O28 
doubly-unfortunate dilapidations of the 
bridges on the river Tyne, 
' Professor Robison certainly inserted 
the Emerson theory in the Encyclopedia 
Britannica, and at first held it in some 
esteem, out of respect to common opi- 
nion; for he really believed, from the 
ymode in which it was introduced to the 
ublic, that it was the * cornmon theory” 
of architects and builders, But from the 
article “¢ Roof,” to the article “ Arch,” 
Ja the Supplement, until we come to the 
conclusions of the latter, we perceive, in 
the gradual unravelling, how difficult it 
is long to deceive men who look beyond 
‘their own speculations and libraries for 
‘knowledge. Speaking of the fallacious- 
ness of the theory, from the test of nu- 
merous experiments which be had been 
at the trouble and expense of making, 
he says, “ But the clearest proof is, that 
arches very rarely fail, where their load 
differs most remarkably from that which 
this theory allows.” And, again speak- 
ing of those arches which have stood the 
test of ages, he says, ‘* Here is a most 
‘yemarkable deviation from the theory, 
for, as is already observed, the load is 
frequently not the fourth part of what 
the theory requires.” The supporters 
of the wall theory say, and itis all which 
can be obtained in explanation of the 
“theory, that they have “ exploded” the 
wedge-theory, and that their ‘theory is, 
“ the true theory ;” and that those who 
cannot ‘perceive the force of it, are 
ignorant and prejudiced.” 
Permit me to recommend to your 
readers to peruse attentively the first 
proposition of Dr. David Gregory’s 
paper on the catenaria; and to dwell on 
the second, and its corollaries; to turn in 
their mind Dr. ELooke’s deduction, (de- 
cyphered) “ ut pendet continuum flexile, 
“sic Stabit contiguum rigidut inversum ;” 
Dr. Johnson’s three Letters; Mr. Simp- 
son’s Answer to the Committee for 
building Blackfriars Bridge; the An- 
~ swer of the Monthly Review, (written, as 
~ Dr. Hutton states, by Mr, Woodhouse, 
now sufticiently eminent,) June, 1802; 
the Letters in Answer to your Review, 
and the Monthly Review, in the Monthly 
“Magazine, August and October, 1802; 
they will then exclaim, in the words of 
the commencement of one of Dr. John- 
son’s Letters, ‘It is the cominon fate of 
efroneous positions, that they are be- 
trayed by defence, and obscured by ex- 
planation; their authors deviate from the 
main question into incidental disquisi- 
tious, and raise a mist where they Should 
On the Emerson Theory of Arches. 
[April 1, 
let in light.” If there were wanting any 
instances of the absurdities which great 
and eminent men have been led into by 
the modern analysis, when they have 
not been sufficiently attentive to deter- 
mine the truth of the first proposition, 
depending merely on the principles of 
mechanics, Emerson’s Extradoses of 
Arches, and especially of domes, would 
stand like the full and perfect warning 
which a wreck offers to the heedless 
mariner; his charts and books lying ne- 
ylected in his chest. Not even those on 
whom the authority of the Woolwich 
Academy has imposed Emerson’s theory 
of arches, can contemplate with a se- 
rious countenance, the monstrosities of 
his conclusions in respect of domes. It 
is difficult for a mason to resist exclaim- 
ing, in this instance, ‘ Multos se deliros 
senes spe vidisse, sed qui magis, quam 
Phormio, deliraret, vidisse neminem !” 
The methods by analysis and geometry, 
resemble the progress of a young and 
an old hound. The former, if he get 
a right scent, and keep it, will soon 
overtake the pursued; but, on a wrong 
scent, his fleetness but removes him 
further from the object: the old hound 
is oftener in at the death, and is always 
near the victim. Notwithstanding the 
importance which has been attached hy 
mathematicians to the question of the 
equilibration of arches, it is a question 
of little worth to the builder, in compa= 
rison with that of the piers. On this part 
of arch-building little has been written, 
and still less understood, except by those 
who have been nursed in the practice. 
Should you think the above worth in- 
serting in your Magazine, and hold of 
any value the theory which has in fact 
been, from the first arch which ever 
stood, the theory which has guided the 
builder, though unconscious of the extent 
of his knowledge, and which I have at- 
tempted to detail from Dr. Gregory’s 
paper, and Dr. Hooke’s conclusion ;-I[ 
shall, at my leisure, send you a paper on 
the subject of piers; shewing their pro- 
perties involved, and dependent on the 
accurate reasoning of Gregory and 
Hooke, in which an ‘extraordinary va- 
nation between false theory and true 
practice, extracted from the second Re- 
port of the Committee of the House of 
Commons onthe Holy-head Roads and 
Harbour, printed June, 1810, may further 
illustrate this subject. Larrea. 
P.S. See letters on this subject, Monthly 
Magazine, Sept. 1809, Nov. 1809, Aug. 
1810, and Noy. 1810. f 
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