130 



LECTURE 



CUSS them more at length than we can in treating of the practical 

 points of their construction. 



There are evidently other matters to be considered in building a 

 bridge than the mere theoretical principles involved in the action of 

 the beam, the arch, or the suspension chain, and of these we shall 

 now treat. In designing a bridge for a certain locality, the engineer 

 must be governed by what are sometimes termed the limits of location. 

 They may be defined as the two horizontal lines above or below 

 which the bridge cannot go. The upper limit is fixed by the nature 

 of the matter which is to pass over the bridge, and by the topography 

 of the place at which it is to be built. If the structure is a foot 

 bridge, the upper limit may be high above the surface of the stream 

 or its banks, since pedestrians can easily mount up to almost any 

 elevation. If it is for pack-horses or mules only, as is the case 

 with some of the bridges of Switzerland and other mountain coun- 

 tries, the ascent may still be steep. For carriages the ascent must 

 be less, and the top of the bridge must be more nearly on a level 

 with the banks. For a railroad the upper line of the bridge must be 

 still more nearly on a level with the surrounding country. 



The other limit, or lower line, will be determined by the level of 

 the stream, by the freshets that occur in it, and by the headway re- 

 quired under the bridge, if the river is a navigable one. In a locality 

 where the stream is deeply sunk below its banks, the problem is 

 simple ; but where the country is low, and the stream apt to be 

 swollen by freshets, the engineer must exercise considerable judg- 

 ment in determining upon the best and most economical limits. 



Fisr. 59. 



Comparing Figures 57, 58, 59, we see that the beam gives for a 



