A R C H I r 
The conditions required in a bridge are, that it be well 
deligned, commodious, durable, and fuitably decorated. 
It Ihould be of Inch a height as to be quite convenient for 
the palTage over it, and yet eafily admitting through its 
arches the velfels that navigate upon it, and all the water, 
even at high tides and floods : the ncgleft of this precept 
lias been the ruin of many bridges. Bridges are com¬ 
monly continued in a direction perpendicular to the dream ; 
though fonie think they diould be made convex towards 
the dream, the better to relid floods, &c. And bridges 
of this fort have been executed in fome places, as Pont 
St. Efprit near Lyons. Again, a bridge fhould not be made 
in too narrow a part of a navigable river, or one fubject 
to tides or floods: becaufe, the breadth being dill more con¬ 
tracted by the building, it will increafe the depth, velo¬ 
city, and fall, of the water under the arches, and endan¬ 
ger the whole bridge and navigation. There ought to be 
an uneven number of arches, or an even number of piers; 
both that the middle of the dream or chief current may 
flow freely without the interruption of a pier ; and that 
the two halves of the bridge, by gradually riling from the 
ends to tire middle, may there meet in the highed and 
larged arch ; and alfo, that, by being open in the middle, 
the eye in viewing it may look directly through. This is 
indeed founded, not merely on the principles of beauty, 
but of philofophy and reafon ; lince, in general, it is found, 
that the dronged current runs in the centre of a river, 
which the more readily clears itfelf in the aperture of a 
bold arch, and renders floods and fpring-tides lefs injuri¬ 
ous to the w'hole fabric. When the middle and ends are 
of different heights, their difference however ought not to 
be great in proportion to the length, that the afeent and de- 
feent may be eafy ; and in that cafe, alio it is more beauti¬ 
ful to make the top in one continued curve, than two draight 
lines forming an angle in the middle. Bridges diould ra¬ 
ther be of few and large arches, than of many fmaller 
ones, if the height and fituation will poldbly allow of it; 
for this will leave more free paffage for the water and na¬ 
vigation, and be a great laving in materials and labour, 
as there will be fewer piers and centres, and the arches, 
&c. will require lefs materials ; a remarkable indance of 
which appears in the difference between the bridges of 
Wedminder and Blackfriars, the expence of the former 
being more than double the latter. 
For the proper execution of a bridge, and making an 
©dimate of the expence, &c. it is neceffary to have three 
plans, three leCtions, and an elevation. The three plans 
are fo many horizontal feCtions, viz. fil'd a plan of the 
foundation under the piers, with the particular circumdan- 
ces attending it, whether of gratings, planks, piles, &c. 
the fecond is the plan of the piers and arches; and the 
third is the plan of the fuperdruCture, with the paved 
road and banquette. The three feCtions are vertical ones: 
the fird of them a longitudinal feCtion from end to end of 
the bridge, and through the middle of the breadth ; the 
fecond a tranfverfe one, or acrofs it, and through the fum- 
rait of an arch; and the third alio acrofs, but taken upon 
a pier. The elevation is an orthographic projection of 
one. fide or face of the bridge, or its appearance as viewed 
at a didance, diewing the exterior afpedt of the materials, 
with the manner in which they are difpofed, &c. 
When bridges form the entrance to or from the principal 
dreet of a capital city, their condruCtion is generally at¬ 
tended with great expence, and a degree of elegance and 
durability is required in their formation, that calls for the 
utmod Ikill and ingenuity of the architect. Palladio, in 
this cafe, tells us, that bridges ought to have the fame 
qualifications that are judged necedary in all other build¬ 
ings, namely, that they diould be convenient, beautiful, 
and lading. The perfection of a bridge, therefore, con- 
fids in its having a good foundation, which majkes it lad¬ 
ing; an eafy afeent, which makes it convenient; andajud 
proportion in its feveral parts, which renders it beautiful. 
