Of the extraordinary summer rainfall, of 25.75 inches, at Rothamsted, less than one-half appeared as drainage, while in all of the other observations the summer drainage was not only very small, but it was less from a soil where grass was growing than from a bare soil. The winter drainage varied from less than one TABLE 24. RELATIONS OF DRAINAGE TO RAINFALL. Summeryear Winter year. Total for the year. April-Sept. Oct.-March. Drain Gauges. Rainfall Drain'ge Rainfall Drain'ge Rainfall Drain'ge inches. inches. inches. inches. inches. inches. half to rather more than three-fourths of the rainfall, with the single exception observed by Mr. Dickinson, in which the heavy summer rainfall of the wettest season increased the winter, as well as the summer, drainage. It was only claimed for the Central Park observations, that they approximately represented the relations of drainage to rainfall, but they are consistent with the more accurate records obtained with drain-gauges, and they may, therefore, be accepted as representing a conformity to the general law. During the summer in which the drainage was observed, once or twice a day after every rain, the maximum discharge of water from the drains followed a rainfall of less than one-third of an inch, and it was at the rate of 0.44 of an inch of rainfall in twenty-four hours, at 9 A. M., August 25th; at 7 P. M. it had fallen to 0.39 of an inch; at 6.30 a. M., Aug. 26th, it was only 0.18 of an inch; and at 6 P. M. it was but 0.10 of an inch. The average of six observations of the drainage, in the three days following the maximum discharge, was at the rate of only one-fifth of an inch of rainfall in twenty-four hours, but this was only eleven days after the close of the month recorded above, in which nearly twice the average amount had fallen, including three rains of from 1.48 to 2.20 inches. The drainage, in this case, must have been influenced by the heavy rains of the preceding month. Rainfalls of two and one-half inches, or over, must be looked upon as extraordinary, and they so rarely occur in the Northern United States, that they need not be considered in estimating the required capacity of drains to secure thorough drainage. From the increased cost of tiles, and the labor required in laying them, it will not pay to provide for the discharge in a few hours of the surplus drainage of extraordinary rains that seldom occur. They are best provided for by deep draining, to increase the storage capacity of the soil, and prevent a rapid transfer of water from the surface to the drains, by the larger mass of soil through which it must percolate, and if the tiles are well laid, with close-fitting joints, on a uniform grade, the drains will not be injured by running full for several days under the increased pressure to which they are subjected. On the other hand, from the evidence already presented, in regard to the behavior of soil water, the indications are that the surplus water of extraordinary rains cannot be disposed of in a few hours, under the most favorable conditions for its discharge by large drains, as time must be allowed for its percolation downwards to the water table, and for its more or less extended lateral movement through the soil between the drains, before it can escape through the tiles. CHAPTER IX. HOW TO MAKE TILE DRAINS. To make an efficient and permanent drain, round tiles must be laid with close-fitting joints, on a uniform slope, without any vertical undulations to obstruct or check the flow of water through them, and, what is quite as important, they must be covered with earth, and the ditch filled, without displacing the tiles or interfering with their alignment. Every detail of the work should be carried out with unwavering attention to these fundamental requirements, which should be secured with the strictest economy in the expenditure of labor. In order to accomplish the desired end the work must be carried on in accordance with a definite, wellmatured plan, and the implements best adapted to the purpose must be provided, before any tiles are laid. Skilled labor, or, at least, skillful and intelligent supervision, is required, to make a tile drain that will prove satisfactory in every way, and keep its cost within reasonable limits. It has been estimated, by those who have given the subject attention, that at least threefourths of the tile drains which have been made, have failed, to a greater or less extent, to give satisfactory results from errors in construction. To one who is familiar with the ordinary methods of draining, it is not surprising that the partial, or total, failures in tile draining are so numerous, as the work is frequently undertaken by those who have no definite knowledge of correct principles; and preconceived notions, or fallacious reasoning upon the facts presented, have often led to easily avoidable faults in construction, and consequent disappointment in the results. Many of the mistakes made in draining may, however, be attributed to a reliance on authorities that are hastily consulted, as the errors of the early writers have, in too many instances, been copied, and even found their way into standard works on draining, without due consideration of their real import and impracticability. After an extended and unsatisfactory experience in attempting to follow the directions for laying tiles found in books on draining, I was compelled to abandon them, and devise new methods to simplify the work of construction and secure a reasonable degree of accuracy in the finished drain. In the first place, it was found necessary, and it proved to be a fortunate innovation on accepted methods, to begin laying tiles at the outlet and work towards the upper end of the drains, instead of keeping long lines of ditch open, and trying to overcome the almost insuperable difficulties involved in following the directions uniformly given by writers on draining. With this change of base, many of the most serious obstacles which had before been encountered, entirely disappeared. In the next place, it was evident that the old methods of determining, or fixing, the grade of the drain, by means of "boning rods," "A levels," and similar devices, were not only inconvenient, but fallacious and unreliable, under average conditions, and attention was directed to an improvement of the methods for establishing the grade of the bed for the tiles. Grade Fixed by a Line.-Judge French* had recommended a line, as "the most accurate and satisfactory method of bringing drains to a regular grade," but his method of adjusting and fixing the line above the ditch proved insufficient and unreliable, as it could not be readily fixed in the proper position, and was liable * Farm Drainage, p. 233. After to displacement in the progress of the work. numerous experiments, the method of adjusting the line, FIG. 28. described below, was finally adopted, as the best rience in the field, and the sides the simple appliances for adjusting the line, which will soon be noticed, and ordinary spades and shovels, a few OLD FORM "draining tools" should be OF PUSH provided before beginning SCOOP. draining operations of any extent. The tools that must be considered indispensable are three sizes of the draining scoop (figs. 22 and 30), for OF PULL SCOOP. two, three and four-inch tiles, and two FIG. 29. OLD FORM or three draining spades, with blades sixteen inches long, and from four to six inches wide at the point. |