MEANS OF COMMUNICATION. 21 
P1. 3, fig. 28, is a plan of the superstructure of the Baltimore and Ohio 
Railroad ; jig. 23 is a longitudinal section ; fig. 27, a cross-section of the 
same. J/g. 26 shows the attachment of the rail to the sill by plates and 
screw-bolts ; jig. 24 is a top view and jig. 25 a side view of the chair. The 
whole forms an excellent arrangement. 
In laying the rails the effect of changes of temperature upon their length 
must be paid attention to. The difference in length at extremes of tempera- 
ture is from ¢ to 3, of an inch in a rail of 18 ft., and if the rails were laid 
close to each other at a low temperature the track would inevitably be torn 
up by an increase of heat. In order to avoid this, pieces of iron gauged to 
thicknesses corresponding to the existing temperature are interposed between 
the ends of the rails while they are being fastened to the supports. 
The distance between the inner edges of two opposite rails is called the 
gauge of the track. On the first railroads in the coal-mines the gauge was 
from 3 ft. to 8 ft. 6 inches, but on the introduction of locomotives the gauge 
was increased, and Stephenson first introduced the gauge of 4 ft. 84 inches 
on the Stockton and Darlington road. The success of his locomotives 
caused them to be employed everywhere, and thus the above gauge of 4 ft. 
83 inches came to be almost universally adopted. A wider gauge was sub- 
sequently considered by Stephenson himself preferable for the more power- 
ful engines built since then; and Brunel proposed a gauge of 7 ft. for the 
Great Western Railway, which was adopted. In Russia and in Baden a 
gauge of 6 ft. was introduced, but in the latter state it was found necessary 
to lay extra tracks for the narrow gauge in order to accommodate trains 
from adjoining roads. The broad gauge is also used on the Atlantic and 
St. Lawrence Railroad in Maine. 
The distance between the rails must be greater by about # of an inch 
than that between the flanges of the wheels, so as to allow a play of 2 of an 
inch for each wheel, without which the friction would be too great. A 
larger play would prove destructive to the road and to the carriages by 
allowing the latter to rock violently from side to side. The flanges (pl. 5, 
jig. 14¢ and jig. 12a) are on the inside of the wheels and guard the 
carriages against sliding off the rails. 
On railroads consisting of a single track provision is made for allowing 
two trains to pass each other by an arrangement called a seding or turnout, 
consisting of a portion of a track laid by the side of the main track, at a 
suitable distance from it, and connected with it at each extremity by 
a curved portion, which is so arranged by means of a movable part that 
the cars can either continue on the main track or enter the turnout, as 
circumstances may require. The curved portion must be composed of two 
arcs of circles, one tangent to the main track, the other tangent to the 
siding, and both tangent to each other midway, but convex in opposite 
directions. The movable portion by means of which the cars may be made 
at pleasure to take either track is called a switch. A simple arrangement 
for turning out to the right is shown in pl. 2, fig. 48, where awa are the rails 
of the main track, 66 those of the turnout; the latter do not come close up 
to the former, but leave a space of 13 or 2 inches in order to allow the 
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