SUPPL. VOL. I. Part I.
not be of long duration. The bridge at Mantz is still more exceptionable, because its piers are tall and slender. If any one of the arches fails, the rest must fall in a moment. An arch of Blackfriars Bridge might be blown up without disturbing its neighbours.
Mr Perronet mentions another mode of striking the A had method of centering, which he says is very usual in France. Every second bridging is cut out. Some time after, every second of the remainder; after this, every second of the remainder; and so on, till all are removed. This is never practiced in this country, and is certainly a very bad method. It leaves the arch hanging by a number of distant points; and it is wonderful that any arch can bear this treatment.
Our architects have generally proceeded with extreme caution. Wherever they could, they supported the centering by intermediate pillars, even when it was a trussed centre, having a tie-beam reaching from side to side. The centre was made to rest, not immediately on these pillars, but on pieces of timber formed like acute wedges, placed in pairs, one above the other, and having the point of the one on the thick end of the other. These wedges were well soaped and rubbed with black lead, to make them slippery. When the centres are to be struck, men are stationed at each pair of the wedges with heavy mauls. They are directed to strike together on the opposite wedges. By this operation, the whole centering descends together; or, when any part of the arch is observed to have opened its joints on the upper side, the wedges below that part are slackened. The framing may perhaps bend a little, and allow that part to subside. If any part of the arch is observed to open its joints on the under side, the wedges below that part are allowed to stand after the rest have been slackened. By this process, the whole comes down gradually, and as slowly as we please, and the defects of every part of the arch may be attended to. Indeed the caution and moderation of our builders have commonly been such, that few defects have been allowed to shew themselves. We are but little acquainted with joints opening to the extent of two inches, and in such a case would probably lift every stone of the arch again (a). We have not employed trussed centerings so much perhaps as we should have done; nor do we see their advantage (speaking as mere builders) over centres supported all over, and unchangeable in their form. Such centres must bend a little, and require loading on the middle to keep them in shape. Their compression and their elasticity are very troublesome in the striking of the centres in Mr Perronet's manner. The elasticity is indeed of use when the centres are struck in the way now described.
These observations on the management of the internal movements of a great arch, will enable the reader to appreciate all the merit of Mr Mynne's very ingenious construction. We proceed therefore to complete our description.
C e
(a) The writer of this article can only say, that, after much inquiry, he has no information of any arch being received from the builder as sufficient that had suffered half the change of shape mentioned by Mr Perronet. The arch of Dublin bridge, built by an excellent, but a very private, mason, Mr Steven, is 105 feet wide, with only 22 feet of rise. It was erected (but not on a trussed centering) without changing one full inch in its elevation; and when the centering was removed, it sunk only 1 1/4 inches, and about half an inch more when the parapets were added and the bridge completely finished.
The gradual enlargement of the base of the piers of Blackfriars bridge enabled the architect to place a series of five posts c, c, c, c, c, one on each flap of the pier; the ingenious contexture of which made it like one solid block of stone (see ARCH, Supplement). These struts were gradually more and more oblique, till the outer one formed an obtuse angle with the lowest side of the interior polygon of the truss. On the top of these posts was laid a sloping seat or beam D of stout oak, the upper part of which was formed like a zig-zag scarfing. The posts were not perpendicular to the under side of the seat. The angles next the pier were somewhat obtuse. Short pieces of wood were placed between the heads of the posts (but not mortified into them), to prevent them from slipping back. Each face of the scarf was covered with a thick and smooth plate of copper. The feet of the truss were mortified into a similar piece E, which may be called the SOLE of the truss, having its lower side notched in the same manner with the upper side of D, and like it covered with copper. Between these two lay the STRIKING WEDGE E, the faces of which corresponded exactly with the slant faces of the seat and the sole. The wedge was so placed, that the corresponding faces touched each other for about half of their length. A block of wood was put in at the broad end or base of this wedge, to keep it from slipping back during the laying the archstones. Its outer end E was bound with iron, and had an iron bolt several inches long driven into it. The head of this bolt was broad enough to cover the whole wood of the wedge within the iron ferrule.
