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HISTORY OF ENGINEERING.

I>uok I.

Though there seems to be in the manner of framing, and in the strength of the materials,abundant provision made to supply this want of equilibrium, yet as the tendency ofso great an increase of pressure at the haunches must be to make the arch spring at thecrown, it may perhaps be expedient to make the wedges near the crown deeper and heavier,in comparison of the rest, than is proposed in the plan. I am aware, at the same time,that the power of the iron bars that compose the framework of the bridge, to resist ex-tension as well as compression, and to draw as well as to thrust (quite different from whathappens in stone, where it is the latter only which takes place), may render this pre-caution unnecessary. Those who have had experience in the construction of arches ofiron arc the only good judges in this case.

Mr. J. Robeson did not think the mode of widening the bridge towards the ends can bematerially improved. This additional width does contribute to increase the load on thehaunches, which are already overloaded; but they are so stiff, if united with moderatecare, that they are vastly stronger than is necessary, and will not be sensibly affected bythis addition. It is in some degree hanging work, that is, hanging by the rest, and not byany regular abutment. This might be given it, as in the Pont Admirable between Calaisand Ardres , which I examined with great attention many years ago. But this wouldrequire a most difficult framing, with oblique angles, each frame and each tie requiring amould for itself, and it would not be one fiftieth stronger.

Dr. Charles Hutton says, there can be no doubt but the bridge will be greatly strengthenedby an increase of its width towards the two extremities or abutments, especially if the coursesor parts be connected together in the manner mentioned in the answer to the first question;for thus the extent of the base of the arch at the impost being enlarged, the strength orresistance of the abutment will be increased in a much higher degree than the weight andthrust of the arch, and consequently will resist and support it more firmly. The architself will thus also acquire a great increase of strength and stability, both from the quantityand disposition of the materials, as well vertically as horizontally, by which, in the latterdirection in particular, the arch will be better enabled to preserve its true vertical position,and to resist the force or shock of any thing striking against it in the horizontal direction ;and for the better security in these particulars, considering the immense stretch of thearch, it will be perhaps advisable to enlarge the width in the middle to 50 feet instead of45 feet, and at the extremities to 100 feet instead of 90 feet, as proposed in the design.As to the form of this width or enlargement, the side of the arch might be bounded by acircular arch or by any curve that will look more graceful, perhaps a very eccentric ellipsewill answer as well as any other curve, or better.

Mr. Atwood observed, that the proposed increase of breadth near the abutment does notappear to affect the strength of the bridge in a vertical direction; but a more effectualresistance will be opposed in consequence of this form to any force which may be appliedhorizontally, and perpendicular to the plane of the arch to alter its position in thatdirection.

Colonel Twiss stated, that the increasing breadth of the arch at the abutments seemsto afford no advantage in the vertical section, on the supposition that no force exceptgravity could act upon it; but considering the length and breadth, it appears a wise pre-caution to resist any force acting in a horizontal direction, such as wind, the masts ofvessels, &c., and as a guard against vibration ; perhaps the strongest form of widening thebridge would be by constructing the sides in straight lines from the centre to the abut-ments, but the present proposal is cheaper, and may answer the object.

Mr. JHliiam Jessop says, that widening the bridge towards the ends will be attended withmany inconveniences, and a great extra expense; it is worth consideration whether it maynot have sufficient strength to counteract any lateral bias by other means. I am muchdisposed to believe that if the braces which connect the ribs with each other, instead ofbeing rectangular with the ribs, were to be connected diagonally’, so as to form triangles,little more would be wanted to give it the necessary stiffness ; but when, in addition to thisI consider that the whole covering of the bridge under the roadway may be of iron plates,or reticular gratings with flanclies, so that they may be put together with screw bolts andnutts of cast-iron, or with eye bolts and cotters, and form one great iron plate or grating of45 feet in width ; I have no conception of its being capable of any sensible flexibility edge-ways ; and by diagonal braces, in a vertical as well as horizontal position, which I conceivemay be applied without much difficulty, and without the aid of malleable iron. I shouldhave no apprehension of its being liable to any material injury from any external violence,and upon the whole I believe, that an arch of 600 feet span, similar to that in question,with such improvements as it may be susceptible of, is practicable, and capable of beingrendered a durable edifice.

Mr. J. Rennie observed, that the strength of the arch will be very much increasedhorizontally, by the increase of width towards its two extremities. But I do not think anyadvantage will arise to it vertically from this increase, but rather on the contrary ; for, asthe additional width will also increase the weight, it cannot be made in equilibrium unless