ROAD-BED.
The road-bed, intervening between the foundation and the superstruc ture (cross-ties, rails, etc.), effects the distribution of the traffic burden over a larger surface of the road-foundation, lessening its liability to give way under the weight. At the same time it serves effectually and rapidly to drain the road-surface.
material that generally is considered the best for ballasting the road-bed is broken stone, as the angular fragments interlock well and thus distribute the pressure. It permits, likewise, of the free passage of water. Coarse riyer-grayel affords an excellent material for this purpose. Pit-gravel, on the other hand, frequently contains too much clay to suit the requirements of the case. It is often difficult to procure a suitable ballast; and when such is not obtainable along the line of the road, the engineer in charge of the construction must employ the best material at hand until something better can be obtained. Huntington, in his s Assistant, recommends inill (furnace) cinder broken fine, when it is procurable, affirming that for the following reasons it has no superior: Besides being elastic, it is dry, clean, free from dust, contains no nourishment for vegetation, and permits water to pass off readily. Its freedom from weeds is a very great economy, a large proportion of the hack-work in summer being avoided on this account, while in winter the road-bed is warmer and clears itself of snow. According to the same
author, engine cinders from the yards have like good qualities, but are reputed to be less durable than mill cinders, blast-furnace slag, gravel, or stone. \\Then broken stone is used, it is recommended that the frag ments should not exceed in size a cube that will pass through a 2Y,-inch ring. Gravel, being liable to work out, is not so suitable as stone- or cinder ballast.
Dtph of the depth of ballast no absolute rule can be laid down, but it is safe to say that it should not be less than one foot, in order to insure the track against displacement from the effects of frost. The ballast is carefully packed under the cross-ties, after these have been placed in proper position, by the use of the tamping-pick (pi. 24, fig. 19). On many railroads in Germany it is the rule to fill in the road-bed with fine ballast up to the level of the head of the rails. The practice is based on the belief that the cross-ties will be more securely maintained in posi tion and the wood more effectually protected against frost. The advantage of this expedient is questionable, and it is not in vogue elsewhere. The provision of proper drainage is of vital importance for the maintenance of a °pod track.