VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 31 



Hook, in his animadversions on the Machina Coelestis of Hevelius, published in 

 the year 1 674, has given an elaborate description of a quadrant, whose divisions 

 were formed, and afterwards read off, by means of an endless screw, working 

 on the outermost border of the limb of a quadrant ; which, he says, " does 

 not at all depend on the care and diligence of the instrument-maker, in 

 dividing, graving, or numbering the divisions, for the same screw makes it 

 from end to end ;" yet he has given us no account of any particular care 

 or caution that he used, in preventing the same screw from making larger or 

 smaller paces, in consequence of unequal resistance, from a different hardness of 

 the metal in different parts of the limb ; nor any method of correcting or check- 

 ing the same ; nor of making a screw, the angle of whose threads with the axis 

 shall be equal in every part of the circumference ; therefore the whole of this 

 business (in which accurate mechanists well know consists the whole of the dif- * 

 ficulty) he refers to the ingenious workman ; and in particular to the then cele- 

 brated Mr. Tompion, who he says was employed by him to make his instru- 

 ment, and who had thereby " seen and experienced the difficulties that do occur 

 therein :" but was any ingenious workman now to pursue the directions of Dr. 

 Hook, so far as his communication extends, we may conclude that he would 

 make a very inaccurate piece of work, far inferior in performance to what the 

 doctor seems to expect from it. But yet I believe it was the first attempt to apply 

 the endless screw and wheel, or arch, to the purpose of forming divisions for 

 astronomical instruments ; for the doctor says himself that the perfection of this 

 instrument is the way of making the divisions ; that it " excels ail the common 

 ways of division :" and in the table of contents it is entitled, " An Explication of 

 the new Way of dividing." 



This method however, of Dr. Hook's, was not laid aside without a very full 

 and sufficient trial : for Mr. Flamsteed, in the Prolegomena of the 3d volume 

 of Historia Coelestis, informs us, that he contrived the sextant, with which his 

 observations were chiefly made, from his entrance into the Royal Observatory 

 in the year 1676, to the year 1689. This sextant was first made of wood, and 

 afterwards of iron, with a brass limb of 2 inches broad, by Mr. Tompion, at 

 the expense of Sir Jonas Moore ; its radius was 6 feet QJ- inches ; it was fu,r- 

 nished with an endless screw on its limb of 17 threads in an inch, and with 

 telescopic sights. Of this instrument Mr. Flamsteed gives the figure at the 

 latter end of his Prolegomena before-mentioned, sufficiently large to see the 

 general design ; the whole being mounted on a strong polar axis of iron, of 3 

 inches diameter. 



Though, in the full description of this instrument, Mr. Flamsteed men- 

 tioned the limb's being furnished with diagonal divisions, distinguishing the arch 

 to 10 seconds ; yet it is pretty clear, that it had not these originally on it; but 



