text:  In regard to the depth of drains it is shown that in order that they may _draw_ they should never be shallower than thirty inches and should always leave a depth of eighteen inches clear of the draining materials in order that the subsoil and trench plough may have full freedom of action without risk of injury to the drain; while of the use of soles he says-- "I am a strenuous advocate for drainsoles _in all cases_; and even when they may really prove of little use I would rather use too many than too few precautions in draining; because even in the most favourable circumstances we cannot tell what change may take place beyond our view in the interior of a drain which we are never again permitted and which _we have no desire to see_