CHAP. VI. SOUTHWARK BRIDGE. 193 



purpose different gauges were made, of brass, iron, and 

 wood. These gauges were firmly attached to the middle 

 or crown of the wooden centres, and divided into 

 sixteenths of an inch, and at each a Fahrenheit thermo- 

 meter was placed; so that, the ends of the arch being 

 fixed, the variation in the temperature would be indicated 

 by the rise and fall in the centre. The observations 

 were made daily in the morning, at midday, and at 

 sunset for several months during summer and winter, 

 when it was ascertained that the arches rose and fell 

 about one-tenth of an inch for every 10 degrees of tem- 

 perature, more or less. 



The whole iron work is covered with solid plates, 

 having flanges cast on their upper side. These plates 

 are laid in the transverse direction and on the top of the 

 spandrel walls, so that they form a solid and compact 

 cast iron floor to support the roadway. The cornice, 

 which is cast hollow, is of the plain Roman-Doric order, 

 and is secured to the roadway-plates by strong stays and 

 bolts at proper intervals. The parapet consists of a 

 plinth, also cast hollow, with a groove at the top to 

 receive the railing, which is cast in the form of open 

 diamonds corresponding with the spandrels. The road- 

 way is 42 feet wide from outside to outside, and formed 

 in the same manner as that over Waterloo Bridge, 

 already described. The total quantity of cast iron in 

 the bridge is 3620 tons, and of wrought iron 112 tons. 

 It has been said that an unnecessarily large quantity 

 of material has been employed ; and no doubt a lighter 

 structure would have stood. But looking at the imper- 

 fections of workmanship and possible flaws in the 

 castings, Mr. Rennie was probably justified in making 

 the strengths such as he did, in order to ensure the 

 greatest possible solidity and durability qualities which 

 eminently characterize his works, and perhaps most of 

 all, his majestic metropolitan bridges. Although the 

 Southwark Bridge was built before the Railway era, 



VOL. II. O 