We presume that the reader, by this time, foresees the use of this wedge. It is to be driven in between the sole and the seat (having first taken out the block at the base of the wedge). As it advances into the wider spaces, the whole truss must descend, and be freed from the arch; but it will require prodigious blows to drive it back. Mr Mylne did not think so, founding his expectation on what he saw in the launching of great ships, which slide very easily on a slope of 10 or 12 degrees. He rather feared, that taking out the block behind would allow the wedge to be pushed back at once, so that the descent of the truss would be too rapid. However, to be certain of the operation, he had prepared an abundant force in a very ingenious manner. A heavy beam of oak, armed at the end with iron, was suspended from two points of the centre like a battering ram, to be used in the same manner. Nothing could be more simple in its structure, more powerful in its operation, or more easy in its management. Accordingly the success was to his wish. The wedge did not slip back of itself; and very moderate blows of the ram drove it back with the greatest ease. The whole operation was over in a very few minutes. The spectators had suspected, that the space allowed for the recess of the wedge was not sufficient for the settlement of the arch; but the architect trusted to the precautions he had taken in its construction. The reader, by turning to the article ARCH in this Supplement, will see, that there was only the arch LY which could be expected to settle; accordingly, the recess of the wedge was found to be much more than was necessary. However, had this not been the case, it was only necessary to take out the pieces between the posts below the seat, and then to drive back the heads of the struts; but this was
not needed (we believe) in any of the arches. We are well assured that none of the arches sunk an inch and a half. The great arch of 100 feet span did not sink one inch at the crown. It could hardly be perceived whether the arch quitted the centering gradually or not, so small had been the changes of shape.
We have no hesitation in saying, that (if we except some waste of great timber by uncommon joggling) the whole of this performance is the most perfect of any of the centering that has come to our knowledge. We doubt not but that several have equalled it, or may have excelled it; but we do not know of them: and we think, that the bringing forward such performances is no less serviceable to the public than it is honourable to the inventor. Nor do we suppose that any views of interest can be so powerful as to prevent an ingenious architect from communicating to the public such honourable specimens of his own talents. We should be happy to communicate more of this kind; for we consider it as a very important article of practical mechanics, and think that it is of consequence to the nation that it should be very generally understood. In every corner of the country bridges are to be built—we have everywhere good masons, who are fully able to execute any practicable project, but too little acquainted with principle to invent, or to accommodate even what they know to local circumstances, and are very apt to be duped by appearances of ingenuity, or misled by erroneous notions of the strains which are excited. We profess more science, and to treat the subject with the assistance of accurate principles: But while we are certain that every circumstance is susceptible of the most accurate determination, we must acknowledge that we have by no means attained an accurate knowledge of all the strains which are produced and excited in a frame of carpentry, which is settling and changing its shape, even though it be not very complicated; far less are we possessed of a clear view of what happens in a mass of masonry in similar conditions. Therefore, though we speak with the strong belief of our being right, we speak with a sense of our fallibility, and with great deference to the judgment of eminent and experienced architects and engineers. We should consider their free and candid criticisms as the highest favour; and we even solicit them, with assurances of thanks, and that we will take some opportunity, before the close of this work, to acknowledge and correct our mistakes. We even presume to hope, that the liberal minded artist will be pleased with this opportunity which we give him of increasing the national stock of knowledge. Let mutual jealousy and rivalry reign in the breasts, and prompt the execrations, of our restless neighbours on the continent—let them think that the dignity of man consists in perpetual warfare, in which every individual feels himself indebted only to himself, freed from all the sweet ties of domestic partiality, of friendship, and of patriotic attachment. We hope that the hearts of Britons will long continue to be warmed and fortified by the thoughts of mutual assistance, mutual co-operation, mutual attachment, and a patriotic preference of their countrymen to all other men. While these sentiments are regulated by unshaken honesty, by candour, and by Christian charity, we shall be secured from the errors of partial attachments, and yet enjoy all the pleasures of unsophisticated nature. Families will still be bound together by the affectionate
ties of blood; and the whole frame of British society will be in harmony with the bonds which connect the members of each family, by their endless crossings and intermixings. In this state, the state of social nature, the man of talents will not look up all the fruits of his exertions in his own breast, but will feel a pleasure in imparting them to a society that is dear to him, and on which he depends for all his best enjoyments. Nothing will hold the good man back when this is in his power, but the virtuous use which he can make of his superiority in the discharge of his own little circle of duties. This is all that is required of true patriotism; and it is not too much to be expected from Britons, who feel a pleasure in viewing their country as the great school of the arts, under the patronage of a sovereign who has done more for their improvement than all the other princes of Europe, and who (we are well assured) is now meditating a plan which must be highly gratifying to every eminent professor of the arts.
