3S4 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November, 



the shoals liclow, where the decreased velocity, caused by the wldenlne of the 

 river, has not force enoii;^h to carry them along with it. There ean be no 

 more striking illustration of this general theory, than the effect of floods upon 

 the river near London Hridge, which is invariably to deejien belneen tlie 

 arche.s. and ai the same time to raise the shoals below the biiilge; therefore, 

 althoMgii the natural effect of the increase of current is \ip()n 'he ^^"holc to 

 deepen its channel, it does it so partially that it has also the eflect. in rivers 

 of unequal current, such as the Thames is, of forming and increasing shoals, 

 and unless guarded against by projier means may therefi re be injurious to 

 the navigation. Now the effect of opening London Bridge Mill be. that tlie 

 cbb-tidc and land-floods, not being checked by the pen of London Bridge, 

 will increase in velocity to the extent vp the river that the eflects of this pen 

 are felt, and produce the consequences we have mentioned, so that an increase 

 of expense in drepening the shoals after floods, and a greater iiier|uality of 

 level in the bottom, will I'e the consequence, and this will be a lasting ex- 

 pense unless means are taken to prevent it. The means we should recommend 

 are, the nearer approsimation to an uniform velocity, which would best be ac- 



complished by producing an rqitolifij of area, such as contracting the width of 

 the river abreast of the shoals, by means of enthanhments or otherwise : as this 

 cannot, however, be done in many places to the required extent without 

 enormous expense, ballasting must be had recourse to until a new regimen 

 corresponding to the existing circumstances is obtained. Finally, althougli 

 we think it might have been desirable that the great change, which the pro- 

 posed opening of the arches in London Bridge will certainly produce in the 

 navigation, had been made, so that ilicir effects might have been felt, and 

 things conformed to the new state by degrees, yet when called upon to give 

 an opinion witheut these experiments, \\c feel little hesitation in saying that 

 if effectual means are taken for preventing the evils to which we have re- 

 ferred, then the proposed alterations will be benefic al to the navigation above 

 bridge, but that without those efleclual means they will be injurious.'" 



No» , the fact is, that the alterations have been made to the full extent 

 stated in this report, and the consequences have been to the lull extent of 

 what is stated, but as yet no means have been taken to remove the evil which 

 was anticipated, and is now fell. 



A Statement showing the .Sec'ional Areas of the River Tliames, taken in the Years 1823 and 1831. 



PROCCSDINGS OF SCIENTIFIC SOCIETIES. 



INSTITUTION OF CIVIL ENGINEERS. 

 /Ipril 14. — Tlie President in the Chair. 



" Des-cripti07i of a Diptamomefer, or an Instrument for meai^uring Hie 

 Friction on Roads, Railways, Canals, Sfc." By Henry CaiT, Grad. Inst. C. E. 

 The object of Mr. Can-'s modification of the djTiamometer is to obviate the 

 irregularity of the common indicator arm, caused by the jerking motion of 

 the tractive power or any inequality of resistance. The instrument consists 

 of a cylinder half tilled with mercury, and containing a piston connected with 

 the spring of the dynamometer, so as to be lowered or raised as the tractive 

 power is increased or diminished. Two tubes of glass, connected by a pass- 

 age with a regulating valve, stand in front of the cylinder, one of them com- 

 municating freely with it, and in this tube the mercury is raised or lowered 

 proportionally to the power applied; while in the other, an average of the 

 variations is obtained as the facility of communication between the tubes is 

 increased or diminished by the opening or closing of the stop-valve. The in- 

 strument must be graduated by actual experiment, and the results of the 

 average power may be read oft' from the scales placed behind the tubes. The 

 paper is illustrated by a detailed drawing of the machine. 



*' .Jn account of a proposed .Suspension Bridge over the Uaslar Lake al 

 Portsnionth." By Andrew Burn, Jun., Grad. Inst. C. E. 



The usual calculation for the maximum load on each sujicrlicial foot of the 

 jilatforms of suspension bridges is 70 lb. ; b\it, as in the event of a crowd of 

 persons assembling the pressure may increase to nearly 100 lb. per foot, and 

 by the passage of soldiers marching in regular time the strain may be greatly 

 augmented, the projector assumed 200 lb. per superficial foot as the amount 

 of load to which the platform might be subjected. The pecidiar feature of 

 this bridge is the substitution of cast-iron chains for the wrought-iron ones 

 generally used. This deviation from the usual practice is adopted as a mea- 

 sure of cconomv, and with a view of increasing their stability and durability, 

 cast-iron being much less iiiflueuccd by atmospheric action than wrought- 

 irou. Cast-iion beams, when well proportioned, will bear a very considerable 

 tensile strain. As these chains would be proved beyond the weight they are 

 intended to bear, no doubt is entertained bv the author of their securitv. The 



platform, which is formed of transverse iron girders carrv'ing cast-iron plates 

 f of an inch thick, with dovetails falling into holes cast in the girders, is sus- 

 pended by WTOught-iron rods 1^ inch square from two lines of chain only, as 

 the strain is more easily brought to bear on them than on a greater number 

 of chains. They are trussed laterally to prevent oscillation, and the balus- 

 trade is so constructed as to prevent the undulation so prejudicial to suspen- 

 sion bridges generally. To insure a perfect bearing, each pair of links of the 

 chains are in manufacturing cramped together, and the holes bored out to 

 receive the pins which are turned to fit them accurately ; they are of a larger 

 size than usual, being 4 inches diameter, and a less numlier are employed. 

 The piers on which the chains pass are of cast-iron, 33 feet high above the 

 level of the roadwav. 



Feet. 

 The extreme length of the bridge is . . • . 032 



The breadth of the roadway l/J 



The clear waterway between the piers . . . 300 



The clear headway of the platform above the high water 



hne 18.\ 



Ditto ditto above low water hne . . 33 



The tension on the chains is calculated as equal to 991'4143 tons. To sus- 

 tain this tension, the section of the chains is 25C square inches, and taking 7 

 tons per square inch as the elastic limit of cast-iron, the resistance of the 

 chains will equal 1792 tons, leaving a surplus of SOO'IJ tons after the calcu- 

 lated strain has been deducted from the real strength of the chains. Three 

 elaborate detailed drawings accompany tliis paper. 



Jlr. Smith, of Deenston, e.rplained a nrni' system of Lockage for Canals 

 proposed by him, a model of which he presented to the Institution. 



To avoid the present expensive construction of locks and their waste of 

 water, Ihc author proposes to divide the canal into a series of basins, the 

 water levels of which should be from 12 to 18 inches above each other. The 

 extremity of each basin is so contracted as to permit only the free passage of 

 a ho.at ; in this is placed a single gate, hinged to a sdl across the bottom, the 

 head pointing at a given angle against the stream, and the lateral faces press- 

 ing against rabbets in the masonry. The gate is to be coustrncted of buoyant 

 materials, or made hollow so as to float and be held up by the pressure of the 

 water in the higher level ; on the top is a roller to facilit.ite the passage of 



