used in that bridge were absolutely perfect, the size of each piece would have to be exactly large enough to bear its part of the strain, and no larger; but as neither in iron or steel, and particularly in wood, can you calculate just exactly how many pounds of strain any particular piece will stand, in order to make it perfectly safe you use, in calculating the size of the bridge members, the load it is to bear multiplied by five, and sometimes even by ten, and then make the bridge theoretically strong enough to hold up this load that is, five or ten times the amount of load that ever can come on it—and this five, or ten, or six, as the case may be, is called the factor of safety; that is, if all material used in the bridge were absolutely perfect, the bridge would hold up five or ten times as much as ever would come upon it; and wherever a great deal of wood is used the factor of safety has to be very large, as the amount of strain that wood will bear is very uncertain, and varies under different circumstances.
You will readily see that the Howe truss can be used either as a deck bridge or a through bridge, and remember that the Howe truss is the type of bridge that was generally used upon railroads so situated that wood was plenty and iron expensive, and without money enough to send a long distance for iron bridges; and there have been some remarkable examples in this country of the durability of Howe-truss bridges designed by ordinary carpenters without any technical education.
As the price of iron decreased, in a very short time the lower chord of the Howe-truss bridge was made of iron instead of wood, as it was found to be much more economical, and it was then what is called a "combination bridge"; that is, of wood and iron.
The next form of bridge is what is called the "Pratt truss" (Fig. 22). The distinctive feature of this is that the compression members are vertical, while the tension members or ties are inclined or diagonal. In this, the amount of iron, supposing the tension members to be of iron and the compression members of wood, is increased and the amount of wood is decreased. This was a very natural result as the price of iron decreased. In a short time the wooden posts were removed and iron posts substituted for them, and we then have an entire bridge of iron, in which the compression members are vertical and the tension members inclined, and it is the most generally used form of iron bridge in this country; it may be called the typical American railway bridge.
The next form of truss that we will examine is what is known as the Warren triangular girder (Fig. 23). You will see that each of the pieces connecting the upper and lower chords acts both as a tie and a brace—that is, is subject to both compression and tension. The only advantage that can be claimed for this bridge