extent of figure. Very rarely can we suspend it from Center points situated as D and E. It is even seldom that we have depth enough of bank to allow the support of the rafters dC, eC; but we can always find room for the simplest truss AFB. This therefore is the most usually met with and practiced.
In the construction, we must follow the maxims and directions prescribed in the article CARPENTRY of this volume, and the article ROOF of the Bagel. The beams FA, FB must be mortised into AB, in the firmest manner, and there secured with straps and bolts; and the middle must hang by a strap attached to the king post FC, or to the iron rod that is used for a king post. No mortising in the point C must be employed; it is unnecessary, and it is hurtful, because it weakens the beam, and because it lodges water, and soon decays by rot. The best practice is not to suspend the beam immediately by this strap, but to let it rest, as in fig. 10, on a beam C, which crosses the bridge below, and has its other end supported in the same manner by the other truss.
It is evident that the length of the king post has no effect on the support of C. We may therefore contract every thing, and preserve the same strength of support, by finding two points a and b (fig. 11.) in the banks, at a moderate distance below A and B, and setting up the rafters aF, bF, and suspending C from the shortened king post. In this construction, when the beam AB rests on a cross bearer, as is drawn here, the struts aF, bF are kept clear of it. No connection between them is necessary, and it may be hurtful, by inducing cross strains on both. It will, however, greatly increase the stiffness of the whole. This construction may safely be loaded with ten times the weight that AB can carry alone.
Suppose this done, and that the scantling of AB is too weak for carrying the weight which may be brought on the parts AC, CB. We may now truss up each of that me-36 half, as in fig. 12, and then the whole will form a handsome bridge, of the simplest construction possible. The intersections of the secondary braces with those of the main truss will form a hand-rail of agreeable figure.
We are not confined to the employment of an entire piece AB, nor to a rectilinear form. We may frame the bridge as in fig. 13, and in this form we dissuade from allowing any connection with the middle points of the main braces. This construction also may be followed till each beam AC and CB is loaded to ten times what it can safely bear without the secondary trussing.
There is another way by which a bridge of one beam may be supported beyond the power of the first and simplest construction. This is represented in fig. 14, and fig. 15. The truss beam FG should occupy one-third of AB. The advantage of this construction is very considerable. The great elevation of the braces (which is a principal element of the strength) is preserved, and the braces are greatly shortened.
This method may be pushed still farther, as in fig. 16.
And all these methods may be combined, by joining the constructions of fig. 14 and fig. 15, with that of fig. 16.
In all of them there is much room for the display of skill, in the proper adjustment of the scantling of the timber.
THE subject which we have been considering is very closely connected with the construction of wooden bridges. These are not always constructed on the sole principles of equilibrium, by means of mutual abutment. They are stiff frames of carpentry, where, by a proper disposition, beams are put into a state of extension, as well as of compression, so as to stand in place of solid bodies as big as the spaces which the beams inclose; and thus we are enabled to couple two, three, or four of these together, and set them in abutment with each other like mighty architones. We shall close this article, therefore, with two or three specimens of wooden bridges, disposed in a series of progressive composition, so as to serve as a sort of introduction to the art in general, and furnish a principle which will enable the intelligent and cautious artist to push it with confidence as far as it can go.
