490

HISTORY OF ENGINEERING.

Book I.

opposed, would cause the sections immediately under the additional weight to descendfrom their places, which cannot happen without the elevation of some other sections,in consequence of which the form of the arch would be altered; but this change is pre-vented by the fastenings which oppose equally the descent of any section, in consequenceof additional weight superincumbent on it, and the ascent of other sections from theexcess of pressure caused by the aforesaid superadded weight. But the form of the archwill be preserved entire, notwithstanding the occasional addition of any weight whatever,until the stress on the joinings should be so great as to cause them to give way, or to breakor crush the substance of the iron frames which compose the sections. This point ofsecurity must therefore depend on the cohesive force of the iron, and affords no data forprecise calculation; but the circumstances of the construction afford some grounds forforming a probable conjecture on the subject. The force necessary to cause a change inthe positions of the sections depends on the following circumstances: — 1. On the massiveweight of the sections; 2. on the strength of the joinings, which resists in a certain degreeany force applied to separate the sections; 3. the pressure between the surfaces of thecontiguous sections, which increases the effectual strength of the ties or fastenings.According to the dimensions stated in the table, the weight of each section is from 100 to120 tons. And since the sections united by the fastenings are bound or pressed togetherwith a force or pressure from 7300 to 8100 tons, if 30 or 40 tons should be the greatestoccasional weight that may probably hear on the same part of the arch, it does not seemprobable that the form of the arch would be sensibly changed by the addition of such aweight. 6. The horizontal force, which might be sufficient to press the arch out of thevertical plane, must depend on many circumstances, some of which can be estimated by con-jecture only. The principal of these are the following : —

1. The motive force, such as the force of the wind in tempestuous weather, which maybe in very high winds at the rate of 8 or 10 pounds on a square foot of surface, when thewind nnfrinyes directly against it, according to the experiments of Colonel Beaufoy forascertaining this point. 2. The magnitude of the areas which may be estimated to besubject to this force of pressure. 3. The angle at which the wind may strike upon it.4. The effect of any force in pressing the arch out of the vertical plane will be the less inproportion as the massive weight of the iron used in the construction is greater. Thestrength of the ties or fastenings will also, as in a former instance, add to the stability ofthe arch, against any force that may press against it.

Mr. John Southern. — An additional weight or force brought upon any part of the archwill distort the curve of equilibration, (an imaginary line, which if the bridge be properlyconstructed passes through the centre of the frames of the arch,) will render it ^the curveof equilibration) prominent opposite the additional force, and will depress it at distantpoints of the arch ; but if these distortions of th‘e imaginary curve keep within the limitsof the frames of the arch, no change of position of the parts will ensue, nor will any tensionbe brought upon the limits of the frames ; but if the distortion go beyond the said limits,the joints of the frame will open under the prominent and above the depressed parts of thecurve, and the bridge fall, unless those openings be prevented by means which bring ontension in the limbs. The force which will bring on this degree of distortion of the equilibrialcurve is in proportion to the weight of the bridge, and nearly the depth of the framejointly; and I am induced to believe from an experiment I have made, that if the framesbe 8 feet deep, the weight which would produce this effect would not be less than 100 feetlong of the road. *

The Sixth Question. — Supposing the bridge executed in the best manner, what horizontalforce will it require, when applied to any particular part, to overturn it or press it out ofthe vertical position?

Dr. Xevil Maskelyne _Suppose the largest ship that could strike against it to be of

80 tons, and the bridge to be 7000 tons. The ship then is an eighty-seventh of the bridge ;let the ship be going at the rate of 5 miles an hour, or 7*2 feet in a second. This strokethen tends to communicate a velocity of an inch in a second to the whole bridge, if it werefree to move. The friction upon the buttresses, or the connection with them, would imme-diately overcome this motion; but suppose the bridge was free to vibrate upon the middleline of its base as an axis, it would swing to and fro like a pendulum, each vibration takingup about 4 seconds, and as in that time the centre of gravity would not be moved by thevelocity acquired by the stroke above 4 inches, it is plain that this small removal from thecentre of the base, which is itself 45 feet, can no way endanger the oversetting of thebridge; nor even if it were ten or twenty times greater, but as was said before, the con-nection with the buttresses will soon bring it to rest.

Rev. A. Robertson. — The power of the bridge to resist any force tending to press it outof the vertical plane will consist of four particulars; the strength of the iron of which itis composed, its width, and the resistance at the abutments, and the inertia of its matters.The absolute effect of the strength of the iron and the width cannot be calculated, for thereasons already mentioned; but, cceteris paribus t an increase of width will give the bridge an