12 Mr. Smeaton’s Obfer nations on the 
which was fixed up at Wanjlead , in j Efex, in the year 1727* 
The very fimple conftruftHon that he adopted for this inftru- 
ment (the plumb line itfelf being the index) did not admit of 
the ufe of a vernier: he therefore contented himfelf with 
dividing the arch of the limb of this inftrument by primary 
points, as clofe as he thought neceffary, that is, by divifions 
of five minutes each, and meafuring the diftance from the Jet 
of the inftrument to the next point of divifion by a micrometer 
fcrew, in the conftrudion of which fcrew he ufed uncommon 
care and delicacy. I have mentioned this inftrument to intro- 
duce this obfervation ; that I think it highly probable, had 
Mr. Graham conftrucled the great quadrant after the zenith 
fe£tor had been fully tried, he would have rejefted not only the 
diagonals but the verniers alfo, as containing a fource of error 
within themfelves which may be avoided by a well-made fcrew ** 
It feems alfo, that Mr. Graham, at the time he con- 
ftruftted both thefe inftruments, was not aware how much error 
could arife from the unequal expanfions of different metals by- 
heat or cold : for in both the radius, or frame of the inftru- 
ment, was iron, while the limbs were of brafs. They, how- 
ever, remain in the Royal Obfervatory, perfect models, in all 
other refpe&s, of every thing that is likely to be attained in 
their refpedlive deftinations, and monuments of the fuperla- 
five abilities oft that great mechanician Mr, Graham ft. 
* This has been found eonfentaneoiis to the experience of my friend Mr,. 
Aubert, who, on my fuggeftion, has long fm.ce laid a-fide the ufe of his vernier,, 
meafuring always by the micrometer fcrews the diftance between the fet of the 
inftrument, and the coincidence of the firft: ftroke of the vernier with the next 
primary divifion of the limb. 
f 1 have been informed, that Dr. Maskelyne has caufed this objection to the 
feftor to be rectified, frnce its remhv&l to the Royal Obfervatory, by fubftituting 
an iron limb iaftead of that of brafs, the points being made upon ft yds of gold.. 
Mr# 
