STONE BRIDGE. In a former article, (13nincE) general historical view has been taken, of the rise, progress, and present state, of bridge-building, exemplified in descrip tions of the most celebrated edifices of the kind, in various parts of the world. Under the present head, the theory of the art will be principally considered ; and we shall avail ourselves of the permission granted to us by the late Thomas Telford, Esq., to make considerable extracts from the able article on this subject, written by him for the Edinburgh Encyclopedia.
" The 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 balancing 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 subject, from the beautiful specimens of their architecture which exist even in our times ; yet, in none of their authors, either prac tical 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 are still the ornament of the chief cities in Europe, and the delight of the architectural antiquary, seem to have fondly indulged in the balancing of arches. They were, without doubt, directed by maxims which had been elicited front a varied and extensive practice ; but what ever these were, they are to us unknown. None of these architects, though many of them were men of learning, seem ever to have committed to writing, either the history of any such erection, 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 initiated 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 mathe
matical sciences were then little known, and we may see front the construction of the bridges of that age, that the priests, who were the only architects, have had in their eye, rather, the successive vaulting of a Gothic cathedral, than to have originally considered of the best way of forming a per manent 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 atten tion of mathematicians. Since that time, volumes have been written respecting the equilibrium of arches. It has been found one of the most delicate, as it is one of the most impor tant, applications of mathematical science. Yet with all due deference to the etninent men who have. prosecuted this sub ject, we are much inclined to doubt whether the greater part of their speculations have been of any value to the practical builder. lle 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 furthest from the deductions of the theorist ; and at all events, lie finds that a much greater latitude is allowable than theory seems to warrant. Ile 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 intelligent prac titioner. 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, with out requiring much previous scientific information. Indeed, though we highly value the sublime geometry, we are inclined to think that the unnecessary parade of calculus in the application of science 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.