MECHANICS AND USEFUL AUTS. 35 



WASHINGTON AQUEDUCT. 



At a meeting held December 6, 1866, Mr. Edward C. Pickering 

 called the attention of the Massachusetts Institute of Technology 

 to one of the greatest of American engineering works, and, at the 

 same time, one of the least known, viz., the aqueduct by which 

 the city of Washington is supplied with water. 



The plan accepted by Congress was to ei*ect a dam across the 

 great falls of the Potomac, conducting the water about thirteen 

 miles through two reservoirs to the city. Gen. Meigs, who had 

 the work in charge, instead of reports, prepared photographs of the 

 working drawings and of the aqueduct itself; a set of these rare 

 photographs he exhibited and explained to the Institute. The 

 supply thus obtained for the city of Washington is 67,000,000 gal- 

 lons daily, twice as much as the Croton, and five and a half times 

 as much as the Cochituate supply. The greatest engineerino- 

 work in the Cabin John branch is the bridge over Cabin John 

 Creek, which has one stone arch with a span of 220 feet, makino- 

 the largest arch now in existence ; the Chester ai-ch beino- only 

 200 feet, London Bridge 152, Neuilly 128, and the Rialto 99 feet. 

 When the centre scaffolding was removed the arch did not settle, 

 the key-stone having been set in winter and the centre struck in 

 summer. Other great arches have settled more or less, according 

 to the excellence of the workmanshii? of the arch and centre. 



From the distributing reservoir the water is conveyed in two 

 30-inch pipes. There were two streams to be crossed, College 

 Branch and Rock Creeks. In spanning these creeks the structi^-e 

 is remarkable, not only for size, but for the ingenious principle 

 of construction. Instead of building a bridge and laying pipes 

 on it, the pipes themselves were cast in the form of an arch, and 

 constitute the bridge. The Rock Creek bridge has a span of 200 

 feet, with two 48-inch pipes ; the College Branch bridge has a 

 span of 120 feet, with two 30-inch i)ipes. The arch is so strong 

 over the Rock Creek that a roadway is placed upon it, continuing 

 Pennsylvania Avenue to Georgetown. The pipes were at first 

 lined with wood. The diurnal rise and fall of the bridge is about 

 two inches ; this constant motion produced slight leakage from 

 droppings ; the wooden lining was then taken out, as it was shown 

 there was no danger of freezing, and now there is no leakage, the 

 pipes remaining at the temperature of the water. 



It was commenced in 1853 and finished in 1863. 



SUSPENSION BRIDGE AT CINCINNATI, OHIO. 



Another gi-eat triumph of American engineering is the suspen- 

 sion bridge over the Ohio River at Cincinnati, from Front Street, 

 in that city, to Second Street, Covington, Ky. It is said to be 

 the longest single-span bridge in the world. Its cost was about 

 $2,000,000. It is strong, ornamental, and affords an easy road 

 of communication between the States. Railroad tracks are to be 

 laid over its span. The following are its dimensions, &c. : 



