Chap. VIII.

BRITAIN.

493

sinned, will principally depend on the quantity of materials used, to be determined by thedegree of strength, arising from massive weight, it may be proposed to give to thestructure; and the distribution of the mass among the several sections will be regulatedaccording to the principle adopted for establishing the equilibrium, which, it appears tome, will be to all practical purposes the same, whatever be the form of the arch.

Colonel Twiss _Increasing the rise of the arch 10 feet, and leaving the span the same,

would strengthen the bridge, chiefly by increasing the width of the back part of thevoussoir; but I cannot state the exact proportion, nor do I believe any theory can ;neither do I think that theory can answer the second part of this question, but I am ofopinion, that when an arch can be built to answer its object, and yet contain less thansixty degrees, that in all such cases the segment of a circle is to be preferred to anelliptical one.

Mr. John Southern .—The span of the arch remaining the same, as also the weight of theroad, the stress or compressing force, which the arch frames would suffer, and also the hori-zontal push against the abutments, would be lessened, by increasing the altitude, or versedsine of the arch, nearly in the inverse proportion of the altitude, or more accurately, in thepresent instance, as eight to seven. It would appear, therefore, that the members of thearch might be lessened in section to sustain the same road, but as the road itself is supposedto be the same weight, the whole quantity of materials of the bridge would not be lessenedby increasing the altitude of the arch in any such ratio as the above; and the force thatwould break up the bridge would rather be lessened, supposing the depth of the archframes the same.

The elliptical arch is better adapted to resist the pressure of a road of uniform thickness,which is supported by pillars, pointing to the centre of the arch, especially if it widentowards the abutments *, but for the same altitude, as it implies a less degree of curvatureat the crown, it necessarily induces a greater compressing force on the arch frames, winch,of course, will be transmitted to the abutments. The degree of ellipticity which the casewould demand is, I think, too trifling to afford greater convenience for vessels to passunder; it cannot be more durable, being made of the same materials, but it will certainlybe much more expensive and difficult to execute well than the arch of a circle.

Fourteen other questions were submitted to these professors and others by the selectcommittee, but they were chiefly of a nature relating to the material and cost, which werenot very satisfactorily answered; and Mr. John Playfair concludes his report by observing,“ That it is not from theoretical men that the most valuable information in sucli a case asthe present is to be expected. When a mechanical combination becomes in a certaindegree complicated, it baffles the efforts of the geometer, and refuses to submit even to themost approved methods of investigation. This holds good particularly of bridges, whenthe principles of mechanics, aided by all the resources of the higher geometry, have not yetgone further than to determine the equilibrium of a set of smooth wedges, acting on oneanother by pressure only, and in such circumstances as, except in a philosophical experi-ment, can hardly ever be realised. It is, therefore, from men educated in the school ofdaily practice and experience, and who, to a knowledge of general principles, have added,from the habits of their profession, a certain feeling of the justness or insufficiency of anymechanical contrivance, that the soundest opinions on a matter of this kind can beobtained.”

Iron Bridges .—Colebrook Dale, over the Severn, was the first erected in England, andwas completed after the designs of Abraham Derby, in the year 1777. It is situatedbetween the villages of Madeley and Brosely in Shropshire , and where the river is bothnarrow and rapid.

The abutments are of stone, and were built up about 10 feet above the common lowwater mark; they are finished off with a platform of squared freestone, in breadth 10 feet,which is made to answer both for a base for the springing of the arch and for a towingpath.

Cast-iron plates, 4 inches in thickness, formed with sockets to receive the ribs, are laidupon this platform, and to save metal the plates are cast with considerable openings. Onthese rest the five principal or lower ribs, which form the arch; they are 9 inches by ;above them is a second row, cut off at the top by the horizontal bearing pieces, by6 inches; and above these is a third row, 6 inches by 6 inches. Between the ribs areupright standards, 15 inches by inches, with an open space in the middle, 5| inchesin breadth. The back standards are 9 inches by 6| inches, with projections for the braces;the diagonals and horizontal ties are 6 inches by 4 inches, and the cast-iron tie bolts arein diameter 2\ inches.

The covering plates, which reach quite across the bridge, are 26 feet in length, and 1inch in thickness.

The great ribs are each cast in two pieces, meet at the keys, and are 70 feet in length,being one-half of the entire circumference of the arch, which has a clear span of 100 feet 6inches, and a rise of 45 feet.