284 Improve^nent in the Construction of Bridges, 6fc. 



thirty years, unless better protected from the weather. It has 

 been stated, that it was left so exposed to the weather to render 

 it secure against wind ; most certainly a more mistaken, absurd, 

 and unphilosophical idea, could not be entertained. There is 

 much less danger from winds to bridges, when covered com- 

 pletely from the weather, than in almost any other kind of build- 

 ing of wood ; because they are, when of considerable length, 

 much secured by combination, into one mass, their whole length ; 

 they are, also, very heavy, compared with most other wooden 

 structures, and have great strength, as well as a long and contin- 

 ued connection of parts, by which means, one part is weight and 

 support to the other ; they are never high enough to present a 

 very deep volume to the wind ; and, lastly, the wind passes un- 

 der them so freely as to give itself vent, and if the length presents 

 a wider resistance to wind, the great length of heavy and well 

 combined materials is an amply sufficient anchor of safety to 

 itself. 



It may well be doubted, whether the covering of a bridge on 

 any construction, with trusses or framed work to support them, 

 for spans of more than 100 feet, presents more surface of obstruc- 

 tion to the winds, than is secured from its action by the inclosure. 

 If not covered, all the timbers have half of their surface exposed to 

 strong winds, in a manner similar to what would be the ca^e if such 

 bridge, with all its timbers, were immersed in a quick current of 

 running water ; it is evident that in both cases, more surface is 

 exposed to the action of the moving fluid, than would be the 

 case if covered sufficiently to keep out the weather. The reason 

 of which is, because half of the exterior surface of the covering, 

 &c. of a bridge is probably much less than half of the exterior 

 surface of all the timber, plank, &c. of the uncovered bridge ; 

 the covering protects the interior timbers, &c. from the action of 

 winds, and presents its own volume only, as one mass, to its force. 



The great exposure to decay, from having the feet of the 

 arches stand below the floor of the bridge, and bear or butt 

 against the abutments and piers, thereby occasioning certain de- 

 cay of their timber, sooner or later, has, in some instances, been 

 obviated by placing the feet of the arches into the tie-string 

 pieces. This certainly does away with the danger of decay, but 

 another greater difficulty succeeds, viz. that in arches of any 

 considerable span, the arch timbers must be, in a segment of a 