The general problem is this. Suppose that a bridge is to be thrown over the space AB (fig. 9.), and that this is too wide for the strength of the size of timber which is at our command; how may this beam AB be supported with sufficient effect? There are but two ways in which the middle point C (where the greatest strain is) can be supported: 1. It may be suspended by two ropes, iron rods, or wooden ties, DC, EC, made fast to two firm points D, E, above it; or it may rest on the ridge of two rafters dC, eC, which rest on two firm points d, e below it. 2. It may be supported by connecting it with a point so supported; and this connection may be formed, either by suspending it from this point, or by a post resting on it. Thus it may hang, by means of a rod or a king-post FC, from the ridge F of two rafters AF, BF; or it may rest on the strut C/, whose lower extremity f is carried by the ropes, rods, or wooden ties Af, Bf.
Whichever of these methods we employ, it follows, from the principles of carpentry, that the support given to the point C is so much the more powerful, as we make the angle DCE, or dCe, or the equivalent angles AFB, or AfB, more acute.
Each of these methods may be supposed equally strong. Our choice will depend chiefly on the facility of finding the proper points of support D, E, d, e; except in the second case, where we require no fixed points but A and B. The simple forms of the first case require a great
timber, and the obliquity of the braces to the lengths of the different bearings. A very oblique strut, or a slender one, will suffice for a small load, and may often give an opportunity to increase the general strength; while the great timbers and upright supports are reserved for the main pressures. Nothing will improve the composition so much as reflecting progressively, and in the order of these examples, on the whole. This alone can preserve the great principle in its simplicity and full energy.
These constructions are the elements of all that can be done in the art of building wooden bridges, and are that can be found more or less obviously and distinctly in all attempts of this kind. We may assert, that the more obviously they appear, the more perfect the bridge will be. It is astonishing to what extent the principle may be carried. We have seen a bridge of 42 feet span formed of two oak trusses, the biggest timber of which did not exceed six inches square, bearing with perfect steadiness and safety a waggon loaded with more than two tons, drawn by four stout horses. It was framed as fig. 16. nearly, with the addition of the dotted lines, and was near thirty years old; protected, however, from the weather by a wooden roof, as many bridges in Germany are.
We recollect another in the neighbourhood of Stettin, which seemed constructed with great judgment and spirit. It had a carriage road in the middle about 20 feet (we think) wide, and on each side a foot way about five feet wide. The span was not less than 60 feet, and the greatest scantling did not appear to exceed 10 inches by 6.
This bridge consisted of four trusses, two of which formed the outside of the bridge, and the other two made the separation between the carriage road and the two foot ways. We noticed the construction of the trusses very particularly, and found it similar to the last, except in the middle division of the upper truss, which, being very long, was double trussed, as in fig. 17.
The reader will find in that volume of Leupold's Theatrum Machinarum, which he calls Theatrum Pontificum, many specimens of wooden bridges, which are very frequent in the champaign parts of Germany. They are not, in general, models of mechanic art; but the reflecting reader, who considers them carefully, will pick up here and there subordinate hints, which are ingenious, and may sometimes be useful.
What we have now exhibited are not to be considered as models of construction, but as elementary examples and lessons, for leading the reader systematically into a thorough conception of the subject.
