4 Mr. Smea?on’s Obfervations on the 
threads in an inch, and with telefcopic fights*. Of this in- 
ftrumeut Mr. Flamsteed gives the figure at the latter end of 
his Prolegomena before-mentioned, fufficiently large to fee the 
general defign ; the whole being mounted upon a ftrong polar 
■axis of iron, of three inches diameter. 
Though, in the full defcription of this inftrument, Mr. 
Flamsteed mentioned the Limb’s being furnifhed with dia- 
gonal divifions , diftinguiftiing the arch to 10 feconds t ; yet it 
is pretty clear, that it had not thefe originally upon it ; but 
that the dependance was wholly upon the fcrew divifions, 
when it came out of Mr. Tompion’s hands. This one may 
reafonably infer from the obfervations themfelves ; for the firft: 
obfervation, fet down as taken with this inftrument, being 
upon the 29th of October, 1676, it was not till the 1 ith of 
September, 1677, that the column which contained the check 
angle by diagonal lines was filled up ; and there was alfo a 
fpace of time, antecedent to that laft mentioned, wherein no 
obfervations are recorded as taken with this inftrument, in 
which time the diagonal divifions might be put on ; and this 
will be put beyond a doubt, as he fays expreffly, that finding, 
in the year 1677, that the threads of the fcrew had worn the 
border of the limb, he divided the limb into degrees himfelf, 
and drew a fet of diagonal divifions j ; and then comparing the 
two fets'of divifions together, he fometimes found them to 
differ a whole minute ; wherefore, for correction thereof, he 
conftruCted a new table for converfion of the revolutions and 
parts of the fcrew into degrees, minutes, and feconds; and 
* u — - Qualern nemo, ccelo adhibens; — Preface to Hiitoria Cceleft. printed 
in one vol, 1712. 
f Prolegomena Hiftor. Ccelelt, vol. III. p. 104. 
;f: Ibid. p. 106. “ Gradns in limbo deflribm acdiagonalesd'uxi,^ 
which 
