of the Equatorial Instrument. 85 
roof,* moveable round its axis upon six friction rollers, of 
about 4 inches in diameter, two of which are seen at s and t. 
The base of this cone consists of an iron ring, about 1 1 feet in 
diameter, and 3 inches wide ; the upper part of the cone at 
s, is terminated by another iron ring 2 \ feet diameter ; and 
these are connected by 12 iron ribs, or rafters, in the direction 
of s, s. Over these rafters lie two coats of extremely thin deal 
planks, not more than inch in thickness each, crossing each 
other in transverse directions of the grain of the wood, and 
over these a covering of copper, of about the thickness of a 
shilling. Over the copper, on the outside, are three good coats 
of white paint, and the wood withinside is lined with stout 
canvas, well painted also ; so that the whole roof is as move- 
able and as light as possible, not more than 200 or 3001b. 
weight, and withal very strong. At s and s, is an aperture, 
in the roof, of about one foot wide, which is opened or co- 
vered, suitably to the occasion, by the two doors 0, p. r is 
another door, (in the plate) open ; by means of these three 
doors, all opened very readily by iron rods withinside, a 
prospect is given of the heavens from the horizon to the ze- 
nith, and even to io° beyond it. q is another aperture, about 
9 inches long and 4 wide, covered occasionally by a shutter z. 
* As I flatter myself that I have now executed what the very celebrated Abbe 
Boscovich seems to have had in view, in the 14th opusculum of his fourth volume of 
Opera pertinentia ad Astronomiam, &c. I cannot restrain myself from citing the 
passage, wherein, after describing the use of the small equatorial, he says ; “ Apparet 
“ igitur egregius usus machine etiam mobilis. Verum machina parallactica metallica 
“ cum circulo, et semicirculo, satis magnis, ac telescopio acromatico, et satis bono mi- 
“ crometro filari, collocata firmiter in turri habente tectum mobile, esset instrumentum 
“ usus immensi et expeditissimi, ac incredibilis ad astronomiam cum maximo fructp 
<£ excolendam utilitatis.” p. 309. 
