THEORY OF BRIDGES.
TEE construction of a magnificent stone bridge is justly looked upon as one of the greatest performances of the masonic art : for if we compare the enormous weight of a great arch, with the strength which the cohesion of the firmest cement can give, we readily admit, that it is only by the nicest adjustment and balanc ing of its parts, that they are hindered from instantly falling to pieces.
Though there can be little doubt that the Romans and latter Greeks had paid some attention to this sub ject, from the beautiful specimens of their architecture, which exist even in our times ; yet in none of their authors, either practical or scientific, is the smallest light afforded us respecting the principles upon which their practice was regulated.
The architects of the middle ages, who constructed those great cathedrals that arc still the ornament of the chief cities in Europe, and the delight of the architec tural antiquary, seem to have fondly indulged in the balancing of arches. They were without doubt directed by maxims, which had been elicited from a varied and extensive practice ; but, whatever these were, they are to us unknown. None of these architects, though many of them were men of learning, seem ever to have com mitted to writing, either the history of any such erec tion, or the principles by which its construction was regulated. Nay, this knowledge seems rather to have been carefully kept secret, and regarded as a sort of mystery ; a craft, which was only to be communicated to the brethren, whose experience and skill had already qualified them to be irritated into the mysteries of the sublime degree.
It does not appear, that a knowledge of this subject could be acquired otherwise than by experience. The mathematical sciences were then little known; and we may see from the construction of the bridges of that age, that the priests, who were the only architect, have had in their eye rather the successive vaulting of a Gothic cathedral, than to have originally considered of the best way or forming a permanent and convenient road. It was only about a century ago, when Newton had opened the path of true mechanical science, that the construction of arches attracted the attention of ma thematicians. Since that time, volumes have been writ
ten respecting the equilibrium of arches. It has been found one of the most delicate,*as it is one of the most important applications of mathematical science. Yet, with all due deference to the eminent men who have prosecuted this subject, we are much inclined to doubt whether the greater part of their speculations have been of any value to the practical bridge builder. He is still left to be guided by a set of maxims derived from long experience, and as yet little improved by theory. In truth, his works seldom fail even where they differ far thest from the deductions of the theorist; and at all events, he finds that a much greater latitude is allowable than theory seems to warrant. He is therefore surely excusable in doubting of the justice of such theories, at least until they are more consonant to the approved practice.
It is our intention, in the present article, to point out a new mode of considering this subject, to which, with great diffidence, we request the attention of the intelli gent practitioner. It may indeed still be deficient, if not in some respects erroneous. But it will, we think, have this merit, that of being readily apprehended, and easily applied, without requiring much previous scienti fic information. Indeed though we highly value the sublime geometry, we are inclined to think that the un necessary parade of calculus in the application of sci ence to the arts, has been one of the chief causes of the dislike, which many able practical men of our country have shown to analytical investigation.
Nevertheless, as many of our readers are well quali fied to comprehend, and will naturally expect that we should point out, the modes of investigation, usually pursued in this interesting subject; we shall previously, and in as succinct a manner as possible, endeavour to lay before them the commonly received theory of equili bration. From which, having cleared away the useless rubbish, if we can extract any proper materials, we may, like economical builders, make good use of them in our future structure.