[ 110 ] 
and ab folate proof that the obliquity of the ecliptic 
has fen fib! y diminifhed during an interval of 2 t, and 
even of i 8 years. 
The obfer rations of 1743 were made with a mu- 
ral quadrant of five French feet, conftrudled by the 
late Mr. Siflbn : but as the linear divifions were 
found to be fomewhat lefs accurate than was 
expe&ed, and as the body of the quadrant was not 
framed with proper ftrength and folidity, Mr. Bird 
was employed in the fummer of the year 1745, by 
the Earl of Macclesfield, (the body of the inftrument 
having been ftrengthened by fcrewing a large and 
broad plate of brafs upon the crofs-bars), to put a fet of 
points upon the limb between the 90 and 96 arches 
of linear divifions. By thefe operations the line of 
collimation was found to have varied, and to be 
z= 6' ,3, by which the zenith diftances were given 
too fmall, by the pofitive divifions, from the end of 
1746 to the end of June 1751, when Mr. Bird bi- 
fedted the fpaces between the points which he had 
formerly added in 1745. But after the year 1751, 
the error of the line of collimation was — 2", 6, as 
appears from obfervations of y Perfei, and y Dra- 
conis, by which the zenith diftances are alfo given 
too fmall j and in that Rate the inftrument continued 
to the year 1 767, when a new fet of wires was put 
into the telefcope, and the line of collimation thereby 
altered. The error of the line of collimation from 
1743 to 1745 cannot dire&ly be afcertained, for 
want of zenith obfervations ; but, from fome indirect 
methods, it fihould feem that the error was as nearly 
as pofiible =2", to be added to the obfervea zenith 
diftances. 
1743 - 