In erecting done-bridges, therefore, there are feveral re. 
E C T U R E. I** 
quifites which peculiarly demand our attention. In the 
fird place, the hutments not only receive the prelfiire of 
the arches, with which tiiey are connected, but they mud 
be capable of redding in tome meafure the force of the 
whole combined. Hence the necellity of a folid founda¬ 
tion at the oppolite (ides of the river, without which the 
feveral arches will be liable at lead to partial rents. The 
direction of the whole druCture ought always to be at 
right angles with the current; and the lize of the piers 
ought not to be larger than what is edentially requidte to 
the fupport of the arches; for the unncceffary thicknefs 
of piers lias the effeCt of contracting the current of the 
water, which, as we ablerved before, increafes its velo¬ 
city, and rentiers the foundation of each pier more liable 
to be undermined. Ladly, we are to decide on the num¬ 
ber and figure of the arches, which are points of great 
confequence to the whole, in relation to drength, ulefuL 
nefs, beauty, arid economy. 
In the choice of arches, we find great difference of opi¬ 
nion among different architects. Some contend, that the 
femicircular arch is in mod cafes to be preferred, becaule 
they prefs more perpendicularly on their piers, and in pro¬ 
portion to their number will relieve the butments. Others 
prefer the elliptical arch, particularly where they are to 
be large, and few in number; becaufe the extenfive radius 
of the femicircular arch would occafion the centre of the 
bridge to be fo high as to render the pallagc of carriages 
exceedingly troublefome, an objection which is completely 
obviated by the elliptical form, wiiofe elevation is conli- 
derably below that of a femicircle; and Mr. Muller con¬ 
tends, “ that the elliptical arch does not prefs againd the 
piers with a greater force than a circular one ; and, being 
lighter, and condruCted with lefs materials, will confe- 
quently be more lading.” There are others, however, 
who prefer the catenarian arch to all others, for the pur- 
pole of bridges. Mr. Emerfon, in his Principles of Me¬ 
chanics, infids, “ that it is the dronged arch poifible to be 
made, for fupporting a great weight.” This is again con¬ 
tradicted by Dr. Hutton, profell'or of mathematics in the 
Royal Academy at Woolwich, who is the lated writer on 
the fubjedt; and, as the drength and folidity of a bridge 
mud unquedionably depend on mathematical principles, 
we (hall here give Dr. Hutton’s opinion in his own words : 
“ For the figure of the arches, fome prefer the femicir¬ 
cle, though perhaps without knowing any good reafon 
why ; others the elliptical form, as having many advanta¬ 
ges over the femicircular; and fome talk of the catenarian 
arch, though its pretended advantages are only chimerical; 
but the arch of equilibration is the only perfect one adapt¬ 
ed to the principles of bridges. This arch, being in exaCl 
equilibrium in all its parts, and having no tendency to 
break in one part more than in another, is therefore the 
fafed and dronged. Every particular figure of the ex- 
trados, or upper fide of the wall above an arch, requires 
a peculiar curve for the under fide of the arch itfelf, to 
form an arch of equilibration, fothat the incumbent pref- 
fure on every part may be proportional to the drength or 
refinance there. When the arch is equally thick through¬ 
out, a cafe that can hardly ever happen, then the catena¬ 
rian curve is an arch of equilibration; but in no other 
cafe : and therefore it is a great midake in fome authors to 
fuppofe that this curve is the bed dgure for arches in all 
cafes; when in reality it is commonly the word. When¬ 
ever the upper lide of the wall is a draight horizontal line, 
as in the following figure, the equation of the curve is 
thus expreded, 
, . <24-.v- 4- v zax-X-xx 
log. of —I—d--- 
j>■=* x- y = ; 
, .a4-r-LV 2 a>-\-rr 
log. of- 
where i~DP, y— PC, r=DQ^ h— AQ^> and »=DK. 
And hence, when a, h , r, are any given numbers, a table 
is 
