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THE POPULAR SCIENCE MONTHLY. 



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 particu- 

 larly 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 uncer- 

 tain, 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 dura- 

 bility 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 in- 

 clined 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 in- 

 clined, 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 ten- 

 sion. The only advantage that can be claimed for this bridge 



