Military/ Bridges, and the Passage of Rivers. 301 



Whence it appears, that Sir John has, by these simplifications 

 of complex formulae, rendered an important service to practical 

 engineers. Sir H. Douglas applies these formulae, or rules de- 

 rived from them, to the construction of a rope bridge, where 

 the span is IBO feet^ the depression 12 feet, the weight of the 

 bridge unloaded 12,490 lb., and when loaded with infantry 

 34,090 lb. From which— 



1. The whole length of the curve, .... =, 132.95 feet. 



2. Weight of one foot of the bridge unloaded, . . 93.94 lb. 



3. Loaded, 256.41 ... 



4. Strain at the lowest point unloaded, .... 16726 



5. Loaded, 45652 ... 



6. The greatest strain, or that at the highest point, un- 



loaded, 17853 ... 



7. Loaded, 48729 ... 



& The angle which the chain makes with the vertical, at the 



highest point, 69° 32' 



Now, it appears, from tables given by Sir H. Douglas and 



others, that — 



A rope, 3 inches in diameter, or 9.42 in circumference, will 



sustain a weight of 27350 lb. 



A rope of 6 inches in circumference, .... 11088 ... 



Hence six floor cables will sustain .... 164100 ... 

 And the two suspension cables, 22176 ... 



These are the tensions which the ropes can safely support 

 •mthout risk; and, consequently, thfe floor cables alone have 

 abundant strength to sustain all the strains to which they may 

 be subject. 

 -The author then adds, that — 



*'^**'One of the principal arches of Trajan's Bridge*, across the Tagus at 

 Alcantara, having been destroyed bj the French, Lord Wellington found it 

 necessary to direct that a communieation across that bridge should be re- 

 established, for the purpose of bringing up artillery and stores from Badajos, 

 for the attack of the forts at Salamanca. Timber of sufficient dimensions to 

 effect this could not be easily procured, and indeed any application of that 

 material to make good such a fracture would have been extremely difficult, 

 and required much labour to be performed on the spot, in fashioning, framing, 

 and setting up the work ; and which, consequently, would have given warn- 

 ing to the enemy, before the campaign opened, of some important movement 

 in that quarter being intended. To obviate these difficulties and objections, 

 (Lieutenant-Colonel Sturgeon, of the Royal Staff" corps), the officer sent in 



• An elegant plate of this bridge, repaired as here described, forms a beautiful frontispiece to 

 the title-page of this work. 



