only may the nature of the changes which are constantly taking place be clearly and unquestionably explained, but the quantities of gases absorbed and evolved be correctly ascertained.

Circumstances have prevented the possibility of my renewing such delightful employments; I therefore leave my observations, unsatisfactory as they may be, as hints to others, who may have both opportunities and abilities to investigate the subject throughout all its bearings.

An Account of the Chain-bridge erected at The Hyde, Ingatestone, Essex, 1829.

HAVING proposed to myself the erection of a bridge over a piece of water in my private grounds, it occurred to me that one of suspension might be constructed upon common chain, if it were sufficiently strong, and would, at the same time, be much cheaper than either in wood or brick.

After consulting with some persons whose practical knowledge of chains was much greater than my own, and having calculated, as well as I was able, the probable weight of the proposed joists, flooring, and side pieces, I procured a proper quantity fit for my purpose, ready made, from one of the principal dealers in London. The breadth of the water is fifty feet, and, consequently, the suspended flooring is the same, and the width of the walk is four feet. By as accurate a computation as I could make, I found the weight would be from 3-4ths of a ton to a whole ton, making the flooring of inch deal-boards, and the other wood work and parts of the dimensions stated below. The suspending wires do not weigh more than two hundred weight.

The chain I have used, I was told, would support, in a perpendicular direction, singly, one ton and a half. The bridge complete is represented in p. 316. The dimensions of the different parts (shewn in the same and following page) are as follows:

The main chains, A, Fig. 1, three pounds to the foot, are seventy-five long each from the places where they join the anchorage.

The joists, B, Fig. 3, are of heart of oak, four feet six inches long, and in substance three inches by two, bearing the weight edgewise. In the

under side of these, two inches from each end, is a notch for the purpose of receiving a stirrup, (in order to keep it in its place,) in which they rest. These joists are fifteen inches apart.

The side pieces, C, Fig. 3.-The joists being placed in their stirrups, there are laid over them, and just within them, four pieces of deal, three inches by four and a half in substance, and twelve feet long each, having notches cut in the lower edges, at equal distances of fifteen inches, to receive the joist, and are made fast to them by a screw passing downward into the joist; these I have called side pieces, and their use is to keep the joists in their places, and steady.

The suspension wires, D, Fig. 1.-The chains being properly placed, and secured, iron wires 3-8th inch in diameter, of proper length, having a hook turned at each end, after the manner of S hooks, were put on; one end of each wire is hung upon a link of the chain, and on the other is hung the stirrup.

The stirrup, E, Fig. 1, is made of the same sized wire, with a loop, or hook, at the top, and receives the end of the joist on its flat side, falling into the notch described before.

APRIL-JUNE, 1829.

Y

The anchors, F, Fig. 5, if they may be so called, are in fact old gardenroller stones, round each of which is a strong bar of iron, in substance one inch square, welded again to itself, so as to form a ring, in which the stone lies horizontally. This bar is brought out to the length of four or five feet, and turned into a very strong hook at the end; the stone with the hoops (for there are two at each stone, being one for each chain) are buried in the ground three to four feet deep, and rammed in, presenting above ground a portion of the iron bar and the hook.

The chains, G, Fig. 4, are fastened to the anchorage by means of plates of iron somewhat similar to the links of the chains in the large suspension bridges. Two parallel plates, of similar shapes and equal lengths, with a hole at the end of each, are placed one on one side and one on the other of the anchor-hook, and a bolt as large as the hook will carry, passed through the three, and screwed close together by a nut. A similar bolt is run through the other ends of the parallel plates, and the last link of the chain being placed between them, are screwed together in the same manner. The plates are ten inches long, and one inch by 5-8ths in substance.

There are two chains on each side of the bridge, and the joists are hung alternately, each on two chains only. Thus, the first joist hangs on the outside chain, on the left hand, and the inside chain on the right hand; the next on the inside chain on the left hand, and on the outside chain on the right hand; the third hangs as the first, the fourth as the second, and so on, so that each chain supports half the weight of one side of the bridge, and, consequently, 1-4th of the whole. The piers (Fig. 2), over which the chains pass, are of 'wood, heart of oak, five inches by six in substance, and set into the ground two feet and a half. They are put with their tops leaning together, at such an angle as to make the base of the triangle half its perpendicular height; they are braced together under ground, and bolted together at three feet and a half from the surface of the land, and also, again, immediately below where they meet. A moveable stone cap protects them, and the grove in which the chains lie, from wet.

They have been now in use nearly two years, and have not varied from their original position the 1-8th part of an inch, as appears from a mark with a chisel, made when first put up.

When the bridge was first erected, the chains were brought in a direct line from the top of the piers to the anchor-hooks. But, then, the angles formed by the descending chains on each

side of the piers, with the perpendicular axis of the triangle, were not equal, and the pressure was necessarily thrown more upon the inner post than the other. To have corrected this by lengthening the chains, would have made them inconveniently long. I therefore took the bridge off the chains, and placed an oaken stay, H, (Fig. 2,) resting its lower end on a strong iron bracket, I, (Fig. 2,) at the foot of the outer post, and put it in a direction parallel to the inner post, making its length just sufficient to raise the chain to an angle, externally, equal to the inner one. At the same time, finding that the vibration of the bridge was very considerable, owing to its length, the suspended flooring (which is exactly fifty feet), and its great lightness, I put an additional stay on each side, in one piece, forty-eight feet long, over the other four side pieces, and bolted them together, which has had the effect of stiffening it, without adding much to its weight, which I conjecture does not now much exceed a ton.

The beauty of the bridge is great, and though not absolutely necessary, being over a piece of water in a private pleasure ground, is very useful and efficient. The whole cost of materials and workmanship did but little exceed thirty pounds, This account I have put together, which, it is feared, is much wanting in technical or scientific accuracy, to shew how easily and cheaply foot bridges of this description may be erected; and of materials always to be had. If the posts which support the chains, the most perishable of all the parts, should in ten or fifteen years decay, so as to endanger the fabric, ten pounds in money and three days in time would set it up again.

Upon a rough estimate, a wooden bridge of the same span would have cost from eighty to one hundred pounds, and a high arch probably from one hundred and fifty to two hundred pounds.

I am, &c.,

JOHN DISNEY.