On Spandrel Groins. 165 
arches in brick-work, than the common one generally adopted 
(termed spandrel groins), with the centring and moulds laid 
down as used for executing the same. These kinds of groined 
arches are better adapted for subterraneous structures, and better 
answer the purpose for large warehouses where immense weights 
or burthens are required above them, than the common method 
of vaulting, they being less liable to fracture at the groined angles 
by any extraordinary impulse operating on the crown of the 
arch. This system of vaulting, I believe, has not yet heen pub- 
lished in any work. As far as I can learn, they were first executed 
by Mr. Alexander, architect, at the London Docks ; and recently 
by Mr. Laing, architect, at the new Custom-House. 
BB represents the ichnography of the caps of the piers with 
the abutments of the arches, as shown at RR. Fig. P. repre- 
sents the scenographie view of the groined vault as executed; 
xxx the elevation of one of the centres made of two-inch 
deal board rising circularly six feet, which is one-third the 
span; the whole opening being 18 feet. rt, rt, &c. represent 
the angle rib of the same thickness traced from A6, 45, &c.; 
mm shows the plan of the ribs, which are two feet apart; nna 
square centring box ; and wea the ichnography of the boarding 
on the centring. These boards are each 14 inch thick: at 
the London Docks 14 inch boards were used, the arches there 
being one brick and half thick. Fig. R shows the mould by 
which to cut or form the ends of the boards to the centring. 
This mould is made in the following manner: Take the hither- 
most divisions 123456, at x, and set them on the line 
123456, at fig. R; these lines draw parallel and at right 
angles to the line 1,6; next take the divisions at the plan hn, 
fn, &c. and transfer them to 6 p, 5 p, &c. and the intersections 
will give the mould sought for. To find the mould fig. T, for 
the spandrel of the groin, first draw the lines aa, &e. at right 
angles to the centre line aa; then take the height A6, h5, &c. 
and set them on the lines ac,ac, &c.; by this means you will 
form the angle curvature or middle rib: next take the separate 
distances cc, &c. and set them on the line ee, &c. at fig. T. 
These lines draw parallel and at right angles to the line ee; 
lastly, take the distances am, an, &c. and set them from e to o 
(the corresponding divisions) seen at fig. T; this will now give 
the length and bevel of each board round the spandrel, which is 
all the moulds required in constructing the centring in this way: 
observe the line 00, &c. at fig. T is a straight line: one side of 
the bricks in the arch (but not visible in this drawing) has a 
concavity on their surface for a key to the mortar; this is formed 
by the mould when made, 
: L3 The 