We cannot quit the subject without taking notice of a very wonderful bridge at Wittingen in Switzerland, slightly described by Mr Coxe (Travels, vol. I. 132.) It is of a construction more simple still than the bridges we have been describing. The span is 230 feet, and it rises only 25. The sketch (fig. 18.) will make it sufficiently intelligible. ABC is one of two great arches, approaching to a Catenarian shape, built up of seven courses of solid logs of oak, in lengths of 12 or 14 feet, and 16 inches or more in thickness. These are all picked of a natural shape, suited to the intended curve; so that the wood is nowhere cut across the grain to trim it into shape. These logs are laid above each other, so that their abutting joints are alternate, like
those of a brick wall; and it is indeed a wooden wall, simply built up, by laying the pieces upon each other, taking care to make the abutting joints as close as possible. They are not fastened together by pins or bolts, or by scarings of any kind. They are, however, held together by iron straps, which surround them, at the distance of five feet from each other, where they are fastened by bolts and keys.
These two arches having been erected (by the help, we presume, of pillars, or a centering of some kind), and well butted against the rock on each side, were freed from their supports, and allowed to settle. They are so placed, that the intended road a b c intersects them about the middle of their height. The roadway is supported by cross joists, which rest on a long horizontal summer beam. This is connected with the arches on each side by uprights bolted into them. The whole is covered with a roof, which projects over the arches on each side to defend them from the weather. Three of the spaces between these uprights have struts or braces, which give the upper work a sort of trussing in that part.
This construction is simple and artless; and appears, by the attempt to truss the ends, to be the performance of a person ignorant of principle, who has taken the whole notion from a stone arch. It is, however, of a strength much more than adequate to any load that can be laid on it. Mr Coxe says, but does not explain how, that it is so contrived that any part of it can be repaired independent of the rest. It was the last work of one Ulrich Grubenhamm of Tuffen, in the canton of Appenzel, a carpenter without education, but celebrated for several works of the same kind; particularly the bridge over the Rhine at Schafhausen, consisting of two arches, one of 172 and the other of 193 feet span, both resting on a small rock near the middle of the river.
While writing this article, we got an account of a wooden bridge erected in North America, in which this simple notion of Grubenhamm's is mightily improved. The span of the arch was said to exceed 250 feet, and its rise exceedingly small. The description we got is very general, but sufficient, we think, to make it perfectly intelligible.
In fig. 19. DD, EE, FF, are supposed to be three beams of the arch. They consist of logs of timber of small lengths, suppose of 10 or 12 feet such as can be found of a curvature suited to its place in the arch without trimming it across the grain. Each beam is double, consisting of two logs applied to each other, side to side, and breaking joint, as the workmen term it. They are kept together by wedges and keys driven through them at short intervals, as at K, L, &c.
The manner of joining and strongly binding the two side pieces of each beam is shewn in fig. 20. The mortise a i c b and d e i o, which is cut in each half beam, is considerably longer on the outside than on the inside, where the two mortises meet. Two keys, BB and CC, are formed, each with a notch b e d, or a i o, on its side; which notch fits one end of the mortise. The inner side of the key is straight, but so formed, that when both keys are in their places, they leave a space between them wider at one end than at the other. A wedge AA, having the same taper as the space just mentioned, is put into it and driven hard. It is evident that this must hold the two logs firmly together.
This is a way of uniting timber not mentioned in the article CARPENTRY; and it has some peculiarities worthy of notice. In the first place, it may be employed so as to produce a very strong lateral connection, and would then co-operate finely with the other artificial methods of scarfing and tabling that we described in the article referred to. But it requires nice attention to some circumstances of construction to secure this effect. If the joints are accurately formed to each other, as if the whole had been one piece divided by an infinitely thin saw, this manner of joining will keep them all in their places. But no driving of the wedge AA will make them firmer, or cause one piece to press hard on the other. If the abutment of two parts of the half beam is already close, it will remain so; but if open in the smallest degree, driving of the wedge will not make it tighter. In this respect, therefore, it is not so proper as the forms described in CARPENTRY.
