144 Recent Advances in Mechanical Science. 



difficulties in connection with tunnels, such as the Severn 

 tunnel, both in their design and construction ; but there are 

 other structures which to my mind are much more interesting, 

 which should at any rate be touched upon. A tunnel can have 

 no grace about its design. You would say anybody that can 

 dig and build could make a decent tunnel. It becomes a 

 different matter when you expose your work to the light of day. 

 Strange as it may appear, the sunlight, or rather the heat which 

 accompanies the light, is a most troublesome matter to deal 

 with in iron or steel bridges, from the effects of the expansion and 

 contraction of the structure. Then, there are the effects of wind 

 and weather, and the critical eye of the profession, and of the 

 even more critical public, to contend against, all of which the 

 tunnel constructor need not trouble himself about. 



To go into even a small outline of the history of the principal 

 types and designs of bridges — tubular, suspension, girder, cante- 

 lever, and others — would be impossible in the time at my disposal, 

 but a most satisfactory evening or two might, and probably 

 will, be spent in the ^consideration of such structures, and how 

 and where the different designs were started. The great cante- 

 lever across the Firth of Forth is a developed form of a rude 

 bridge of this type, of one span, built across a small river in 

 China, designed many years ago, to overcome a difficulty where 

 the river was too wide for the construction of a single span with 

 the available materials. The history of how the various types 

 of suspension bridge developed is particularly interesting, cul- 

 minating in the Brooklyn Bridge, which I think is the last big 

 bridge of that type we, or even the Americans, shall see built. 

 Most interesting also have been the illustrations and descriptions 

 of the various stages of construction of the Forth Bridge which 

 have appeared from time to time in the engineering journals. 

 This structure is simply gigantic, and dwarfs the Eiffel and the 

 proposed Chicago towers into nothing. Even a child can build 

 up bricks one on the top of the other to a considerable height, 

 but it becomes a different story when such a structure has to be 

 built out horizontally under the open firmament of heaven from 



