G ft} 
signal for doing this, is the rising of air-bubbles from 
the bottom of the pond, which never takes place uti- 
til a decomposition of the plants below begins. In 
winter, this tendency to decomposition is corrected 
by cold; and the submersion may, of course, be con- 
tinued for weeks, and months, and the water permit- 
ted to freeze, not only without injury, but with great 
benefit to the plants, particularly if they have been 
closely pastured in the fall. 
Filtration is a process requiring, id general, more 
labor and science than the other ; because, besides a 
dam to raise a sufficient head of water, you must 
have your canal of derivation, your reservoir, your 
cuts or ditches, and lastly, your fosse of discharge— 
which, to be useful, must be well constructed and 
judiciously placed. The canal and reservoir. will 
necessarily oceupy the highest ground, and be pro- 
' portioned to the quantity of water to be conducted 
and retained ; the cuts or ditches, supplied from the 
reservoir, will be parallel to each other, of nearly 
equal descent, but of diameters diminishing in pro- 
portion to their length, s6 as to give to the water the 
same swiftness it had when its volume was greatest. 
Stops or gates must be made in tlie cuts or ditches, in 
such number as may be necessary so to pond the 
water as to make it overflow the lower sides of the 
ditches, and at such points’ as will, from the shape of 
the ground, diffuse it most generally. In this way, 
small streams, occasional showers, and dissolving 
snows may be turned to great account, and with this 
additional advantage, that they require no reservoirs, 
and little if any draining, and only cuts or ditches 
formed with a plough or a hoe. 
