18 TECHNOLOGY. 
As all embankments settle more or less according to their depth, it 
becomes necessary to re-adjust the level of the stone blocks by packing 
gravel underneath, which, on account of their weight, is very expensive. 
In order to avoid this, wooden cross-sills are generally first used on embank- 
ments for the time of their duration, after which they are replaced by 
stone blocks, as the road will have become settled by that time. F%g. 29 
shows one of these wooden sills. They have great advantages when placed 
sufficiently near each other. The best kinds of wood are used for the pur- 
pose, generally oak, which sometimes is kyanized. They should be twelve 
inches wide, from 4 to 6 in thickness, and 6 feet long, and are generally flat- 
tened on top, or else only notched to receive the rails. They are laid on 
beds of broken stone, and should not be more than 3 feet apart from 
centre to centre. Opinions differ as to the proper height of the filling 
between and outside of the tracks. Some keep it below the top of the sills 
in order to keep the rails clear of earth, and to air the wood, which they 
suppose assists its preservation ; while others prefer to fill up as high as 
can be done without interfering with the flanges of the wheels; because 
wood, especially oak, is in fact better preserved by being entirely covered 
with earth than when partially exposed to the air; and because such filling 
protects the wood from being set on fire by coals dropping from the loco- 
motives: and besides, in case of the locomotive or any carriage running off 
the track, the revolution of the wheels will be gradually stopped, diminish- 
ing very much the breakage and danger attenaant upon such accidents. 
The fastening of the chairs on stone supports is shown in pl. 2, fig. 33, 
which illustrates fhe method used on the London and Birmingham road. 
First holes are drilled of 14 inches diameter, to correspond with those in 
the chair; on the bottom of the holes iron or wooden wedges, e, are placed 
with the edges upwards, and oaken pins, split at the lower end and tarred, 
are driven into the holes and cut off even with the chair. The iron spikes 
d, chisel-shaped at the lower end, and sometimes barbed, are then driven 
home, and confine the chair firmly to the support. The rail ais then placed 
into the chair and fastened to it by the wedge c. We must not omit to 
mention that the stone-blocks are frequently split by the successive driving 
of the pins and spikes, and afterwards by the swelling of the pins by 
moisture. Between the chair and stone-block must be placed a plate of 
wood, or else a piece of felt, 4 inch thick, and soaked in oil, in order to 
break the rebounding which would otherwise be intolerable and ruinous to 
the cars. 
The first rails were of cast-iron, and it was not until 1820 that at Birking- 
shaw, under the direction of J. Stephenson, wrought-iron rails were pro- 
duced. Those of cast-iron had the double disadvantage of being necessarily 
very short, and so brittle as to break readily when not continuously sup- 
ported. They can be used only on roads where the superstructure is made 
as is shown in jigs. 25 and 26, where the rails, d, are supported by longitu- 
dinal sills, 6, which rest on the cross-sills, a ; or as in jigs. 27 and 28, where 
the rail-stringers, ¢¢, rest on stone-beds, a, which have supporting walls at 
the junctions of two rails. The use of cast-iron rails has been almost 
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