42 
to ie put upon thdsaty Now it is evie 
dent, by inspecting the medel, that these 
ribs: (springing not from a <xnall corner, 
er edge, ef a pier, but from the very heart 
and substance of it; made still more firm 
and immoveable by the counteracting 
agency of the opposite yi ibs, added to the 
weight of the solid span sdvils above) must 
possess extraordinary strength and dura- 
buity. They abut avainst’a stout square 
key-stone placed in the central intersee- 
tion: and by having to bear the largest 
portion of the weight (according to our 
previous lemma, they are to be worked 
throughout mite solid unmutilated bricks, 
except such cuttings'as may be necessary 
for hond atone; also, by receiving addi- 
tional ¢ aid and support from the four fill- 
qg-in arches, one brick and half in thiek= 
Ress, end the whele beme well ber added, 
and thoroughly drenched and cemented 
together with strong flowing mortar, 
would, I have no dout it, be capabie ei 
susialping upwards of twice the weight 
that tiie “prsseut common evroined-arches 
of equal Unickness could support. 
As the sum of the diagonal of the 
square is grearer than the sum of one of 
the sides, so it is eG juaily obvious that the 
givth of the alipiie crvss-ribs must be 
reater than the giith o the semicircular . 
arch ;-on which account, I found that it 
was ok possible to kee b the joints, of th € 
brickwork of the fill ling-in arehes upon 
the same level’as the jomts of the brick- 
work composing the ribs, without cutting 
away and ‘di a inishing the real body and 
substance of the bricks, and thereby 
falling inte one error by shunning ano- 
ther. But, as it frequentiy hap pens, 
that which at first appeared to be neconal. 
€d with considerably citieutty, ended in 
a novel expedient of con necting the ribs 
and thé filling-in arches tove ether, in a 
tich better and more substantial man- 
ner than could possibly have been the case 
by any contrivance of making them co- 
icide-at the angles. ‘Fhat 1s, ] was in- 
duced toset back the fil livig-4 m arches 
four inches from the faceef the ribs, which 
yeuders the cutting of eroins torally un- 
necessary, and thereby ocensions a con- 
siderable s saving of expence,-im pont of 
labour to every arch, w Riete est sai 
an extensive warchouse, would aiOUi Itt 
a large sum of mon =a 
But what is still more de sirable, and 
payamoint to pecuniary co:siderations, 
is, that this method atiords to.the filling- 
-* The filling-im arches: may also be made 
thicker than specified by the model, whet tever 
it may be thought necessary. 
On an improved Method of Building groined Arches. 
[Septo 3, 
m arches a most excellent and solid 
shoulder or abutment against the whole 
body of the ribs, as well as a hetter op= 
portunity of bonding and tying the ribs and 
Hilling-in arches tovether in the interior 
part ofthe work, aud occasionally on the 
face, as wi]l more convincingly appear by 
consulting the separate fragment (on a 
larger scale) appertaining to the model 
first alluded to. By the inspection of 
which, every workman will perhaps 
be instantly satisfed that all the bond 
and connection that can be either tse- 
ful or neces nt to the work, may be 
obtained with the greatest ease, notwith- 
standing the Rohe of the brickwork of 
the filing-in arches and those of the ribs. 
are not, generally speaking, upon the same 
level ; ‘though many of the courses, m 
the progress of the work, will aeciden- 
tally come so. Now, had the filling-im 
arches’ been brought forward, so as to 
meet the angles of “the ribs, there would 
inevitably have appeared, a continued 
joint, or ‘line of separation, m the ver 
worst place 1 in which it could have haps 
pened, viz. the angles of the ribs; because, 
as before observed, the courses of the 
brickwork cannot be preserved altoge- 
ther upon the samelevel. Therefore by 
setting back the filling-in arches, this prae= 
tical deformity 3 is not only avaideth, but 
the arches themselves, mstead of thrust- 
ing ayainst the weakest part imaginas 
bie, that of an extreme angle, will, 
by this contrary disposition, exert all 
their maps .e where there is the greats 
est power and substance to resist. it. 
Further, 1 may not be improper to eb- 
serve that Dr. Hutton, in his Treatise on 
the Principle of Bridges, recommends - 
the stones of the wail (or spandril) over. 
the extredos of the voussous of an arch 
to be bonded mto the stones ef the pier, 
and with one auother; because the 
pier will then carry part of them weight, 
and thereby oppose a greater power of” 
resistance io the thrust of the arch. For 
he same reson, it would be equaily ads 
viseable, alluding to the new method of 
building groimed arches, to earry up, at 
the same tine, the diagonal ribs, Amg-in 
arches, and intervening spandmils, to ‘the 
level shewn by the model, well bonding 
the whole together; andthencethe arch is 
continued round till it meeis the same sub- 
stantial abutment on the other side; and 
thus, by the meanus.of assuming a new 
chord,. the central part of the fourteen= 
inch arch will bear nearly an equal pre- 
portion, even singly considered, with the 
thickness assign ned by the ablest engineers 
te the arches, “of powder-magazines, and 
wherg. 
