BEIDGES. 



plates which form the top and bottom portion 

 of the girders, to which they are attached by 

 two of the angle-irons, the other two serving 

 to join the flanges to the bars, which form the 

 vertical web. The cross-sectional area of the bot- 

 tom chords consists of three thicknesses of iron, 

 well riveted, forming together a plate, in the 

 centre part 2 J inches thick, and 27 inches wide, 

 the balance of the construction being similar 

 to the top chord. The vertical web is com- 

 posed of lattice-work, which is framed with 

 two sets of bars six inches wide, crossing each 

 other and inclined to the top and bottom at an 

 angle of 48 degrees, and forming a network, 

 the distance of the crossing apart being 2 feet 

 1 inch between the centres of the rivets, meas- 

 ured along the bars, the thicknesses of the bars 

 increasing from \ inch at the centre part, to 

 $ inch at the ends of the girders. They are 

 secured at their crossings by two rivets. The 

 lattice-work is stiffened by vertical angle-irons 

 fixed double on both sides, at distances of about 

 8 feet apart. The central girder has about one- 

 third more iron in the cross-section than the 

 outside girders. The principle of the construc- 

 tion is known as the latticed-girder plan with 

 vertical stiffenings. - 



Bridge Strains. Mr. W. Airy has invented 

 and applied a very novel and ingenious 

 method of determining bridge strains. The 

 problem was on the determination of the 

 strains on every one of the intermediate bars 

 connecting the top and bottom members of a 

 bowstring bridge as ordinarily constructed, 

 the said strains being those due to various ar- 

 rangements of weights upon the bridge. To 

 "solve this problem practically, Mr. Airy has 

 availed himself of the sense of hearing, or 

 rather of comparison of musical sounds. It 

 is well known that a steel wire, if stretched 

 and sounded, will give forth a note dependent 

 on its weight, length, and tension. If, there- 

 fore, two wires of the same steel, and of equal 

 lengths, and at equal tensions, are sounded, 

 they will give forth the same note, and, con- 

 versely, if they give forth the same musical 

 note, it is to be inferred that the tensions are 

 equal. In order to turn this principle to ac- 

 count, a model of a bowstring girder was con- 

 structed with great care, of which the inter- 

 mediate ties were of thin steel wire. If, 

 therefore, a load were hung from the string, 

 these ties would at once go into tension, and 

 each one, if sounded, would give forth a musi- 

 cal note due to its length and tension. To de- 

 termine this tension, a wooden frame was con- 

 structed, and in this frame was hung a wire of 

 the same steel as the ties ; a sliding bridge was 

 provided by which the wire could be cut off to 

 any required length, by merely leaning against 

 the bridge, and a small scale-pan was fastened 

 to the end of the wire. This frame was 

 moved close up to the model, and the wire in 

 the frame being cut off by the bridge to the 

 exact length of any one of the ties, the scale- 

 pan was gradually loaded with weights till the 

 VOL. vm. 6 A 



BROOKE, JAMES. 



81 



musical note of the monochord exactly coin- 

 cided with the note of the tie under compari- 

 son. In this condition, since the two wires 

 are of the same steel and of the same length, 

 and give forth the same musical note, their 

 tensions are equal, and the weight in the scale- 

 pan is the tension of the tie. In this way 

 every tension was determined separately and 

 independently. Mr. Airy reckons that he deter- 

 mined all the tensions on his model to the ex- 

 tent of T^g-th part of the whole tension of each. 



An ingenious artifice was employed to elim- 

 inate the unavoidable errors of original ad- 

 justment of the wires, and at the same time 

 to obtain expressions for the strains of thrust 

 as well as strains of tension ; it is clear that 

 there must be errors of original adjustment of 

 the wires, for, even if it were possible so to 

 adjust the lengths of the wires that each 

 should have the strain due to a certain disposi- 

 tion of load, yet this very condition assumes 

 that the problem is already solved, and that 

 we know the exact strain that each wire ought 

 to have; consequently, the determination of 

 the strains is throughout a differential process : 

 a certain load is laid upon the model, and each 

 wire acquires a degree of tension, the precise 

 amount of which is determined by the method 

 of comparison with the monochord; these 

 tensions are booked, and an additional load is 

 then introduced ; this will cause an alteration 

 of the tensions on every wire, and the ten- 

 sions of all are again taken. The differences 

 of the tensions, before and after the introduc- 

 tion of the additional load, are formed for 

 every wire, and these represent the strains on 

 the wires due to the additional load only. The 

 same process determines also the strains of 

 thrust, for thrust is but negative tension, and 

 if the tension of a wire, after the additional 

 load is introduced, be less than it was previous- 

 ly, the difference is negative, and represents" a 

 thrust strain of that amount on that wire. 

 Mr. Airy also discovered a ready method for 

 throwing all the wires into tension, by a uni- 

 form distribution of the preliminary load 

 along the whole length of the model. 



BROOKE, Sir JAMES, K. 0. B., D. 0. L., 

 Rajah of Sarawak, and Governor of Labuan, 

 an English knight-errant, philanthropist, and 

 governor, whose love of adventure made his 

 career one of the most remarkable of modern 

 times, born in Bandel, Zillah Hoogly, Bengal, 

 in 1803 ; died at Burrator, Devon, June 11, 

 1868. His father, who was a government 

 officer of considerable wealth and prominence, 

 returned to England when James was a child, 

 and he received his education there. His 

 father's influence secured him a cadetship in 

 the East-Indian army, and he soon distinguished 

 himself by his bravery and daring. He was 

 in most of the engagements of the Burmese 

 War, under Sir Archibald Campbell, but in the 

 storming of a stockade, in 1826, received a 

 severe gunshot wound in the chest, and was 

 forced to return to England for surgical treat- 



