Graduation of Afr gnomical Injlruments . 3 
angle of whofe threads with the axis ftiallbe equal in every part 
of the circumference ; therefore the whole of this bufinefs (in 
which accurate mechanifs well know confifts the whole of the 
difficulty) he refers to the ingenious workman ; and, in particular* 
to the then celebrated Mr. Tompion, whom, he faj^s, he em- 
ployed to make his inftrument, and who had thereby feen and 
experienced the difficulties that do occur therein : but was any in- 
genious workman now to purfue the dire&ions of Dr. Hook, 
fo far as his communication thereof extends, we may con- 
clude, that he would make a very inaccurate piece of work, 
far inferior in performance to what the Dodtor feems to expert 
from it *. But yet, I believe, it was the jirf attempt to apply 
the endlefs ferew and wheel, or arch, to the purpofe of form- 
ing divifions for aftronomical inftruments; for, the Doctor fays 
himielf, the perfection of this inftrument is the way of making 
the divifions; that it excels all the common ways of div if on t 
and in the table of contents it is intituled, An Explication of the 
new Way of dividing . 
This method, however, of Dr. Hook’s was not laid afide 
without a very full and fufficient trial : for Mr. Flamsteed, 
in the Prolegomena of the third volume of Hiforia Ccdefis , in- 
forms us, that he contrived the fextant, wherewith his obfer- 
vations were chiefly made, from his entrance into the Royal 
Obfervatory in the year 1676 to the year 1689. This fextant was 
firft made of wood, and afterwards of iron, with a brafs limb 
of two inches broad, by Mr. Tompion, at the expence of Sir 
Jonas Moore; the radius thereof was 6 feet 9 1 inches; it 
was furniffied with an endlefs ferew upon its limb of 17 
* This was indeed verified in an attempt upon the fame plan by the Due db 
Chaulnes, published in a Memoir of the Royal Academy of Sciences at Paris, 
for the year 1765. 
B z 
threads 
