[ 100 ] ' 
obfervations be as much depended upon as the fore 
oblervations. The caufes of this feem to have been 
principally thefe two, the difficulty of adjufting the 
back horizon glafs, and the want of a method of 
direding the fight parallel to the plane of the qua- 
drant. The back horizon glafs, like the fore-one, 
requires two adjuftments : the firft, or common one, 
difpofes it at right angles to the index glafs, when 
the index ftands at (o) upon the arch ; which is ufu- 
ally performed by fetting (o) of the index of the arch 
of the quadrant by double the dip of the horizon of 
the fea, and then holding the quadrant vertical with 
the arch downwards, and turning the back-horizon 
glafs about, by means of its lever or perpetual fcrew, 
till the refleded back horizon appears to coincide 
with the fore-horizon feen diredly. Rut this ope- 
ration is fo difficult in pradice with the back-horizon 
glafs wholly filvered, except a fmall tranfparent llit 
in the middle, as it has been ufually made, that few (if 
any) perfons have ever received proper fatisfadion 
from it. If the back-horizon-glafs was filvered in 
every refped like the fore-horizon-glafs (which it 
•ught to be) the upper part being left unfilvered, and 
a telefcope was applied to it, perhaps this adjuftment 
might be rendered fomewhat.eafier and more exad ; 
but it could not even thus be made fo exad as the 
adjuftment of the fore-horizon-glafs may, by making 
ufe of the Sun’s limbs. 
The fecond adjuftment of the back-horizon- 
glafs, in the common conftrudion of the quadrant, 
is ftill more troublefome, fince it cannot be exe- 
cuted without fetting the index 90 degrees off; 
the arch, in order to place the index-glafs par— 
'I* ' allel. 
