M r. G. Fox on the Construction of the Oblique Arch, 169 



and having considered the external ring as removed, have 

 proceeded to develop the outside surface of the remaining 

 one : this I shall hereafter speak of as the intermediate de- 

 velopment, as it is the development of a surface midway be- 

 tween the extrados and soffit or intrados. 



" Upon this intermediate development I place the approxi- 

 mate line, and then draw all the courses square to it, by 

 which means we obtain a line in the centre of each stone ex- 

 erting its force in the true direction, as in proportion as the 

 one half of this bed exerts its force in an oblique direction 

 on the one hand, the other half acts in the opposite direction, 

 and is therefore always producing a balance of effect, which 

 resolves the various forces into one exerting all its power in 

 the true direction, which is the object to be obtained." 



It will be seen that in all the elevation^ of oblique arches 

 given by Mr. Nicholson, the bed-joints in the face are straight 

 lines^ whereas by my rule they are necessarily curves (see 

 fig. 5 of the plate illustrating my paper.) 



In corroboration of my statement that "many practical 

 men have experienced considerable difficulty in the construc- 

 tion of skew bridges," Mr. Welch has furnished several in- 

 stances; and the fact which he adduces, that "the executive 

 engineer of a very extensive public railroad has very prudently 

 applied to a competent person for a definite development of your 

 (Mr. Nicholson's) ^nVic/p/^," is really admitting that that prin- 

 ciple is not developed with sufficient clearness for practical 

 men in Mr. Nicholson's Treatise. Mr. Welch also seems 

 aware of some latent defect in Mr. Nicholson's rule, for he 

 says, " I am firmly of opinion, after an attentive observation 

 of the practical worJcing of the best'* (meaning of course Mr. 

 Nicholson's) " rule, that it is very injudicious to adopt them," 

 (oblique arches,) "except in cases of absolute necessity;" and 

 so giving it to be understood that oblique arches built on 

 Mr. Nicholson's principle are not so substantial as square 

 ones. Mr. Welch will find it difficult to point out any such 

 want of stability in arches built on my principle. 



Every one will be ready to admit that oblique arches should 

 be avoided, from being more expensive, and not equal in appear- 

 ance; but that they are not inferior from any fault in their 

 *^ practical working,'' my own practical working has established. 



I trust that what has been brought forward shows that 

 Mr. Nicholson's rule and mine are not alike, and therefore 

 that he has no " prior" " claim" to mine ; and that my rule 

 possesses advantages not to be found in his. 



I am. Gentlemen, your obedient servant, 



Park Village East, London, Jan. 19, 1837. CllARLES Fox. 



Third Series. Vol.10. No. 60. March \837. Z 



