412 Mr . Pigott’s Determination of 
The line of collimation was deduced from / 3 , ^and/stDraconis ; 
half of each -let obferved with the face of the quadrant to the 
eaft, and half with its face to the weft. This, as well as the 
other methods, is very tedious, particularly wheii required to 
be often repeated, as is the cafe in travelling; I ftiall therefore 
propofe the following invention, the idea of which was im- 
proved on by Mr. Sm eaton, and flatter myfelf it will prove 
of the greateft facility. 
The error of the line of collimation includes the fixed errors 
of the inftrument, and thofe that are fubjeft to change, occa- 
lioned by the wires and glaffes, &c, of the telefcope moving* 
The error of thefe laft may be found by making the telefcope 
turn on its center, fo that the fun, ftars, or terreftrial objects 
may be obferved on the horizontal wire in two manners ; firft, 
when the wire is in its natural pofition, and then inverted, 
which is performed by turning the telefcope 180 degrees, or 
half round : thus, this part of the error can always be known 
with the greateft eafe ; and in order to find the fixed errors, it 
is requifite for a Jingle time to get the whole error of the line of 
collimation by one of the common methods, from which the 
error of the telefcope being dedudled, the fixed errors become 
known ; and as they are unchangeable, if any alteration fhoulcl 
take place, it proceeds from the telefcope, and may eafily be 
detected as fhewn above. Perhaps, inftead of the whole tele- 
fcope, it would be fufficient only to make that part turn con- 
taining the eye-glafs and wires. 
