414 A Resultant System for the [Oct. 



the motion of a far smaller weight, as of a company of soldiers march- 

 ing in step, as occurred to the " Broughton" Bridge, near Manchester, 

 nay, the great "Manai" Bridge which was calculated to he equal to a 

 load of 1245 tons in excess of its own weight, and the "Brighton" 

 Chain Pier, (vide Frontispiece and description thereof), to an extra load 

 of 100 tons, have both been nearly destroyed by merely their own 

 weight when put in motion by a violent wind. The large suspension 

 Bridge at " Montrose," which when first put up was proved by a dead 

 weight of 970 tons, being the greatest it would have to bear, was des- 

 troyed in a similar manner. 



4. The disastrous effects which have already occurred, and may still 

 be apprehended from such causes,, to bridges on the uniform chain sys- 

 tem, are so universally admitted, that they need not here be further 

 dwelt on ; it will suffice to notice that no bridge of large span in any 

 exposed locality, is ever put up without some special arrangement to 

 counteract the vibratory and undulatory, tendencies of the structure. 

 This protection is sometimes attempted by means of guy-chains, some- 

 times by a system of side and under trussing, (as in the Hammersmith 

 Bridge,) at others by counter chains, (as in the Brighton Pier), the 

 latter being intended to enable the platform to resist the lifting power 

 of the wind from below. 



5. From the result of the opinions on the disastrous effects of gales 

 on the Menai Bridge in the years 1826, 1836, and 183.9, and especially 

 when during the latter, 148, or one-third nearly, of the suspending rods 

 were torn asunder, no other conclusion can be drawn, than that the 

 tubular rods introduced between the chains, the trussing of the road- 

 way, the small brace chains, &c. did not preserve the bridge from the 

 effects of the combined motions of the vibration, and undulation, of the 

 chains,* which were the primary cause of the injuries sustained, and the 

 reason is evident, viz. that these accessories contended against the effect, 

 without attacking the cause. It will be therefore evident, that, some- 

 thing more than strength to resist a known strain in a certain direction, 

 is required, and however true the main position demonstrated by the 

 Rev. Mr. Pratt may be, it still remains an open question whether, in 

 order effectually to meet the varied strains and trials to which Suspen- 



* Vide Report by Mr. Provis, resident Engineer. Trans : Civil Engineers, Vol. 3. 

 page 357. 



