EVOLUTION OF THE MODERN RAILWAY BRIDGE. 467 



iron rod, and all the weight upon these ends of the beams passes 

 up the rod, and then conies down the two diagonal beams to the 

 abutments. The one thing necessary in this is that the lower 

 ends of the two diagonal beams shall be so fixed as to make it 

 impossible for them to slip out in the direction of the arrows, and 

 this object is usually attained by making the floor stringers serve 

 as a tie to hold them together. In the drawings, the full black 

 lines are in compression and the dotted lines are in tension. Thus, 

 you see the vertical rod or rope in the center is in tension that 

 is, a weight being at W, all of that weight comes directly upon 

 the rod and is carried to the apex, A ; then half of it passes to 

 each side down the inclined braces, and they are in compression. 

 The tendency at the foot of these braces is for them to slip out in 

 the direction of the arrows. They are held together by the tie- 

 rod or floor stringers, which are in tension. In regard to tension 

 and compression, you may get a better comprehension of them if 

 you understand that a cord or rope can be used for any member 

 of a bridge that is in tension, while a post or some stiff piece of 

 timber or iron is necessary for anything in compression that is, 

 in all these diagrams the dotted lines could be replaced by ropes or 

 cords, while the full black lines are obliged to be iron or wooden 

 posts or braces. 



You thus see that we have the simplest form of a framed 

 truss. This form of truss is called the king-post truss. Now, as 

 the width of the opening increases, the height of the posts would 

 also have to increase, and in a very short time would get so high, 

 and make the inclined braces so long, as to become unwieldy. In 

 order to overcome this, after a certain height has been reached, 

 instead of continuing the king-post higher, we simply cut it off 

 and substitute two posts or rods in its place (Fig. 12). In this the 



QUEEN POST Fig. 12 



length of span that can be covered with the same sized material 

 is one half larger, and the bridge is divided into three panels, as 

 they are called. A panel is one of any number of equal parts into 

 which the truss of a bridge is divided by means of the posts or 

 rods. This second truss is called the queen-post truss ; here also 

 the full black lines are in compression and the dotted lines are in 

 tension. As you will notice in this truss, which is also the case in 



