546 
reverie of fome young man who knows 
not what he is about. It is under this 
apprehenfion I prefume to offer to your 
notice fome obfervations on the defigns I 
have this year exhibited at Somerfet Houfe, 
to vindicate myfclf from the charge of 
rafhnefs and infuficiency. 
Three difficulties have concurred to 
prevent architeéts from imagining a bridge 
of fo large a,fpan as 600 feet. The firft 
is that of procuring ftone hard enough to 
bear the very great preffure which it mutt 
neceffarily have to fuftain at the crown of 
the arch; the next is that of procuring 
‘abuttments fufficiently ftrong; and the 
“third the neceffity of fuflaining the arch 
“nearly in equilibrium, for in {mall arches 
much may be truited to the cement, but 
the powerof the cement will bear fcarcely 
‘any proportion to the weight of the mate- 
‘rials in an arch of fo large a. fpan. 
The firft objeétion is eafily obviated ;— 
‘the preflure at the crown of the arch in 
‘the defign now exhibited will not exceed 
-7o tons on each fquare foot, a weight 
which poffibly might crufh Portland ftone, — 
but would be very inadequate to the de- 
‘ftru&tion of Granite. 
There is no apprehenfion that in a 
‘bridge like this, the abuttménts cannot be 
“made fufficiently weighty to balance the 
‘Jateral thruft of the arch, becaufe they 
muft extend very far on each fide ;—the 
“ queftionis whether, if made thus weighty, 
the foil will be able to bear them ? 
The weight to be fuftained at the point 
where it is the greateft, is that of a co- 
lumn of granite 79 feet high, together 
with a portion of the weight of the arch 
itfelf. This will be diffuled over a {pace 
fomewhat wider than the bridge, as far as 
‘the point at which the lateral preffure 
ceafes to be exerted, which, if the abutt- 
ment were made folid, would perhaps, 
theoretically, be about, 140 feet from the 
opening, and in the prefent defign fome- 
what further, but in praétice would not be 
fo much, as a mafs of ftone 79 feet high, 
would not flide over its bafe without con- 
fiderable refiftance from friétion even if no 
cement were uled. 
The fgundation on the Surry fhore is ac- 
knowledged not to bea good one. Mr.Ren- 
nie * inttances in favour of it the tower 
of St, Saviour’s church, which has ftcod 
very firmly ; but in buildings in general 
ihe wall diminifhes as it afcends, and 
Tpreacds confiderably at the foundation, fo 
that the {pace of ground preffed upon bears 
* Third Report of the Committee of the 
Hicuife of ‘Commons, 
Explanation of a Defign for a Bridges 
~ (July 1,- 
i = 3 
a much greater proportion to the average 
thicknefs of the wail than it can do in the 
pier of a bridge like this; and therefore 
the permanency of a wall or tower is not. 
fufficient to prove the fafety of erecting a 
folid body of mafonry to the fame height. 
On the other ‘hand, it ought to be ob- 
ferved, that, though the weight or preffure 
on the ground of two columns of different 
bafes, but of the fame materia] and alti- 
tude, is certainly the fame on equal areas, 
yet the propenfity of the larger column to 
fink into the earth is lefs than that of the 
fmaller, for if we fuppofe two columns of 
ftone, A, B, one four times as large as 
the other, and the larger to be divided 
into four equal parts, the preffure of B 
will be equal to that of 2, b, ¢, coshe 
fourth of the other column, but neither 
column can fink without forcing the earth 
out laterally from underneath it, and the 
earth is free to move laterally from under 
B in all directions; but in a, b,c, d, it is 
confined on two fides by the other parts of 
the column A, and therefore the column 
a, 6, c,d, will not fink fo readily as the 
column BR. and as the fame may be faid of 
each part of the column A, it follows that 
this will not fink fo readily as B. 
With refpeét to the equilibrium, I find 
that the elliptical curve I have adopted, 
nearly produces it; and as perhaips it might 
be thought, that if in fuch a curve the joints 
were formed at right angles to it, even a 
large mais of ftones about the centre 
would not have enough of the wedge fliape 
to fuftain itfelf, Ihave made the angles 
of the wedges there fomewhat larger than ~ 
they would be in that cafe, and increafed 
them proportionally towards the haunches, 
and by thefe means the equilibrium is ftill. 
more nearly attained. A deficiency in 
this refpect is not however of fo much 
confequence as might be imagined, for 
though the reffiance of the cement be 
fmall, that ariing from friction would be 
SO eee 
Ses Se 
very great, as each two fquare feet of 
ftone would be prefled together by a 
weight of more than 50 tons. 
Attentive only to the mechanifm of the 
building, I have not ins my drawing at- 
tempted to detail the oraament of which 
: . a 
