7© Sir George Shuckburgh's Account 
folio, has given us a description and wooden plates, of no less 
than four different astrolabes, under the names of armillce 
zodiacales et equatoricc , of different sizes, from 4-f to 10 feet 
diameter, divided into degrees and minutes, and some of 
them into every 15 or 10 seconds, but furnished only with 
plane sights. These large instruments were placed in towers 
appropriated to each, with moveable roofs, one half of which 
was taken away at the time of observation. A circumstance 
that it is curious to remark, is, that Tycho, who was atten- 
tive to every thing that could improve the accuracy of his ob- 
servations, made the axis of his 10 feet circle hollow, “ Axis 
“ ejus e chalybe constans, et undiquaque apprime teres ; in- 
“ terius tamen cavus, lie pondere officiat, in diametro est 
“ trium digitorum a principle that has been very prudently 
re-adopted in these later times, as will be presently seen. 
(6.) After Tycho 1 meet with no instrument of this sort 
till the time of Christopher Scheiner, about the year 1620, 
who made use of a small telescope, moving upon a polar axis, 
with an arc of 47 0 of declination, to observe the sun's disc 
commodiously, and examine his spots ; an account of which 
will be found in his Rosa JJrsina, folio, Bracciani, 1630, p. 
347. But this instrument can hardly be considered as an 
astronomical one, being merely a contrivance to follow the 
sun with a telescope, by means of one motion only, similar in 
its object with the heliostate, described by Dr. Desaguliers, 
(Mathematical Elements of Natural Philosophy, lib. 5. c. 2.) 
(7.) Again, Flamsteed's sector, which he has described 
in the prolegomena to the third volume of his Historia Cce~ 
lestis , p. 103, though mounted upon a polar axis, and very in- 
geniously contrived for the purpose it was intended for, viz. 
