oo 
ON RAILWAY SUSPENSION BRIDGES. 157 
will be one ewt., or 112 lbs. to the square foot; if 27 feet wide, it will be 
83 Ibs.; and if 30 feet wide, 75 lbs. to the square foot. The Kieft Bridge 
is 521 feet wide, and therefore a passing load of one ton per lineal foot 
spread over this area, is only 43 lbs. per square foot, whereas the test-load 
was 84 Ibs. to the square foot, which is about double what would have been 
the weight of the heaviest railway train; or taking 42 feet, exclusive of 
footpaths, the railway-load would have been 52 lbs. per square foot, or less 
than two-thirds of the test-load, which, it may be remarked, has remained 
on forty-eight hours without the platform showing any deflection visible to 
the eye, although some deflection really took place. 
It appears therefore most undoubted, that suspension bridges of modern 
construction may be perfectly adapted to sustain the passage of railway 
trains, and that the chief consideration has to be given to the character and 
dimensions of the platform; and as a general rule I would suggest, that not- 
withstanding the advantage to be gained by depth, this should not be carried 
too far, more especially if the lattice-girder system be adopted, as it presents 
too much surface to the wind, and thus induces increased lateral oscillation. 
Also, that the breadth of the platform for a single line should not be less 
than 25 feet, in order to spread the load and reduce the insistent weight per 
square foot of platform. 
It might be interesting to establish a comparison of the expense of various 
descriptions of platform, but this would lead too much into detail, and the 
materials for this purpose have yet to be collected. Still, as a contribution, 
and by way of illustration, the present opportunity may be taken to state 
the cost of the platform of the Kieff Bridge, already mentioned as so re- 
markably stiff, and capable of sustaining the transit of a railway train. 
In a length of 12 feet of the whole breadth of 524 feet of the platform, 
the quantity of materials was as follows :— 
Timber, 600 cubic feet ..........0-005 £150 0 O 
Mra SUN) CNUs hele ataieis cise/ pie isiinveim ¢keiel = 4 Beg OO) VO 
Total....£180 O O 
for a length of 12 feet, or £15 per lineal foot of the whole breadth of the 
platform, which is something less than six shillings per square foot of a 
platform such as that at Kieff (of which the drawings were shown). 
, 5th. Prevention of Undulation, e.—The effects upon a suspension bridge 
of passing loads and of strong winds, cause vibration, oscillation, and un- 
dulation. Of these, the undulation is considered to be the most serious. 
The vibration may be assumed as produced by what may be called the per- 
eussive action of the passing load, and when the platform is not sufficiently 
stiff, and the passing action is irregular over the surface, as, for instance, by 
the impetuous rush of a drove of cattle, or of a multitude of people, oscil- 
lation and undulation ensue; the first producing a lateral swing of the 
_ platform, the latter arising from the bending of the platform in its longi- 
tudinal direction. 
- The remedy for vibration and oscillation is provided by a sufficiency of 
stiffness, not to say absolute rigidity, in the platform, which will also, toa 
certain extent, counteract the propagation of the undulation, but not entirely. 
_ The experience, however, of four years on the Kieff Bridge, has proved 
_ that the mode adopted in that construction of disposing the suspension rods 
_ alternately (in the manner shown on the exhibited drawings) has completely 
counteracted the undulation; and many very heavily-laden carriages together, 
—artillery, cavalry, and large bodies of troops,—haye been continually 
