213 



was quite insufficient. It was at first supposed that the water ought to find 

 its way directly from the surface to the drain, hence the idea of a two-foot 

 drain, but this view was found to be erroneous, and the true principle 

 decided to be as follows : — land is saturated with water rising to a certain 

 height ; when the water reaches that height, it will run off" if opportunity 

 offers. If no outlet appears, it will rise above the siirface, and form a lake or 

 a swamp. If the water table be lowered by the construction of drains, an 

 outlet is offered at a lower level for the subsoil water, or water of capillary 

 attraction, which forms tJie chief supply of water in the drains. This water 

 being kept flowing, makes room for rain water to get down to the subsoil, 

 instead of lying stagnant on the surface or on the upper soil. No doubt, at 

 times, rain water may pass direct into a drain, but it is not in the usual course. 

 It is found, therefore, that by keeping the subsoil water running at a depth of, 

 say, four feet, that the heaviest rain cannot leave water resting for any time 

 on the surface, but that it must find its way down to the subsoil, thus 

 percolating through, and improving the soil, instead of running violently over 

 the surface, and washing away the finer parts of the ground. 



Art. LI. — On the Surface Fall of Water, as a guide for Under Drainage* 

 By James Baber, C.E. 

 [Bead before the Auckland Institute, July 5, 1869.] 

 The practical part of drainage is an agricultural subject, but the principles 

 from which rules for practice are deduced, belong to general science. In this, 

 colony it is of importance that drainage of land be conducted on proper 

 principles. To examine the properties of one of these is the object of this, 

 paper. 



Water on the surface, descending from a higher to a lower level, follows 

 the general law of bodies in motion, moving in the line of least resistance. 

 At any point in the descent, this line will be found to be at right angles to the 

 level or contour-line of the surface at that point. Water drains off an even 

 surface in straight lines perpendicular to the contours, or in eurves having 

 chords in the same direction. So if the courses of water over any land be 

 carefully marked, and lines be drawn at right angles to these courses, the line 

 so drawn will form parts of the contours of the surface.* 



A drain laid in the line of these courses will possess the following 

 properties : — 



Water will enter it on both sides with an equal pressure, the depths from 

 the surface being equal. It will drain equally an equal distance from each 

 side, for if any two equidis+ant points be supposed at the depth of the drain, 

 on what may be termed the drainage surface, on opposite sides, and opposite to 

 the line of drain, these points and the drain will be on the same level. 



The drain will not leak, water will not enter on one side and escape 

 through the joints on the other side, for having sunk through the soil to 

 the level of the drain, it must descend through the pipe, that being in the 

 line of least resistance. 



The forces which chiefly act on water descending from the surface of land 

 to the drainage level are: — impulse from water in motion, and gravitation, The 

 capillary and molecular attractions, and the absorbent powers of the soil, vary 

 so much, that they need not be calculated for general rules. These two prin- 

 cipal forces will operate in the line of descent, at right angles to the contour. 



The deduction from this principle is, that the nearer a line of drainage 

 approaches the perpendicular to the contour, the more efficient that drainage 

 will be. 



* A diagram to illustrate this has been omitted. — Ed. 



