2 
Trunk Drainage. 
Cumbrian, Wclsli, nnd other western mountains, occasion a fall of 
rain in the western counties 50 per cent, greater than in the 
midland and eastern districts : some impervious rocks also shed 
off the rains and melting snows in torrents, while fissured strata, 
cleaved slate, and absorbent chalk or sandstone, imbibe a large 
portion of the downfall, throwing nmch of it out again upon tlie 
clay valleys. The great surplus of water, not lost by evaporation, 
escaping from the western or central watershed runs toward the 
sea in a generally easterly direction : the largest English rivers, 
excepting the Severn, emptying upon the low east coast ; and 
those of Scotland, all but the Clyde, following the same course. 
Of English rivers, the principal points of delivery are the 
estuaries of the Humber, Wash, Tliames, and Severn ; their re- 
spective drainage areas being very large. The Yorkshire Ouse, 
the Trent, and other Humber rivers radiate into Westmoreland, 
Staffordshire, Warwickshire, Leicestershire, including also Derby- 
shire and Nottinghamshire ; traversing broad meadows through- 
out their course. The sluggish Wash rivers, the Ouse, Nene, 
Welland, tScc, embrace in their system of flat valleys parts of 
Lincolnsliire, Rutland, Leicestershire, Northamptonshire, Bed- 
fordsliire, Buckinghamshire, Hunlingd(mshire, Cambridgeshire, 
Suffolk, and Norfolk. The Tham.es and its feeders extend inland 
into Buckinghamshire, Oxfordshire, Gloucestershire, Wiltshire, 
Berkshire, Hampshire, besides Surrey, Middlesex, Kent, and 
Essex, — fringed with peaty or more solid wet lands. The Severn, 
stretching back through the counties of Gloucester, Worcester, 
and Salop, into Warwickshire, Staffordshire, and Montgomery- 
shire, with the Wye and other tributaries ramifying through the 
counties of ^Monmouth, Hereford, Radnor, and Brecknock, bring 
down the rapid floods from the Welsh mountains, and frequently 
deluge large tracts of the wide-spreading plains. In addition to 
these chief drains of the central counties we have streams from 
the Lake district, the Welsh high lands, and the southern, eastern, 
and northern provinces, aggregating into considerable estuaries 
round the coast-line ; such as the Eden, Lune, Ribble, Mersey, 
Dee, Conway, Towy, Taff, Usk, Avon, Parret, Taw, Tamar, 
Dart, Exe, Test, Arun, Rother, Stour, Medway, Crouch, 
Blackwater, Colne, Orwell, 1 ar. Tees, Wear, Tyne, and Tweed, 
not including an innumerable succession of outlets of more 
or less importance. These, with an immense network of 
channels branching from them, together with all the accessories 
of the main rivers already named, form our natural drains for the 
whole of England and Wales. Now, the capability of the prin- 
cipal outfalls to evacuate any amount of water likely to flow to 
them, may be pronounced upon, because in most instances, where 
they are not naturally sufTicienf, the exigencies of their alluvial 
