the 'Trigonometrical Operation . 1 6 1 
ftations from the extremities of the bafe, it would be abfo- 
lutely neceflary to raife the inftrument, by fome means or 
other, to a confiderable height above the ground. For this 
purpofe the portable fcaffold, whofe plan and elevation are 
reprefen ted on the left hand fide of Plate VII. was conftru&ed. 
It confided, as may be feen, of an inward fcaffold for fup- 
porting the inftrument, and an outward one for the obfervers, 
wholly free and independent of each other, the platforms of 
both being framed about thirty* two feet above the lower ends 
of the fcantlings, which reft on the ground. Thefe being 
made of fquared deal, and the feveral parts being bolted and 
fcrewed together with many iron fcrews fecured by nuts, the 
whole could be readily taken to pieces, carried in a waggon (for 
which it made a complete load), and replaced again in any new 
fituation. This fcaffold anfwered very well the purpofe for 
which it was intended ; for the ftep-ladders, or flairs leading 
to the platform, being attached to the outward frame, the in- 
ward one that carried the inftrument remained undifturbed by 
the motion of thofe who went up and down, or walked around 
the top. The filk thread, that fufpended the plummet, was 
fecured from the effe&s of the wind by a fort of funnel or 
trunk, compofed of three deals (one fide being left open), and 
fo contrived as to be eafily turned round to any quarter of the 
heavens, whereby the open fide was always prefen ted to lee- 
ward. The inftrument was covered from the weather by a 
canvas canopy, about feven feet fquare, to which fide walls 
could be hooked for fcreening it from the wind, as occafion 
might require. By referring to the elevation it will be feen, 
that the fcaffolds, both outward and inward, might be divided 
horizontally into two parts, fo as to permit the uppermoft half 
alone to be ufed when it became un neceflary to raife the inftru- 
Vol. LXXX. Y xnent 
