166 On Spandrel Groins. 
Thebrick-work for leveling above the groinsis grouted: in some 
eases groined vaults contain but a single arch ; flooring joists are 
laid over their tops and filled up between with dry rubbish. It 
is necessary to say, the piers of the former groins are built with 
Aberdeen and the caps with Dundee granite, it being found the 
hardest and best calculated for this purpose : observe, the joints 
of the stones must be left open round the piers until the groined 
arches are well settled, otherwise the stones at the joints will 
flush or chip off. This has been found to be the case in practice. 
The drawing in outline marked fig. S shows a better and 
more advisable method of centring these groius, where consider- 
alle labour and wood may be suved. For example, PP, &e. are 
piers of the groins, LLL ledgers of deal quartering running hori- 
zontally along each bay, and supported on shores; cec, &e. 
plans of the centres, as ranged and fixed on the ledgers: each 
bay or passage being centred in this manner; next board them 
over, forming them into a semicylinder, as a common vault or 
area. This being done, next get out a mould for marking the 
common intersections of the cross arches: to do this, take 
the divisions 012384, and set them on the line 01234 at 
fig. M; these lines draw parallel to each other and at right an- 
gles to the perpendicular line 04. Next take the distances ad, 
ab, &c. and transfer them to 47, 37, &e. this will then give 
the curvature of the mould, and which mould is seen further 
applied on the ichnography of the centring. Jack ribs, as rr, 
&c. are next to be regularly fixed on the cylinder, and boarded 
over, making the whole of the centring by this means appear 
as the centring of a common equal pitch cylindric groin. To 
form the spandrels of the groins, first take the divisions ¢f, &c. 
and transfer them to the line Ah, &c. at fig. N53 next take the 
measures ed, ed, &e. and transfer them to A i, hi, &e. at fig. N: 
this will then give the mould whereby to mark out the spandrels 
on the centring, and which mould is seen applied thereon. Lastly, 
bridging pieces, as shown at gg, &c. are to be fixed across the 
angles and boarded, which form the spandrels and complete the 
centring for receiving the brick-work. Thus is the whole suf- 
ficiently and practically explamed, 
XXXVI. An- 
