» 
the 
oc 
.. 
GR 
arches four inches from the face of the ribs, which renders 
cutting of groins totally uoneceflary, and thereby 
ons a faving of 3/. in point of labour to every arch, 
which, in an extenfive warehoufe, would amount to a large 
fum of money ; and it ought, at the fame time to be recol- 
lefted, that in many fituations it is not eafy to find a work- 
man who. perfeétly underftands the principle of cutting the 
groined angle; fo that it muft either be executed in a very. 
imperfect manner, or elfe the work delaye » and perhaps 
with great inconvenience, until a more experienced brick- 
layer can be procured. 
But what appears to be more defirable than any pecuniary 
advantage is, that this method of fetting back the filling-in 
arches affords to them a moft excellent fhoulder or abut- 
ment againft the whole body of the ribs, as well as a better 
opportunity of bonding and connedting the ribs and filling- 
in arche: er, in the interior part of the work, and 
onally on thé face, as will be more practically illuf- 
trated, by confulting the feparate fragment (on a larger 
{cale) appertaining to the model of the new improved me- 
thod ; by the infpection of which, every perfon will be con- 
vinced, that all the bond and conneétion which can be either 
b 
fo as to meet the angles of the ribs, there would inevitably 
have appeared a contirued joint or line of feparation in the 
very worft place of which it could have happened, viz. the 
angles of the ribs; becaufe, as before obferved, the courfes 
of the brick-work cannot be preferved altogether on the 
fame level. Therefore, by fetting back the filling-in arches, | 
this practical deformity is not only avoided, but the arches 
themfelves, inftead of thruttin againft the extreme corner 
of the ribs, will, by this difpofition, exert their force againtt 
the whole fubftance of them, where there is evidenily a 
greater power to refitt it. 
. _. All arches require a certain thicknefs for fupporting their 
own weight, yet how to determine it is not eafily done. In- 
deed th ft able enginee d architeéts differ widely in the 
dimenfions they give, becaufe it is a fubject which has never 
been reduced to mathematical inveftigation, like the equili- 
brium of arches, but wholly depends upon practical experi- 
ence, juft as the thicknefs of walls in houfes and other build. 
ings. But as the {trength of all arches, whether femi-circu- 
lar, fegmental, or elliptical, when of equal thicknefs at the 
crown, is in an inverfe proportion to. their radil, as that of 
ey ig 1 mh eee Nay Bd . . lengths; and fince 
h 
oe is fu 
when firengthened by horizontal fpandrils above, and capa- 
ble of bearing great Sates Yea as is the cafe in the warehoufes 
ure 
readily found; or any other thicknefs, 
‘for the caleulation of the thicknefs of arches of diffecent 
' pres Bess in the groined ich i 
3 ie ; } our : ti ‘i i 
evident that if the weight of the filling-in arches which reft 
O I N. 
rib as could well be placed upon piers meafuring only four 
feet eight inches from fide to fide, and thereby a great itie 
creafe of ftrength is gained over the old grouied arch, where 
the groin is only fourteen inches fquare, uo thicker than the 
filling-in arches, 
© To conclude this diflertation, it may not be improper to 
obferve, that D F r 
of Bridges, recommends the ftones of the wall (or {fpandril,) , 
over the extrados of the v 
{pandrils, well bonding the whole together in one folid mafs. 
Laitly, the author feels confident, that if the»prejudice of 
old cuftoms be not permitted ta overcome truth and ufeful 
improvement, the feveral tuperior properties of his new me- 
thed of ere¢ting groined arches in brick-work, will be the 
means of bringing it into general practice in works of mag- 
nitude, to the utter feclufion of the old fyitem ; {ince it has 
been proved, that the octagonal piers occupy lefs fpace on the 
pian than the fquare ones ; alfo the new arch, by the intro » 2 
duction of the diagonal ribs, is two and a half times ftronger 
than the old arch ; and there will likewife be-a confiderable 
faving in point of labour, by the omiffion of the groins 5 
fo that under the impreffion of the 
has extreme fatisfaGtion in prefenting thefe explanatory re- 
- B. The difference of expence between the cene. 
tering of the old and new method is fo fmall, that it has 
not been contidered worthy of computation in the foregoing 
treatife «=. 
Theory upon which the conftruétion of croined arches 
in brick-work is founded: with a comparifon of their itrength 
according to the prefent manner of buildin 
new methed invented by M 
Lnveftigated by John Narien.—In any ground arch, the dia 
upon the ribs with their proper fp; Is and the loadir 
them, 
In the improved arch defigned by Mr. Tappen, the filling» 
b diag 
iagonal. : 
SETA ORC See ee a ayia” By eet ene eee A ee Lp ee gee 
Ft 
ee 
on is page 43, for an arch of 
e >ratioi he he load-. 
ng over them, be made to throw upon is 3 part of the ribs _ 
a weight proportional to that which is nece 
in ey i 
conttrn 
on both thefe accounts, 
But, as f iag arches are 
houfés, an 
rio; hence, in whateve 
a certain defect in at 
= es are_abfolutely neceffary in ware- 
many- other buildings, for the purpole of g: 
> ‘Yr way groined arches are— : 
Ged, there il always exift — in them 
eae: 
