50 ANNUAL OF SCIENTIFIC DISCOTERT. 



haunches, which abut upon the concrete skcwbacks. Tlie material 

 of which the bridge is made is formed of gravel and Porthind 

 cement, blended in the proportions of six to one, carefully laid in 

 mass u{)on close boarding set upon the centring, and enclosed at 

 the sides. In testing the l)ridge, rails were laid upon sleepers 

 over the arch, which brought a load of two seventy-fifthsof a ton 

 per foot run upon the structure. Seven trucks, weighing, together 

 with tiieir loads, 49 tons, were formed into a train, havini^ a wheel- 

 base of 57 feet; hence the rolling load amounted to forty-nine- 

 fifty-sevenths of a ton per foot run. The deflection produced by 

 tiie i)assage to and fro of this train four times was noted upon a 

 standard, cemented to the side of the arch, at a distance of one- 

 third the span from the abutments. When one side of the bridge 

 was loaded, the extreme rise of the brancli on the opposite side 

 M-as al)out one-sixteenth of an inch, which was produced by a 

 maximum strain of 10 tons 14 cwt. per square foot. At a subse- 

 quent trial, a mass of gravel, 10 feet wide and 3 feet thick at the 

 crown, and 6 feet deep at the haunches, was laid over the bridge, 

 and upon this, ballast was placed the permanent way. After an 

 interval of a few days, the trucks, loaded as before, were passed 

 over the bridge, at first in pairs, and finally all together. In this 

 test the strain upon the concrete was as follows : — 



The weight of the arch, as before, 7 tons 17 cwt. 



170 tons of ballast 4 tons 8 cwt. 



Strain per square foot from dead load, .... 12 tons 5 cwt. 



Strain per square foot from passing load, ... 2 tons 17 cwt. 



Total strain per foot, 15 tons 2 cwt. 



After repeated transit, the load was left upon the bridge all 

 night, and the arch, upon examination, showed no signs oi failure 

 or distress undtn- the severe strains to which it had been exposed. 

 From these trials it is fair to assume, that a thoroughly well-con- 

 structed arch of concrete is absolutely stronger tlftm a similar one 

 of l)rick; but in practice the danger arises that it would be difficult 

 to ensure so high a (jualitv of concrete as that employed in the 

 pre-^cnt instance, and the proper supervision of the contractor's 

 work by the engineer would be almost impossible in structtires of 

 this material, whilst the inspection of brick-work is an easy mat- 

 ter. The utter uselessness of inferior concrete was shown by the 

 failure of the bridge which was previously erected on the site of 

 the present one, which yielded under its own load when the cen- 

 tres were struck. • 



Blackfriars New Bridge. — Black friars bridge is altogether formed 

 of wrought iron, so far as the main structure is concerned, the (>m- 

 bellishmcnt only being of cast metal. J*reparatory to the actual 

 commencement of this important undertaking, the erection of a 

 temporary wooden substitute, as well as the demolition of the old 

 bridge, was neeessar3\ The first ])iles, for the recjuisite gantry, 

 — one-third of which is now removed, — Avere; driven in ,Fune, 

 1SG4. As it is generally considered in the London district that the 

 London clay must be reached to obtain a sure foundation for largo 



