280 Improvement in the Construction of Bridges, §c. 
accomplished or claimed for any other arrangement, viz. of 
having all the strain cr 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 
ue 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 ; buit 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 bave 
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 oF 
compression of timber ; in this arrangement of construction, there 
is not the least tendency whatever to the compression of any 0 
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 oer 
sioned by the mode of construction, as is the case in all bridges 
of other modes of construction, where posts are introduced fot 
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 ita vibIe 
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 cone 
