172 On the Value of Water as a Moving Power. 
single day, as great a body of water as commonly flows in ten. 
To intercept or retain a part of these flows, especially when they 
happen during the dry season, is the principal object of the pro- 
jected reservoir. Such floods are considered by Mr Chapman 
as operating beneficially, by cleansing and scouring the harbour 
of Leith ; and he therefore concludes, that embankments ab- 
sorbing part of the torrent, and diminishing its force and volume, 
must so far prove hurtful to the interests of navigation. But 
pressing this argument, he has overlooked or misconceived the 
true principles which occasion the gradual forming of banks or 
bars in aestuaries. It is always where turbid and agitated water 
becomes still, that deposits are made. The stillness, again, is 
produced, either by the pressure of a swelling tide, or by the 
countervailing influence of two parallel and opposite currents 
along these mutual boundaries. The size of the particles of 
foreign matter which a stream is capable of carrying with it, de- 
pends nearly on the velocity. A slow motion is sufficient for 
suspending the very minute particles of clay and soft earth. A 
quicker flow will transport the finer sand ; a greater celerity will 
sweep the coarser sand, and even gravel ; while the rapidity of 
the torrent is capable of hurling onwards gravel, and even stones. 
But the river of Leith never acquires any great velocity. Heavy 
rains or melting snows may wash the moorish surface of the Pent- 
land-ridge, and bring down mixed with clay the finer parts of the 
peat earth ; but, as the waters swell by accumulation, they find 
a wider vent in their passage through the low-grounds, and ad- 
vance with a moderate velocity to the sea, carrying clay and 
mud, but seldom any grosser matters. In the harbour of Leith, 
the motion is impeded by various causes, by the bridges, the 
ships, and, above all, the pressure of the flowing-tide, and a slow 
deposition of the suspended particles is thus made. When the 
tide ebbs, the turbid waters, no doubt, escape beyond the har- 
bour ; but they are partly brought back again by the returning 
tide. The floods which, at times, descend from the Pentland 
Hills, so far from helping to clean the harbour of Leith, have a 
decided tendency to choke it. The erection of the proposed re- 
servoir would hence be of real service to the harbour of Leith, 
since it would detain the greater parts of the dry and soft earthy 
matter which are now carried down successively, and deposited 
in the eestuary. 