In order that the method now described may have the effect of drawing the halves of the beams together, and of keeping them hard squeezed on each other, the joints must be made so as not to correspond exactly. The prominent angle aio (fig. 21), formed by the ends of the two half mortises, must be made a little more obtuse than the angle asf of the notch of the key which this prominence is intended to fill up. Moreover, the opposite side est of this key should not be quite straight, but a very little convex. With these precautions, it is easy to see that, by driving the wedge AA, we cause the notch asf to take hold, first at the two points a and s, and then, by continuing to drive the wedge, the sides as, sf, of the notch gradually compress the wood of the half beams, and press them on each other. By continuing to drive the wedge, the mutual compression of the key and the beam squeezes all together, and the space asf is completely filled up. We may see, from this process, that the mutual compression and drawing together of the timber will be greater in proportion as we make the angle aio more prominent, and its corresponding angle asf more deep; always taking care that the key shall be thick enough not to break in the narrow part.
This adjustment of the keys to the mortise is necessary on another account. Supposing the joints to fit each other exactly before driving the wedge, and that the whole shrinks a little by drying—by this the angle aio will become more prominent, and the angle asf will become more shallow; the joint will open at a and s, and the mutual compression will be at an end.
We may also observe, that this method will not give any additional firmness to the abutments of the different lengths employed to piece out the arch beam; in which respect it differs materially from the other modes of joining timber.
Having shown how each beam is pieced together, we must now show how a number of them are united, so as to compose an arch of any thickness. This is done in the very same way. The beams have other mortises worked out of their inner sides, half out of each half of the beam. The ends of the mortises are formed in the same way with those already described. Long keys BB, CC, (fig. 19.) are made to fit them properly, the notches being placed so as to keep the beams at a proper distance from each other. It is now plain that driving in a long wedge AA will bind all together.
In this manner may an arch be extended to any span, and made of any thickness of arching. The bridge over Portsmouth river, in North America, was more than 250 feet in length, and consisted of several parallel arches of beams. The inventor (we think that his name is Bludget) said that he found the strength so great, that he could with perfect confidence make one of four times the span. Center.
We admire the ingenuity of this construction, and think it very effectual for bringing the timbers into firm and uniform abutment; but we imagine that it requires equilibration, because it is extremely flexible. There is nothing to keep it from bending, by an inequality of load, but the transverse strength of the beams. The keys and wedges can have very little power to prevent this bending. The distance between the beams will also contribute little or nothing to the stiffness; nay, we imagine that a great distance between them will make the frame more flexible. Could the beams be placed so near each other that they could be somehow jogled on each other, the whole would be stiffer; but at present they will bend like the plates of a coach-spring. But nothing hinders us from adding diagonal pieces to this construction, which will give it any degree of stiffness, and will enable it to bear any inequality of loading. When completed in this manner, we imagine that it will be at least equal to any construction that has yet been thought of. One advantage it possesses that is very precious: Any piece that fails may be taken out, and replaced by another, without disturbing the rest, and without the smallest risk. On the whole, we think it a very valuable addition to British carpentry. The method here practised, both for joining the parts of one beam and for framing the different beams together, suggests the most firm and light constructions for dome-roofs that can be conceived; incomparably superior to any that have yet been erected. The whole may be framed, without a nail or a spike, into one net-like shell that cannot even be pulled in pieces. We may perhaps consider this in another article; at present we return to the consideration of trussed bridges.
When the width of the river exceeds what is thought practicable by a single truss, we must then combine either by simple addition, or by composition, different by simple trusses together. We compose a bridge by simple addition when we make a frame of carpentry of an unchangeable and proper shape, to serve as one of the archstones of a bridge of masonry. This may easily be comprehended by looking at fig. 22. Each of the frames A, B, C, D, must be considered as a separate body, and all are supported by their mutual abutment. The nature of the thing is not changed, although we suppose that the rails of the frame B, instead of being mortised into an upright vv, unconnected with the frame C, is mortised into the upright ee of that frame, the direction and intensity of the mutual pressures of the two frames are the same in both cases; accordingly this is a very common form of small wooden bridges. It is usual, indeed, to put diagonal battens into each; but we believe that this is more frequently done to please the eye than to produce an unalterable shape of each frame.
