100 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
the ground, but from 1 in 150 to 1 in 200 is quite sufficient to 
ensure a steady flow, which is less liable to injure the sides of the 
drain than a more rapid fall. The depth should be from 3 to 4 feet 
according to the nature of the soil, the depth it is intended to 
make the sub-mains and laterals, and the quantity of water it will 
be likely to receive. The sides of the drain should have a slope of 
45° if the soil is at all loose or friable, but less than that will be 
sufficient in clay or adhesive soil, and it should have a width of 
1 foot at the bottom, the sides being neatly dressed, and made firm 
with the back of the spade. The sub-mains should be cut on the 
same principle as the main, but will not require to be so deep or 
wide, their dimensions being in proportion to the number of 
laterals running into them, and the state of the ground. The fall 
for these should be slightly greater than that of the mains, as their 
efficiency depends on their carrying off the water rapidly and 
thoroughly, and they are more liable to get choked with leaves and 
débris than the larger drains. Two drains from opposite sides of 
the main should never run into it at the same point, as when the 
flow is at all rapid, leaves and rubbish are brought down and 
deposited at this point, and tend to prevent the free flow of the 
water ; but a slight curve should be given them just before they 
enter the main, where there is any probability of a rapid flow, 
in order to prevent the water from washing into the opposite 
bank. 
The method of draining the main portion of the ground by 
means of the lateral drains, must depend upon the cause of wetness 
and the nature of the soil. When the superfluous moisture is 
caused by springs, thrown out by an impervious substratum, and 
spreading over the surface of the ground, a deep drain should be 
cut along the line of the outbreak (which can easily be seen by the 
state of the ground), so as to catch and carry away the water 
before it has spread over the surface. This will sometimes succeed 
in drying the ground below the drain, but much will depend upon 
the depth and arrangement of the strata through which the drain 
is cut. Sometimes the moisture is due to the impervious nature 
of the surface soil, such as clay or moor-pan, and should this rest 
on a porous bottom a few deep drains reaching to the porous bed 
will carry off much of the water, and render a main unnecessary, 
This is an important consideration on flat ground, where there is 
a difficulty in getting a proper fall, and should always be taken 
advantage of when possible. When the wetness is due to the 
