MEANS OF COMMUNICATION. 17 
building will give the details on this subject. Drains and small water- 
courses are crossed by means of culverts, which are also bridges on a small 
scale. An arched culvert is represented in pl. 2, figs. 8, 9, and 10, in 
front view and cross-sections. When the elevation of the track is not 
sufficient to admit of an arched stone culvert, iron plates may be employed ; 
and when locomotives only are used, it is not necessary to cover the drain, 
longitudinal string-pieces being laid across the opening to support the rails. 
When the line of a railroad is interrupted by rocks or hills where an 
excavation is impracticable, and the location of the road cannot be changed, 
it becomes necessary to pierce the obstacle by tunnels, which are driven 
according to the principles of mining, and which if very long require to be 
ventilated by shafts from above. /%g. 23 shows the mouth of a railroad 
tunnel in a mountainous region. 
b. The Superstructure. The durability and safety of a railroad are 
altogether dependent on the quality of the rails, and on their being firmly 
fastened to solid supports imbedded below the roadway surface. These 
supports may either be of wood or stone, and may continue without inter- 
ruption along the track, or support the rails only at certain intervals. The 
first railroads had continuous supports and flat bar rails, called plate or tram 
rails ; but the difficulty of procuring the large quantities of timber required 
for that kind of superstructure, and its great cost, together with the exten- 
sive manufacture of iron in England, soon led to the adoption of rails 
of sufficient strength not to require a continuous support, but capable of 
bearing the load when sustained only at intervals by stone or wooden sills 
to which they were attached by iron chairs. 
The stone used for supports should be of the densest and hardest kind; 
a block intended for the support of one chair should measure at least 2 feet 
each way, but generally the top face only need be dressed. Those blocks 
on which the ends of two rails meet should be still longer, as on them the 
load is not sustained by the rigidity of the rails. To fasten the chairs to 
the stone supports, holes are drilled by machinery into the blocks 6 inches 
deep and 1 or 14 inches in diameter, to correspond exactly with the holes 
in the chairs. The blocks are generally not simply sunk into the roadway, 
but a bed of dry masonry 1 to 3 feet thick is carefully laid under each track, 
of the width of the supports. On this bed the blocks are accurately 
adjusted to the level of the road and firmly packed with gravel, after which 
the road is filled up with earth, gravel, or broken stone, to the level of the 
blocks, and well rammed. The top layer is made with a transverse con- 
vexity for the better drainage. 1. 2, fig. 30, shows a ground plan of this 
arrangement, jig. 31 a section. The distance from centre to centre of the 
supports varies from 3 to 5 feet; it appears, however, unadvisable to exceed 
the measure of 3 feet 4 inches, by which a rail of 15 feet length has five 
supports. A distance of 3 feet is still preferable, but more expensive. In 
order to prevent the lateral displacement of the rails more effectually than 
could be done by a large number of supports for single chairs, large stone 
sills have been employed extending entirely across the track and receiving 
two chairs, as may be seen in jigs. 30 and 31. 
597 
