Scientific Lectures. 193 



and we sliall soon find it in common use wherever strengtli or security 

 is demanded. The Seven TVonders of the ancient world were : 

 1. The Eg}-ptian Pyramids. 2. Tlie Mausoleum of Artemisia. 3. 

 The Temple of Diana at Ephesus. 4, The walls and hanging gardens 

 at Babylon. 5. The Colossus at Khodes. 6. The statue of Jupiter 

 Olympus ; and, 7. The Pharos of Alexandria, If seven were popularly 

 selected from the works executed in our day, they would be : 1. The 

 Tliames Tunnel. 2. The Great Eastern steamship. 3. The Atlantic 

 Cable. 4, The Britannia and Niagara Bridges. 5. The Erie Canal. 

 6. The modern Ordnance ; and, 7. The Pacific Railway. If the 

 engineer was called upon to name works in which the highest degree 

 of professional skill has been exhibited, he would probably make some 

 changes in this list. 



BRIDGES. 



Probably few of the audience are aware of the hours of thought 

 and study which are required of the engineer in the calculations and 

 preparation of the plans for an important public work. For an 

 illustration of this I will refer to the Britannia Bridge and Eobeii; 

 Stephenson. Mr. Stephenson began his investigation as follows : 

 There are but three kinds of bridges — 1. The arch, depending wholly 

 upon the compression of its own materials ; 2. The suspension, 

 depending wholly upon the tensile strength of its cables; and, 3. The 

 girder, in which some of its members are subjected to strains of 

 compression, and some of them to tension. Bridges are often built 

 combining two of these principles, but the difliculty of producing 

 unity of action between them has led engineers to generally confine 

 themselves to but one of them. Mr. Stephenson's first design was a 

 bridge with arches of cast iron, of 450 feet span and fifty feet rise, 

 with the center placed at an elevation of 100 feet above tlie level of 

 the sea channel which they spanned. But the Admiralty, which 

 had the legal control over such sti-uctures, declared that "no 

 bridge should be erected which did not leave a clear headway 

 of 100 feet for the lohole width of the channel." The design of 

 this bridge has been greatly admired, and many regrets have 

 been expressed that the structure was not allowed to be built 

 upon that plan. Mr. Stephenson's next design was a suspension 

 bridge, with a stiffened platform; but the difiiculty of combining 

 two such opposite principles in the same structure, led him to dismiss 

 the system of suspension as a permanent support, and his next design 



[Inst.] 13 



