466 THE POPULAR SCIENCE MONTHLY. 



simply the angles have been changed. Now, suppose we divide 

 this rectangle, by means of a brace or tie, into two triangles. 

 Then not one of these timbers can be moved, or the form of the 

 rectangle changed in any way, without lengthening or shortening 

 the diagonal which divides it into triangles, and, therefore, the 

 rectangle with the brace and tie forms a perfectly rigid figure 

 (Fig. 9). 



In other words, the triangle is the only figure the form of 

 which can not be changed without changing the length of one of 

 the sides ; and thus any truss, to be perfectly braced and able to 

 withstand any strains that come upon it, must be framed so as to 

 be divided into a series of triangles. 



Returning to our original beam thrown across an opening, we 

 will suppose that we have a beam long enough and strong enough 

 with the required load to cross an opening eighteen feet wide, and 

 that we have an opening thirty-six feet wide which we wish to 

 cross. That could be done by building a pier in the center of the 

 opening and dividing it into two openings, each eighteen feet, as 

 shown in Fig. 10 ; but, in the case of a bridge over a road or over 

 a small river, it would not be advisable to block up the way by 

 this pier, and some other method must be found to support the 

 two inner ends of the beams. The simplest plan of doing it is 

 shown in Fig. 11. Taking two beams that are each slightly 



Fig. 10 



a 1 



rz^si m jm > . 



KING POST Fig. 11 



longer than eighteen feet, we throw them across the opening, 

 as shown in Fig. 11. These two beams meet at the angle, the 

 apex, A, of which is up, and, if the two lower ends are kept from 

 sliding apart, will stand in that position. Now, if from the angle 

 where these two beams meet we let down a rope or iron rod, run 

 out the floor beams eighteen feet long, and connect the inner end 

 of each to this rope or rod, we have a bridge covering an open- 

 ing thirty-six feet long that is, one end of each floor beam 

 rests upon the ground, the other end is sustained by the rope or 



