82 ANNUAL OF SCIENTIFIC DISCOVERT. 



WATER VELOCIPEDE. 



An ingenious application of the principle of the velocipede to 

 water locomotion has recently been made in Parid. A pair of 

 hollow water-tight pontoons, about 12 feet long, 10 inches wide 

 in the thickest part, and tapered to a point at each end, are 

 fastened together about 20 inches apart b}'^ transverse bars near 

 the extremities. The seat is placed in the centre, and is raised on 

 iron rods about 2 feet above the water. In front is the paddle- 

 wheel, about 3 feet in diameter and 8 inches wide, provided with 

 16 floats, the axle turning on stout iron uprights, the rotary mo- 

 tion being obtained from cranks worked by the feet. The vessel 

 is steered by rudders at each of the sterns, moved by lines. If 

 made of light wood the whole has little weight, and may be pro- 

 pelled with astonishing rapidity. It was invented by M. Thierr}^ 

 an architect of Paris. 



LONG vs. SHORT SPAN BRIDGES. 



There seems to be a complete change of opinion going on 

 with regard to bridges possessing very long spans. As soon as 

 the advantages of wrought iron over older materials for construc- 

 tive purposes were demonstrated, it vvas imagined by some, that 

 all our rivers, over which it was necessary to carry a bridge, 

 would be crossed at a single bound. It is well known now that 

 some of the few existing long-span railway bridges could have 

 been built in a more economical manner^n a different principle. 



Disregarding the somewhat treacherous nature of the material, 

 the limits of cast-iron girders, especially for railway purposes, are 

 soon reached; the arch is not really an exception to this rule, 

 unless for light travel, and the real objection to its employment 

 on a large scale is the impossibility of affin'ding a sufficient 

 amount of rise to enable a curve to be obtained endowed with the 

 property of correct equilibrium. Engineers then began to look at 

 wrought iron for the accomplishment of their stupendous designs ; 

 but, curiously enough, the arch was abandoned at the introduc- 

 tion of the new material, it being generally impracticabh? to em- 

 ploy this form. The centre spans of the new Blackfriars bridge, 

 which are 185 feet across, will, we believe, be the representative 

 of the largest example of a wronght-iron arch. The horizontal 

 girder appeared to offer, especially in the matter of headway, 

 considerable advantages over the arch form, and consequently the 

 first specimens were instances of that principle. This determined 

 its application to the crossing of the Menai Straits, which contains 

 at present the longest single span of any railway biidge in the 

 world. 



There is no question but that the multi-span principle has proved 

 itself in point of economy superior to the system of crossing an 

 intervening space in one gigantic leap. The whole question 

 rests upon the relative economy of piers balanced against super- 



