of 
the circle and the centre of the ftatio 
D, the diftance of the obje& to ae right. 
G, . diltance of the obje& to the left. 
If (O + y) exceed 180°, the firt term is fubtra@tive. 
If the angle y exceeds 180°, the fecond term becomes 
additive 
When the obfervation of the 2 O is terminated, the up- 
per telefcope is always directed towards the left-hand object 
d the inferior towards the right-hand obje& A.— 
Vo obferve the 2 y, the obferver fhouid pupae to keep 
the lower telefcope on the obje&t A, and let the upper one 
revclve from the right towards the left, till it ert to the 
centre 
This 
diftance ig one on) et 
other; as when 
snl in poleaatoes ie determining ‘the azimuths of fta- 
correAion will be reduced toa fingle term, when the 
‘ bec mo 
Fs 
fo) 
a 
vv 
@ 2 
7) 
5 
Bat this correGtion may become nothing without either of 
the diftances becom! ms inGnite ; by which means it is 
obferver 2 to place paca 
cafe 
Thi 
if it be on the ee which conics the given ke 
ABC. 
I G 
(it: cae - y) = 
and in the triangle A BC 
fin. A fin. A 
fin. B fin.(A+C) 
fin. y _ fin. A 
i. C 1p = ina £0) 
ae the denominators 
For then 
=o: and fince C= O 
fin. A 
~ fin. Acof.C ++ cof. A fin. C 
fin, 
tin. C col. 2 + ae Cin. ro 
fin. 7 cof. y cof. C ~ cof C+ ai A fin. C 
and tang. y = tang. A = tang. 180° + A. 
From this it appears, that if the obferver has his choice 
of fituation, he may place himfelf in fuch a manner as to 
render any reduction unueceffary ; and to do this, he mutt 
be on the circumfcribing c'rcle, or its tangent, at the point 
C, fo that the angle y may be equal to A, or its fuppie- 
ment, 
however generally happens eee the la cia which 
exclude the obferver from the cen of the fation, at the 
fame time prevent him from choofing the exact point as de- 
nds on the fine 
from the centre 
e divifions are numbered from right to left, then rigd¢ 
mutt ie fubttituted for deft, and vice verfd in the preceding 
reafoning. 
Ba has been pee how, by having the oblique angle of 
objects, , as feen from a point C, we can obtain 
he horizontal or azimutha "8 e; but it is neceflary, for 
the truth of our aeneluian hat the point C remain the 
fame = the zenith diltance ian as for the oblique 
» XI, 
angle, If we examine the conftru&ion of the repeatin 
circle, we fhall find that its centre Ci is — in ~ vertical 
to 
centre of the eircle o apply this Saat we are to 
nfider what would be the zenith diftaece of an objeG if 
the centre e inftrument were eet or de preffed a 
a: 
The correétion is as follow 
Let dH be the difference a ani of the two pofitions, 
D the diltance of the obferved fignal, 3 the zenith diftance 
required to be corrected ; then the correGted diltance will be 
ye 
D? fin. 
If the ateument is advanced os the fignal, the dif. 
r being the diftance of 
of. 
tance corrected = 3 + area “h 
the centr 
In Mr. Troughtan’s circles the centre of the circle is 
5.4, OF §.5 inches lower in the vertical than in the horizon- 
tal pofition. 
When the Signal is unequally illuminated. 
When the fignal is ay recat by the fun, the 
obferved point is aaa the axis, nor in the direGiion af the 
axis. For example. jig. 63. let abc ny be a feétion of a fignal of 
four fides, if ad or ac on is Suipeaes enlighténed to be vifi- 
ble, the obferver will dire&t the optic axis of his telefeope to 
A or B inttead of thecentre M. and the angle obferved will re- 
quire a corre€tion equa, to AOMo r BOM. etP= 
peadicular MA, A = angle AM 0, D equal the di ree 
O M in 
D fin, 4” 
This correétion is fubtraétive, if the obferved point is to 
the frighe} of the centre in the obje& to the {let t ; 
It is additive, if the obferved point is to the { ngtt 
of the centre in the obje& on the ta : 
If the fignal be a round tower or maft of fenfible dia- 
meter, the eat on is fomething longer, becaule the azi- 
the correGiion e will then be c = 
(fg. 62.) AE 
not ilursnated by the fur 
middie of the iuminated ot inftead of the centre of the 
wh 
Let N M be the meridian line, MCS the azimuth is 
A will be illuminated by th 
fun to "the ex ity A, of the enughtencd part, draw 
O A, and on the other fide the vifual ray O E, tangent to 
the tower; the vifible part SE will be unequally di- 
f x be the azimuth oft the obferver, x the 
vided by OG. 
’ the correc= 
azimuth of the fun, d= 
dfin.? 3 (x—2 
D fin. 1” 
x — 2% may de either calculated or obferved. By calcula- 
tion 
Cof. z = fin. L cof. 1 — ———— 
fin. 
lat., B the declination of the fun, and H the hour angle. 
To obfery take tn horizontal angle between 
the 
tion will be C = 
cof, Ltang. B- 
L being the 
CIVe KN -= By 
