280 Improvement in the Construction of Bridges, 6^c. 



accomplished or claimed for any other arrangement, viz. of 

 having all the strain or weight, of every description, which the 

 bridge can be made to receive or sustain, whether it be its own 

 weight, which is generally the greatest, or any other, such as 

 droves of cattle, or trains of cars, with locomotives, &c., so dis- 

 tributed, that in all cases such strain or weight is sustained, in 

 due proportion, by every piece of plank composing the sides or 

 trusses, in a direct end-grain strain, viz. either a tension or pulling 

 strain, or a thrust or pushing strain. In both instances, of course, 

 therefore, the strain is exactly in the direction of the length of 

 the pieces. The great advantages of this one particular point, in 

 the construction of bridges, is very important ; and in wide spans, 

 this importance is increased to a degree that can only be duly 

 appreciated by the most experienced and sound practical engi- 

 neers. 



That the plank composing the trusses is liable to shrink, is 

 true ; but as the strain is, in all important respects, in the direc- 

 tion of the length of such planks, it is therefore evident, that 

 such shrinking cannot produce any effect, unless it be to do good, 

 by holding more firmly to the three or four tree-nails, which pass 

 through each plank, in several places, and, of course, cannot be 

 affected in any other manner than to be more tight on the tree- 

 nails, in the direction of the width of the plank, but without 

 alteration in the direction of their length, which alone could have 

 the least effect to do injury. What is stated in regard to shrink- 

 age, is also true, to a greater extent, in that of the mashing or 

 compression of timber ; in this arrangement of construction, there 

 is not the least tendency whatever to the compression of any of 

 the plank composing the trusses, by any strain to which they are 

 liable in their own support, or the support of any other weight ; 

 except, only, where the trusses rest upon the piers, and this only 

 by its own gravity, and not by any strain or compression occa- 

 sioned by the mode of construction, as is the case in all bridges 

 of other modes of construction, where posts are introduced for 

 the insertion of braces, by tenons and mortices, and where, of 

 course, the accumulation of shrinkage, and the compression of 

 the posts, by a great strain on a few points, both contribute to op- 

 erate towards the weakening of the bridge, so as to give it a vibra- 

 tory motion, which, in time, is sure to do violence to a bridge, 

 and, in the end, destroy it, or occasion large repairs, and the con- 



