The mains should be from three to six inches deeper than the minors. The fall from one to the other may usually be made most conveniently, by a gradual descent of three or four feet to the point of junction; but with branch-pipes, the fall may be nearly vertical, if desired, by turning the branch upward, to meet the small pipe. It will be necessary, in procuring branches for sole-tiles, to bear in mind that they are "rights and lefts," and must be selected accordingly, as the branch comes in upon the one or other side of the main. The branch should enter the larger pipe not level with the bottom, but as high as possible, to give an inch fall to the water passing out of the branch into the main, to prevent possible obstruction at the junction. DRAINAGE INTO WELLS, OR SWALLOW HOLES. In various parts of our country, there are lands lying too flat for convenient drainage in the ordinary methods, or too remote from any good outlet, or perhaps enclosed by lands of others who will not consent to an outfall through their domain, where the drainage water may be discharged into wells. In the city of Washington, on Capitol Hill, it is a common practice to drain cellars into what are termed "dry wells." The surface formation is a close red clay, of a few feet thickness, and then comes a stratum of coarse gravel; and the wells for water are sunk often as deep as sixty feet, indicating that the water-table lies very low. The heavy storms and showers fill the surface soil beyond saturation, and the water gushes out, literally, into the cellars and other low places. A dry well, sunk through the clay, conducts this water into the gravel bed, and this carries it away. This idea is often applied to land drainage. It is believed that there are immense tracts of fertile land at the West, upon limestone, where the surface might readily be relieved of surplus water, by conducting the mains into wells dug for the purpose. In some places, there are openings called "sink-holes," caused by the sinking of masses of earth, as in the neighborhood of the city of St. Louis, which would afford outlets for all the water that could be poured into them. In the Report of the Tioga County Agricultural Society for 1857, it is said in the _Country Gentleman_, that instances are given, where swamps were drained through the clay bottom into the underlying gravelly soil, by digging wells and filling them with stones. In Fig. 7, at page 82, is shown a "fault" in the stratification of the earth; which faults, it is said, so completely carry off water, that wells cannot be sunk so as to reach it. Mr. Denton says that in several parts of England, advantage is taken of the natural drainage existing beneath wet clay soils, by concentrating the drains to holes, called "swallow-holes." He says this practice is open to the objection that those holes do not always absorb the water with sufficient rapidity, and so render the drainage for a time, inoperative.