of the Equatorial Instrument. 87 
This arch is filled with solid masonry * to the height i, k, 
into which, as upon a plane brick floor, the iron frames that 
support the instrument are firmly fixed with plaster and 
lead at 1, 2, and 3, 4, and hence the instrument seems as 
steady as upon a rock, b, d, is a boarded floor, that has no 
connection whatever with the instrument, or its supporters. 
u, u, are the two mahogany cases that cover up the apertures 
in the floor, and serve as bases to the supporters C and D, 
but do not tcuch the iron frames within ± inch ; so that any 
yielding of the floor, b, d, by passing to and fro thereon,, 
communicates no motion whatever to the instrument. 
(18.) To the north-east wall-f c, d, by means of timber 
laid into it, and long screws, is firmly fixed the clock, or 
sidereal regulator, but so as to have no communication with 
the floor. As it is peculiar in its construction, it may de- 
serve some notice. Whereas most astronomical clocks shew 
sidereal time in hours and minutes, which is afterwards, in the 
course of computation, reduced to degrees and minutes ; this 
machine shews the degree and minute of the equator, that is 
upon the meridian at any given instant, directly without reduc- 
tion. This is of considerable convenience in observations, out 
of the meridian, with an equatorial instrument ; inasmuch as 
the equatorial circle and the clock, by these means, speak the 
* The bricks of this arch were laid dry, and then grout, consisting of gra- 
vel and hot lime, was poured upon them ; when the arch had stood two years, the 
haunches were filled up with bricks, laid in mortar. I mention these circumstances, 
as attention to them may be of use upon a similar occasion, whenever it shall be ne- 
cessary to erect an arch bearing so great a weight, viz. near 30 tons, upon such slight 
buttresses. 
f The walls of this room do not correspond with the cardinal points; the section 
here given having an aspect 53 0 S. east. 
