CURVE OF ADJUSTMENT.

109

When a sharp curve, say of 8 or 10 chains, joins a straight line, the necessary super-elevation of the outer rail is considerable, amounting perhaps to 4 or 5 inches, and the full amount of super-elevation is required at the commencement of the curvature of the line. In such cases the rail on the straight portion of the line joining the outer rail of the curve has to be elevated above its opposite rail for some distance prior to the commencement of the curve, so that the proper amount of superelevation may be attained at the commencement of the curve without too sudden an incline on the outer rail, which would injure the springs and impart a lurching

In sharp

movement to the carriages as they leave the straight part of the line and run on to the curve. reverse or S curves, it is desirable that a piece of straight line should be laid between the two curves, on which the super-elevation of one rail may die out and the superelevation of the other rail may attain its proper amount.

If the piece of straight line cannot be given, the junction of the two curves should be laid with check rails, but the proper plan in all cases where a curve joins a straight line, or where two curves join one another, is to ease off the one curve into the other, or into the straight line by a 'curve of adjustment,' that is to say, by a change of curvature so graduated that the super-elevation of the rails not only varies gradually, but at the same time is also at each point suitable to the curvature.

LECTURE IV.

POINTS AND CROSSINGS-POINT RODS-TRAILING AND FACING POINTS -SINGLE-TONGUE POINTS-MANUFACTURE OF CROSSINGS-SLIP POINTS- CONTRACTORS' POINTS AND

CROSSINGS

OUT-DOOR

SIGNALS-AUDIBLE

SIGNALS-INTERLOCK

ING POINTS AND SIGNALS-DETAILS OF INTERLOCKING-SPRING
CATCH ROD-INTERLOCKING GATES-COMPENSATION FOR TEM-
PERATURE-SWITCH LOCKING BAR-GENERAL APPLICATION OF
INTERLOCKING.

In my last lectures I completed the description of the inert portion of a railway proper. I now come to what may be termed the mechanism of a railway. Of this I will first consider points and crossings which enable vehicles to pass from one line to another.

FIG. 60. Plan of a pair of points-the points standing right for the straight line. Points, which are often called switches, are movable rails pivoted at one end. They are placed at the junction of one line of way with another line of way, as shown in fig. 60, in which the movable point rails are marked with the letter P. These rails, as will be seen, are tapered to allow them to fit closely against the rails which do not

move, which are called the stock rails, and are marked with the letter S.

Fig. 60 shows by the etched or shaded lines the position of the movable points when adjusted for vehicles to run along the straight line, and fig. 61, by the same means, the position of the points when adjusted for vehicles to run along the curved or diverging line.

Point rails are usually about 14 feet long, but where the diverging curve is very sharp, as is often the case in sidings and similar positions, the point rails are made much shorter. The heel or pivot end of the point has to be fully 2 clear inches from the stock rail, in order to let the flanges of the wheels pass freely through, and it therefore follows that at points one of the lines must of necessity diverge rapidly from the other. This rapid divergence is always placed on the least important line of the two, and

FIG. 61. Plan of a pair of points-the points standing right for the diverging line. trains running over the points when they are set for the diverging line should go slowly.

On the other hand, when the points are so placed as to cause a train to continue its course along the straight line, there is no necessity for a limitation of speed provided that the points are accurately adjusted and are held fast in their position. It is to be remembered also that owing to the construction of the points, and in consequence of the intersection of the different lines of way, little or no super-elevation can usually be given to the outer rail of the diverging line, and thus there is an

additional reason for a cautious rate of speed in the case of trains travelling on the diverging line. If the points are made shorter than 14 ft., the rate of divergence is of course greatly increased thereby, seeing that the clearance at the heel of the points must always be the same and sufficient to allow the flanges of wheels to pass between the stock rail and the point rail.

Figs. 62 and 63 show what is called a cross-over road, which is a short diagonal line with a pair of points at each end joining two lines of rails together. The cross-over road is shown in two positions; in the first (fig. 62) trains would pass along the two straight lines, and in the second (fig. 63) a train which was being backed

FIG. 62. Plan of a cross-over road-both pairs of points standing right for the straight lines.

on either line would cross over from one straight line to the other straight line, along the diagonal line. The difference between these two sketches, it will be observed,

FIG. 63. Plan of a cross-over road-both pairs of points standing right
for the diagonal line.

is only that the ends or points of the diagonal line are altered in position.

When a pair of wheels travels on the diagonal line, the flanges of the right-hand wheels will have to cross the rails on the left hand, or near side, of the straight line. This is effected by making a gap in the straight rail at least as deep as the projection of the flange below the tread of the wheel, and to allow wheels on