362 Reply to Lapicida on the T. heory of Arches. 
Mr. Editor, if Lapicida had understood, 
before he wrote, he would have saved 
himself this exposure. Is he so déplora- 
bly ignorant of the subject, as to be per- 
fectly unconscious that, when David 
Gregory says, “arches of other figures are 
supported because in their thickness 
some catenaria Is cluded,” he actually 
points out the principle on which the 
theory adopted by Hutton and Emerson 
rests? If he be, 1 would refer bim to 
Gregory’s Mechanics, Book 1, Chap. vi. 
p- 189, &c. where he will find this theory 
jegitimately deduced from the remark 
of David Gregory to which he points 
as establishing a contrary theory! To 
write with such extreme ignorance, as 
Lapicidu evinces, and to suppose authors 
contradict one another, whose principles 
are in unison, is, indeed, to borrow 
Shakespeare’s words: 
‘« To fortify on paper, and in figures, 
Using the names of men, instead of men: 
Like one that draws the model of a house, 
Beyond his pow’rs to build,: wno half 
through, 
Gives o’er, and leaves his part created lost, 
A naked subject to yhe weeping clouds, 
And waste for churlish winter’s tyranny.” 
Lapicida next adiduces the evidence 
of the late Dr. Robison, of i’dinburgh, 
from the article arcu, in the Supplement 
to the Encyclopedia Britannica: an arti- 
cle which Lapicida has read without 
comprehending, and quoted disingenu- 
ously. Dr. Robison adopts this very 
theory, and no other, in the article in 
question ; as well as in every other place 
where he treats the subject theoretically: 
and he assigns expressly as a reason, 
why arches built according to this theory 
sometimes fail; that the space from the 
flank, to the roadway, instead of being 
filled up with solid matter, is often filled 
with sand, gravel, fat mould, &c. which, 
before it becomes thoroughly consoli- 
dated, acts with a kind of hydrostatical 
pressure, which causes, the. deviation, 
This, however, no more indicates error 
yn the theory, than the bending on the’ 
expansion of a pendulum-rod, argues er- 
ror in the theory of the centre of oscil- 
lation, or the burstivg of a grind,stone 
turned with extreme rapidity, marks 
the fallacy of the received propositions 
_ in the theery of rotatory motion. 
Another reason why Lapicida disap- 
proves of a theory he does not compre-’ 
hend, is because Dr. Hutton “ acknow- 
ledged it to be hastily composed.” Now 
this is extremely. disingenuous, Dr. 
Hutton’s acknowledgement does not ap-~ 
pondent, 
[ Nov. 1, 
ply to the theory, but to the little treatise 
in which it was exhibited. Me expresses 
no doubt as to the correctness of bis priv- 
ciples, and chief deductions ; but depre- 
cates the severity of criticism upon his 
work, considered as a literary perform. 
ance, by stating, that it was drawn up 
with freat haste upon a particular occa- 
sion. ‘ Lastly, under this head,” as our 
sermonisers at St. Mary’s say, Lapicida 
insinuates, that only ‘* one solitary indi- 
vidual,” is inconsiderate enough to call 
this ** the true theory.”- 1 know not to 
whom he ailudes ; but this E know, that, 
besides the authority of Gregory, im his 
Mechanies, before-mentioned, we have 
that of the author of the articles, Arch 
and Bridge, in Dr. Rees’s Cyclopedia, 
in support of this theory ; and that nearly 
all the gentlemen'who gave their opini- 
on, as to the expediency of an ifon 
bridge, over the Thames, at London, and | 
whose reports were printed by the Com- 
mitlee of the House of Commons, for 
improving the port of London, founded | 
their investigations on that- theory. 
Was Lapicida ignorant of this?. Or 
did he know it, and unjustly suppress 
it? : 
In the third place, Lapicida calls the 
attention of your readers in a very curi- 
ous manner, to sofne new method of de- 
scribing the catenaria. He says, ‘* Let. 
it be admitted, that the mode by which 
the author of a. Treatise of Arches and : 
Abutment Piers, descrites the catenaria, 
is correct ;” and then, by some very unin- 
telligible deductions from a-remarkably- 
complex diagram, he makes it appear, 
“that this new mode of describing the 
catenaria, does produce the same curve, 
- called the catenaria, by Bernoulli, Leib- 
nitz, and Gregory.” Notwithstanding 
the cogency of this very logical method 
of proving a thing by taking it for 
granted, I must beg leave to demur a 
little. I cafinot “let it be admitted - 
that the mode-is correct;” and for'this: 
plain reason, that Lapicida has given us | 
no tangible clew, by which we can exa- 
mine it: he has neither described the 
method, nor told as where to find it. Iv 
this ease, L can only express my opimion, 
that the curve he points to, is no catena- 
ria; and add, that whetherit be, or nor, it 
cannot be of the least possible utility in 
the theory of arches. What a strange 
writer, Mr. Editor, is this your corres= 
He calls it “ aspersion”™ to say, 
that David Gregory recommended the 
inverted catenaria as the best figure for — | 
an arch of a bridge; and yet expects, that 
: A MR cre oe oslo 
