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Lapicida on the Emerson Theory of Arches. 
34 
information concerning abutment piers?» 
He found it prudent not to agitate the 
question, as the gentlemen in the report 
alladed to, did in getting over the 11ih 
question of the select committee of the 
House of Commons. Sir, I believe in a 
great measure [ repeat your own senti- 
ments, In stating that the Emerson theory 
does not in any way take into consider- 
ation the arch, but applies to a wall 
with a hole in it, composed of materials 
united by cement, either wholly or 
yound the curve:* whether an arch of 
any thickness is to be placed in this 
hole} remains-to be explained. Through 
the whole of the Principles of Bridges, 
except in the Iasi ten lines of the last 
page, the wer] voussoir is not menti- 
ened; and then, in the dictionary, merely 
fo state that there are such things. Dr, 
Hiutton’s definition of an arch, viz. “an 
opening of a bridge through which, or 
under which, the water passes;” esta- 
~blishes the opinion which is universally 
held of the theory, that nothing more is 
required than a curved intrados, or ma- 
thematical arch, er arch of no thickness. 
‘Hence it is a mis-nomer to eall the 
Emerson theory, a theory of the equili- 
bration of arches: it 1s literally, when 
applied to bridges, a theory of the form 
ef the fat mould, &c. on the extradosses 
of the arches of bridges. Ef Mr. 
Mylne’s practice, in regard to the 
voussoirs, be just, and Mr. Atwood, and 
the French piniosophers, are not depto- 
rably ignorant, the Emerson theorists 
have to begin again upon a new series of 
jntradasses for their walls. 
The defenders of the Emerson theory, 
it may be apprehended, are unacguaint- 
ed that the word extrados, as applied to 
arches, has but one meaning: it is proba- 
ble their errors may have arisen originally 
from a misconception which they now 
think proper to maintain. If Philo- 
veritas will condescend to clear up any 
of the inconsistencies of the true theory, 
er Te 
* See the diagrams, and explanation, in 
Emerson’s Mechanics. 
+ And if of any thickness, whether equally 
thick throughout, or whether the intrados of 
the Emerson wall is the extrados of an arch 
of equilibration, to be guessed by the mason ? 
¢ How is it that the wonder of this 
theory, viz. the curve for a horizontal extra- 
gos, approaches so near to a semi circle, and 
differs so materially from an ellipse ; and the 
properties approach so nearly to those of an 
ellipse, and differ so materially from those of 
a semiecircle ? Are not the details as curious 
2s the results in the 5th Prop. Principles of 
Bsidges? - 
| is 
[Aug. }, 
Lapicida will be obliged; he is not desi= 
rous of having an account of the amours 
of a college, that being the only part of 
the history left out. Lapieida has 
always been of opinion, however the 
lives of some few of the “ ald fellows” of 
the universities may have deviated from 
the stoic regimen, that they never forget 
to maintain the characters of gentlemen. 
The Lapicide, and the Lignicide, are an 
obstinate race: no. persuasion ean induce 
them to adopt what is diametrically op- 
posite to experience and practice; and 
they presume to assert, in opposition to 
the learning of the schools, that they can 
discover what is false, though they cannot 
exactly define what is true. The publi- 
cation on arches, &c. was not referred to 
through friendship to the author, as 
Philo-veritas insinuates; but now still 
more so, as the dire Philo-veritatis 
acumen, without having any know- 
ledge of it, has already devoted it 
to those purposes from which the repu- 
tation of the Monthly Magazine will 
preserve his own farrago. Lapicida has 
seen the article “ Bridge” in the New Cy- 
clopedia, in which he finds the following 
notable passage:—‘‘ A mere arch con- 
structed in this way, viz. according to 
De la Hire, Belidor, Varignon, Parent, 
other French philosophers, and Mr, 
Atwood, would remain in eguilibrio as 
long as the constituent vonssoirs had 
liberty to slide without friction down the 
respective inclined plenes on which they 
lay:” and among other extraordinary ~ 
lights thrown upon their theary, “ that 
the voussoirs of such arches must be cut 
to different oblique angles.” He then 
ejaculates: “ But even this is not all! 
architects contrive to have the butting 
side of their wedges (voussoirs) so rough, 
as to occasion a great-deal of friction 
between them.” These architects must 
have been the workmen who told Philo- 
veritas that arches sink at their 
haunches, or it must be a new precept 
established on purpose for the Emerson. 
theory.* Lapicida cannot but suspect, 
when he observes the industry which has 
been displayed in the historical part of 
this article, and the number of bridges 
which are brought into view, that the 
former part was intended as a body of 
practical evidence to confute the Emer- 
son theory, and that the theoretical 
account was intended, ironically, to fur- 
ther that object. LapPriciDA. | 
EE ait tie Ma 
* The authorities in favour of the Emer- 
‘son theory, will remind many of your readers 
of the story of Elizabeth, Betsey, and Bess. 
| £0 
