Refilling may be done by hand or may be done by a drag scraper at the end of a rope, so that the team of horses may be on one side of the trench and draw into it from the other side the excavated material. This costs only five cents per cubic yard. If the dirt is thrown back by hand, the cost will be that of shovelling, namely, about fifteen cents per cubic yard. If the dirt has to be tamped in the trench, the cost will then be that of another man, and backfilling will often add thirty cents a cubic yard to the cost of excavation. As a summary, it may be said that excavation alone in earth may cost from fifteen cents to forty-five cents a cubic yard, and that backfilling may add to this from five to thirty cents a cubic yard, the entire cost, therefore, varying from twenty cents to seventy-five cents a cubic yard for excavation and backfilling together. Nor is it possible to be more definite in explaining the proper price to put on excavation since the character of the material and the nature of the excavation are of such importance in fixing that cost. If the excavation is for a tank, it is often possible to rig a derrick with a long arm on the side of the excavation and, by means of a bucket, transfer the excavated material from the hole to the bank cheaper than by repeated shovelling or by carrying out the dirt in a wheelbarrow. Wheelbarrow work is always expensive, the cost of transporting earth in a wheelbarrow a distance of fifty feet being about ten cents a cubic yard. Sometimes a horse may be used to great advantage to lift the bucket and operate the derrick in place of a hand-worked windlass, although the use of the horse is hardly worth while unless the excavation is more than ten feet deep. The excavation for the trenches of a sub-surface irrigation system cannot be estimated on the same basis as for a larger trench. More time is required proportionally in trimming and grading the sides and bottom, so that the cost per cubic yard is much increased. Thus, while such trenches contain about one cubic foot of earth per lineal foot, and on the basis of twenty-seven cents per yard would cost only one cent per lineal foot to dig, it is probable that, under ordinary conditions, this amount would be doubled. The cost of underdrains must be made up from the cost of the pipe used and the cost of the necessary excavation. In the bottom of artificial filter beds, the latter amounts to little or nothing. In natural filter beds, the trenches are deeper and the cost of the underdrainage depends largely on this excavation cost. The cost of the pipe varies from two to ten cents per foot, depending on the kind of pipe used and its unit cost.