end of the base-line, and scratching with a knife on the tack where the fifty-foot mark on the tape comes, then starting from this point to measure another forty-nine or fifty feet, and so on until the centre of the hub at the far end of the base-line is reached. The next time that the line is measured the first length should be thirty-nine or forty feet, so as to avoid using the same tacks; and each succeeding first length should be ten feet shorter. This not only involves the use of fresh tacks for each measurement, but also prevents any manipulation of the tape so as to make the partial measurements agree with those made previously.

In case that a perfectly level line cannot be obtained, the line should be divided into level stretches, and where each break occurs the length should be measured on the incline and corrected afterwards for the effect of the rise or fall so as to obtain the true horizontal distance.

For further directions as to measuring base-lines with a steel tape, the reader is referred to Johnson's Surveying.

The ends of base-lines, as well as all intermediate points from which triangulation operations may be conducted, should be marked by solid and secure hubs. In protected places these may consist of six-inch by six-inch timber, three feet or more in length, driven in the ground and cut off about an inch above the surface, the centre being marked with a tack, across which are cut two intersecting lines at right angles to each other.

If the ground be subjected to hard freezing, the timber should be increased in section to eight inches by eight inches, and the length should be such that it will penetrate the ground, if possible, about three feet below frost. The earth around the hub location should be excavated to the greatest depth of frost, then the timber should be driven in or sunk like a post and well tamped, after which a stout timber box with an open bottom and a strong cover should be placed around the hub, and the earth should be packed around the outside thereof. Finally, the box should be filled nearly to the top of the hub with sawdust or dry sand.

In case that the ground be very hard, or if the bed-rock be