144 HARBOUR CONSTRUCTION. 



the iron to the mixture of fresh and salt water, to which the 

 lock was peculiarly subjected, as the soft- water was "locked" 

 down direct into the sea- water. He considered that cast and 

 wrought iron similarly exposed, and for the same length of time, 

 in the Thames would be scarcely affected. 



Mr. Mallet stated that he had devoted a good deal of atten- 

 tion to the important subject of the durability of iron, and the 

 result of observations induced him to consider wrought iron, 

 upon the whole, a more durable material than cast iron. It 

 was true that some wrought iron corroded faster, but only a 

 little faster, than some cast iron ; but the amount of decay and 

 the rate of weakening could be ascertained by measurement in 

 the former, whereas in cast iron the nature of the corrosion 

 was entirely different, and the mass of the material suffered a 

 molecular and chemical change to a considerable and uncertain 

 depth. In sea- water, especially if mixed occasionally with 

 well aerated fresh water, there was the maximum rapidity of 

 corrosion, both of wrought and of cast iron, except that pro- 

 ducible by the admixture with sewage-water, which often 

 contained matter that corroded iron almost as fast as diluted 

 vitriol. The protection produced by even a thin film of mud, 

 covering up immersed iron so as to screen it from fresh supplies 

 of water unexhausted of its combined air, had a marvellous 

 effect in preserving it. With regard to artificial modes of 

 preservation by various coatings, in his opinion there was nothing 

 equal to coal-tar, well boiled, with a mixture of finely powdered 

 dry caustic lime. 



The iron to be coated should be previously heated to about 

 600 or 700 Fahr. and be dipped into the mixture. It was 

 useless merely to pay over cold iron with coal-tar, as was often 

 done ; the iron must be heated almost to a black-red, and be 

 immersed for some time in the hot tar, which then perfectly 

 dried upon it, and produced a varnish almost as persistent as 

 the japan on a tea-tray. If the iron were previously painted 

 with oil-paint, the coal-tar would not adhere, nor become hard 

 and dry upon it. 



Mr. Redman mentioned, with reference to the decay of iron 

 structures exposed to the action of salt or brackish water, that 

 in the estuary of the Thames there were several works which 

 had been completed from twenty to twenty-five years, where 

 the pillars were of cast iron, and which yet remained intact at 



