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16 TECHNOLOGY. 
latter rest the timbers ee which support the track; ff are the stringers on 
which rest the longitudinal sleepers of the track or the rails; gg are side 
railings. 
As railroads frequently cross common roads, regard must be had to these 
crossings in arranging the grades of the road. If the highway is to pass 
above the railroad, which consequently is in excavation, the depth of the 
cut, as well as in all cases the importance of the road, will determine the 
manner of bridging. The clear space between the bridge and the rails 
should in general not be less than 16 feet, in order to allow a free passage 
for the chimneys of the locomotives. When the cut is of a less depth, the 
required elevation must be attained by making an embankment on the 
highway on each side of the bridge, the grade of which must not be steeper 
than 1 in 15. <A separate chapter will be devoted to the construction of 
bridges; but to illustrate road-crossings, we have given in pl. 2, figs. 16, 17, 
and 18, a viaduct of masonry; jig. 16 is a side view on the left, and a 
longitudinal section on the right; jig. 17 is a ground plan of an abutment, 
and jig. 18 a horizontal section below the roadway. 72g. 19 shows a per- 
spective view of a viaduct of very similar construction. 
Whea, on the other hand, the grade of a railroad is at a considerable 
elevation above a highway, the former must cross on a bridge, which, 
whether built of wood or stone, must have strong abutments and wing walls 
of stone to sustain the embankments on both sides. /%g. 20 shows a 
viaduct of this class ; jig. 21 is the ground plan of an abutment, and jig. 22 
a horizontal section below the roadway. In cases where the railroad crosses 
a valley on a viaduct, no especial construction will be required for a road- 
crossing, except perhaps a slight change in the direction of the highway, in 
order to make it pass through one of the bays of the viaduct. When 
the elevation of the railroad is not sufficient to allow the highway to 
pass under it, the latter is brought to the level of the former by means of 
embankments. Road-crossings on a level are prohibited by law in England. 
They are, however, very frequent in the United States and in Germany, 
and no accidents appear to have occurred at such crossings where proper 
care has been used. An elevated pavement of wood or stone must be laid 
at such crossings, even with the top of the rails, as shown in jigs. 45, 46, and 
47. The edges of the pavement next to the rails are covered with flat iron 
bars, 60’ ; they must not approach the rails on the inner side nearer than 
about two inches, in order to leave the spaces, ¢ (jig. 47), for the flanges 
of the wheels. They are either even with the rails (jig. 46) or elevated 
above them as in jig. 47; the latter arrangement has the advantage that 
the wheels of the carriages crossing the railroad will not touch the rails, 
while on the other hand it has the disadvantage that dirt accumulates 
easily on the rails, causing great friction, and sometimes even throwing 
the cars off the track; constant attention, therefore, is required in such 
places. 
Rivers and streams are crossed by railroads on bridges built either of 
stone, wood, or iron, and requiring various modifications of construction 
according to the length and angle of the crossings. The chapter on dridge- 
596 