To an unskilful carpenter this bridge does not seem essentially different from the centering of Mr Hupeau for the bridge of Orleans; and indeed, in many cases,
it requires reflection, and sometimes very minute reflection, to distinguish between a construction which is only an addition of frame to frame till the width be covered, from a construction where one frame works on the adjoining one transversely, pushing it in one part, and drawing it in another. The ready way for an unlettered artist to form a just notion of this point, is to examine whether he may saw through the connecting piece b b from one end to the other, and make them two separate frames. Whenever this cannot be done without that part opening, it is a construction by composition. Some of the beams are on the stretch; and iron straps, extending along both pieces, are necessary for securing the joint. The bridge is no longer a piece of masonry, but a performance of pure carpentry, depending on principles peculiar to that art. Equilibration is necessary in the first construction; but, in the second, any inequality of loading is made ineffectual for hurting the edifice, by means of the stretch that is made to operate on some other piece. We are of opinion, that this most simple employment of the distinguishing principle of carpentry, by which the beams are made to act as ties, will give the most perfect construction of a wide bridge. One polygon alone should contain the whole of the abutments; and one other polygon should consist entirely of ties; and the beams which form the radii, connecting the angles of the two polygons, complete the whole. By confining the attention to these two simple objects, the abutments of the outer polygon, and the joints of the inner one, may be formed in the most simple and efficient manner, without any collateral connections and dependencies, which divide the attention, increase the complication, and commonly produce unexpected and hurtful strains. It was for this reason that we have so frequently recommended the centering of the bridge of Orleans. Its office will be completely performed by a truss of the form of fig. 23.; where the polygon ABCDEF, consisting of two layers of beams (if one is not sufficient), contains the whole abutments, and the other A b c d e f is nothing but an iron rod. In this construction, the obtuseness of the angles of the lower polygon is rather an advantage. The braces G c, G d, which are wanted for trussing the middle of the outer beams, will effectually secure the angles of the exterior polygon against all risk of change. The reader must perceive that we have now terminated in the construction of the Norman roof. We indeed think it the best general form, when some moderate declivity is not an insuperable objection. When this is the case, we recommend the general plan of the centering of the bridge of Orleans. We would make the bridge (we speak of a great bridge) consist of four trusses; two to serve as the outsides of the bridge, and two inner trusses, separating the carriage-way from the foot-paths. The road should follow the course of the lower polygon, and the main truss should form the rails. It might look strange; but we are here speaking of strength; and evident, but not unwieldy, strength, once it becomes familiar, is the surest source of beauty in all works of this kind.
Center of Friction, is that point in the base of a body on which it revolves; into which, if the whole surface of the base, and the mass of the body, were collected, and made to revolve about the centre of the base of the given body, the angular velocity destroyed
by its friction would be equal to the angular velocity destroyed in the given body by its friction in the same time. See FRICTION in this Supplement.
Center of Gyration, is that point in which, if the whole mass be collected, the same angular velocity will be generated in the same time, by a given force acting at any place, as in the body or system itself. This point differs from the centre of oscillation, in as much as in this latter case the motion of the body is produced by the gravity of its own particles; but, in the case of the centre of gyration, the body is put in motion by some other force acting at one place only.
Center of Oscillation, is that point in the axis or line of suspension of a vibrating body, or system of bodies, in which, if the whole matter or weight be collected, the vibrations will still be performed in the same time, and with the same angular velocity, as before. Hence, in a compound pendulum, its distance from the point of suspension is equal to the length of a simple pendulum whose oscillations are isochronal with those of the compound one.
Center of Pressure, of a fluid against a plane, is that point against which a force being applied equal and contrary to the whole pressure, it will just sustain it, so as that the body pressed on will not incline to either side.—This is the same as the centre of percussion, supposing the axis of motion to be at the intersection of this plane with the surface of the fluid; and the centre of pressure upon a plane parallel to the horizon, or upon any plane where the pressure is uniform, is the same as the centre of gravity of that plane.