of
observations in Great Britain, the ratio, or per cent. of the rain-fall
which drains off is 65-1/2, or nearly two-thirds the rain-fall.

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