Colonel Byng now remained behind to hold Philippolis, while Colonel Williams returned to the railway at Prior Siding. At this time a new system of blockhouses was inaugurated for the defence of the railway, which had the effect of releasing for active operations some six thousand of the troops previously required to guard it. The whole of these blockhouses were constructed of wood and iron. The walls consisted of two skins of corrugated iron (six inches apart) filled with sharp sand. On the _complete_ filling of these skins the proof of the blockhouse against rifle fire entirely depended. The filling was done from the inside at the eaves and through holes in the sills of the loopholes, and was kept complete from time to time by order of the officer in charge by working new sand through the holes in the sill by means of a shovel and piece of wood. To prevent blockhouses from being seen through from one loophole to another, screens of blankets or sacking were suspended at right angles to each other crossing the centre of the blockhouse. A barbed wire spiderwebbing and fence surrounded the blockhouse, the entrance to which was firmly closed at night. A 200-gallon bullet-proof cased tank for water (rain water or that brought by water-carts) was kept outside the house, but on emergency water could be obtained by digging a few feet deep within the wired area round most of the houses. These preparations and precautions were none too many, as the Boers were constantly at mischief, and on the 18th they managed to derail a train at America Siding--an incident which cost the life of Major Heath (3rd Battalion South Lancashire Regiment). [Illustration: A TYPICAL BLOCKHOUSE]