1570
THEORY AND PRACTICE OF ENGINEERING,
Book II.
3
Fig. 2931.
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sand or gravel was driven or packed under them by means of pointed beaters, which wascontinued until the tim-bers were strained up-wards of half an inch ormore in the middle andmade to curve; they werethen adzed down to alevel, and a plank of hardAmerican oak, 8 inchesbroad and inch inthickness, was laid upona bed of tar, the wholelength of the longitudinaltimbers, and nailed downin two rows with 2-shil-ling nails, their headsbeing well punched in.
The oak plank wasthen planed to a surfacethroughout, and the ironrails laid on it with greatcare, each joint beingfitted very nicely, withdue allowance for ex-pansion and contraction;the iron rails were fasteneddown by screw-bolts pass-ing through a piece of felt,the American oak, andthen into the longitudinaltimber : the screws on theoutside of the rails hadsquare heads, but thoseon the inside were counter-sunk, so as to avoidany interruption to theflanges of the wheels asthey passed along. Whenthe outside screw hadbeen so tightened as todraw the rail close downto the oak plank, and theinner screws also made astight as possible, a heavyroller, weighing 10 tons,was moved several timesover the rails, and as itpassed the screws wereagain turned until per-fectly tight. The gaugeof these rails is, fromcentre to centre, 7 feet2| inches, and the width
Fig. 2932.
Fig. 2933.
Fig. 2934.
between each pair of rails in the middle 6 feet;so that from the centre of the outside rails,where the double line is complete, is 20 feet5 inches.
The piles are for the purpose of holding downthe sleepers, and the packing under the timberswhich cross the railway is to give them a pres-sure upwards, to enable them in some degreeto resist the weight of the carriages; it hasbeen stated that this springing of the timbers addsconsiderably to their stability ; and, undoubtedly,the pressure upon a curved timber would throwthe weight towards the ends, and deposit itagainst its abutment, which would be the sec-
r~fe»
Fig. 2935.