Seven series of observations in America are cited as follows: ========================================================================== |Name |Annual |Drainage|Ratio, or | |of |rain-fall,|flowing |per ct. of | No.|Drainage Area. |in inches.|away, in|the rain | Authorities. | | |inches. |which | | | | |drains off.| ---+---------------+----------+--------+-----------+---------------------- 1 |Schuylkill | | | | | Navigation | | | | | Reservoirs | 36 | 18 | 50 | Morris and Smith. 2 |Eaton Brook | 34 | 23 | 66 | } 3 |Madison Brook | 35 | 18 | 50 | }McAlpine. 4 |Patroon's Brook| 46 | 25 | 55 | } 5 | " " | 42 | 18 | 42 | } 6 |Long Pond | 40 | 18 | 44 | Boston Water Com'rs. 7 |West Fork | | | | | Reservoir | 36 | 14 | 40 | W. Milnor Roberts. ---+---------------+----------+--------+-----------+---------------------- | Totals | 269 | 134 | 347 | | Averages | 38 | 19 | 50 | ========================================================================== These examples show an average rain-fall of thirty-eight vertical inches, and an annual amount, collectable in reservoirs, of nineteen inches, or fifty per cent. The per-centage of water of drainage from land under-drained with tile, would be greater than that which is collectable in reservoirs from ordinary gathering-grounds. If a soil were perfectly saturated with water, that is, held as much water in suspension as possible to hold without draining off, and drains were laid at a proper depth from the surface, and in sufficient number to take off all surplus water, then the entire rain-fall upon the surface would be water of drainage--presuming, of course, the land to be level, and the air at saturation, so as to prevent evaporation. The water coming upon the surface, would force out an equal quantity of water at the bottom, through the drains--the time occupied by the process, varying according to the porous or retentive nature of the soil; but in ordinary circumstances, it would be, perhaps, about forty-eight hours. Drains usually run much longer than this after a heavy rain, and, in fact, many run constantly through the year, but they are supplied from lands at a higher level, either near by or at a distance. If, on the other hand, the soil were perfectly dry, holding no water in suspension, then there would be no water of drainage until the soil had become saturated.