SITS

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, SURVEYING, the practice of measuring the areas and. ^ning the boundaries of plots of land, estates, houses, &c.,such a manner as to be able to transfer a correct repre-Ration of the same to a plan showing the relative positione “ Proportionate dimensions of the various parts with refer-Ce to each other. In other words, the plan is made to pre-nt > on a small scale, a delineation of the surface of the areaS ^veyed.

tl ■ l ‘ 1 e principal instruments employed in surveying are the' f; odolit e and chain ; the former for measuring angles, andY Tatter for lines. It is usual to divide the plot to be sur-into a number of triangles, as convenient, and, whene chain only is used, which is probably the more accuratemod, to measure the three sides of each triangle, which at^ Ce determines its form and dimensions. If, however, thee odolite be also used, one or more of the angles are mea-^ ca thereby, and one or more sides determined by the1(1 8lR > in this case, the form and dimensions of each trianglej tl 9 he determined by trigonometrical formulas. In survey-hu-ge tracts of country, it is usual to fix or determinel Native position of some of the more conspicuous points

Jj. ~ • V V Vft UV1UV Vfr btAV 1I1V1 v vvuu^/ivuvui; |^VU1 ig

5 b 8 ^ stem triangulation, and from these fixed points, as§ as >3, to determine or fill in the remainder of the plan.[,^1 er;l T other instruments are employed in these operations,'lie two above-mentioned are the principal.

Purveying Wheel. See Perambulator.

SUSPENSION BRIDGE . This kind of construction is,‘ a Ps, the simplest and most easily erected of all bridges;ai , hud examples of it spoken of in remote times, and therecountr * es * n the world where it may not be seen under(jjj ® form or other. In England we now have many splen-of ^ridges of this description, in which the combined laboursduhectical experimentalists and scientific theorists have pro-

•Ronuments of surprising skill and admirable daring.

u «ced .

i, ^he principle on which suspension bridges are constructed,Vid Xl ' Ctly the reverse of that on which the stability of archedby, depend. In the latter case, the force of gravity is,I,, ‘l’ 1 ingenious arrangement, made to counteract the natural))>■„ e . tlo y of all bodies to fall towards the earth ; and a com-ui' ( ^ lVe force is called into play, by which the several parts

ii, 1 ‘® bridge are kept in their proper positions, provided thetl f fcl ‘Rls have been properly disposed, and are sufficiently

j n S to bear the requisite crushing force.

Slit)" Sl,s Pension bridges, on the contrary, the roadway isl, ’ Ported by ropes or chains attached to towers and abut-80 t ' iat s' 161 tension of the materials employed is prin-although not altogether, acted upon, and the platformVh| Stll ' llec * ^ the excess of their tensile powers above the’ll, 't (their own included) with which they are loaded.hi e ae bridges were not constructed to any great extent untiltl,, ^'"tnencement of the present century; up to that periodof ' "’ere of rather a primitive character. We hear, indeed,built in China about the year 65 A.D., and of severalt|| e ® lll Ted bridges in South America and the Indies, in which

cli

Rite,.

ains were formed of ropes or of barks of trees plaited

'"Woven together, so that the roadway was in the form

e »tenary curve; still, even in those countries we haveiit)(j' IIIC R8 of the roadway or platform being placed horizontally,A.i (1() 8lls l >(;l . 1 ded from the ropes or chains by intermediate rods.t|. v "g the first I ron Suspension bridges put up in this coun-the

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iljjj'.'the Winch Bridge crossing the river Tees nearo„i ®t°n. It was merely intended for foot-passengers, beingSu 8 *® e t in width, although 70 feet long. In 1816, a Wire®F l(v!' 8 ' on bridge was built over the Gala Water, of upwards’f\Y e Tbct span, at a cost of only £40; and another over theRtiq , j ilt Peebles in the following year, also of iron wire,Vo, ° ut *be same span, for £160.

6 Gt

In 1820, Captain Sir Samuel Brown built a SuspensionBridge over the Tweed, in an ingenious and novel manner,for which he had obtained a patent in 1817. Instead ofmaking the main iron chains, intended for the support of thebridge, in the form of ropes, as had been hitherto done, heconstructed those chains by links of iron several feet inlength, with holes formed at either end, through whichhe joined them together by means of bolts. This bridge,called the Union, is situate close to Berwick-on-Tweed, andhas a span of 449 feet. It is supported upon 12 chains dis-posed in pairs, and placed one above the other 3 deep, formedby links of round iron 2 inches in diameter, and each 15 feetlong, connected with one another by intermediate short linksalso of iron. The suspending rods are of an oval section,and are placed 5 feet apart, every one being attached to ajoint of one of the three main chains, which are made tobreak joint, so as to admit of this arrangement. These sus-pension rods support the roadway, which is formed of timber.This bridge cost £50,000. In the following year, this engi-neer built the Newhaven Pier, which is constructed on thesame principle, having three openings, each 209 feet span;and afterwards the Chain Pier at Brighton, which runs outinto the sea a distance of upwards of 1,000 feet, having fouropenings, each 225 feet span. In the year 1823, a bridge ofthe kind was erected by Sir Isambert Brunei, with twospans, each 122 feet ; and the year after (1824) TiernyClarke constructed the much-admired Suspension Bridge overthe Thames at Hammersmith, spanning the river by an open- jing of 422 feet, which has stood remarkably well.

Next in order of time is the far-famed Suspension Bridge ]erected by Telford over the Menai straits, for the purpose ofcarrying the Holyhead road, and thus connecting the Island of Anglesea with the mainland. This splendid work of artwas completed in the year 1825, having been about six years iin course of construction. It consists of one opening of 570 Jfeet span. The roadway being 100 feet above the level ofhigh-water. The chains are formed of flat bars of iron10 feet long, 3^ inches deep, and nearly an inch thick. Thereare four rows of chains, each consisting of four tiers of barsranged one above the other, so that, in all, there are 16 chains;to these, which are passed over the tops of two lofty towersplaced at either side of the straits, and fastened at eithershore into the solid rock, bars an inch square are attached,

5 feet apart, for the purpose of supporting the roadway, whichhas two carriage-ways, each 12 feet wide, and a central foot-way 4 feet in width. The entire weight of this suspendedplatform, including the 16 chains, is about 2,186 tons. TheConway bridge, over the river Dee, is also a SuspensionBridge, and was built by Telford about the same time ; it hasa span of 327 feet, and likewise carries the great Holyheadroad. Sir Samuel Brown then erected, in the year 1829, theMontrose Suspension Bridge over the Esk in Scotland , with ja span of 412 feet, and a roadway 12 feet wide. The Hun- igerford Suspension Bridge, over the Thames, at IlungerfordMarket, was built in 1811—’45 ; it has three spans; the cen- itral one 676£ feet.—There is also a neat Suspension Bridgeover the water in St. James’s Park, completed in 1857 ; anda very fine one across the Thames at Chelsea, opened in 1858.

In the year 1829, M. Navier, an able French engineer, to !whom we are indebted for a valuable work on Suspension jBridges , constructed the Pont des Invalides, on the suspen- jsion principle, over the Seine at Paris , with a span of 236 1feet, and a deflection in the catenary formed by its chains, ofabout 26 feet. The Fribourg Bridge, which spans the valleyof the Saline in Switzerland , is one of the most splendidwire bridges ever built. It was constructed by Mr. Challeybetween the years 1832 and 34. It has a span of 870 feet, and