ON PHYSICAL AND ENGINEERING FEATURES OF THE MERSEY. 557 
from the record of the self-registering tide-gauge at George’s Pier, Liver- 
pool, during ten years’ observations, 1854-63, are as given below, viz.— 
Above datum 
Ft. in. 
An extraordinary high tide, as marked on the Leasowe Lighthouse. 25 0 
An extraordinary high tide, January 20,1863 . - 3 B wz D 
Average high-water mark of equinoctial spring tides : ¢ Ar) gan 
Average high-water of spring tides, including equinoctial tides - 19 02 
Average high-water mark of ordinary spring tides, excluding 
equinoctial tides . : ‘ ; : 4 18 10 
Mean high-water level - ; , ; . 15 6 
Highest high-water mark of neap tides. : 14 8 
Average high-water mark of ordinary neap tides a ely 
Lowest high-water mark of neap tides Spur 
Mean tide level . 5 0 
Ordnance datum level F ° 4 8 
Highest low-water mark of neap tides 4 1 
Below datum 
Average low-water mark of ordinary neap tides 4 “ X BPE she 
Lowest low-water mark of neap tides - 5 : : “ - 310 
Mean low-water level 5 - 5 62 
Average low-water mark of ordinary spring tides, exclusive of 
equinoctial tides . - A ; 7 : - ° 2 ences 
Average low-water mark of spring tides, inclusive of equincctial tides 8 10 
Lowest low-water mark of equinoctial spring tides . 5 - - 10 4 
The abnormally high range of tide in this locality, as shown by the 
foregoing figures, is sufficiently interesting to warrant a brief explanation 
of its causes. It is, shortly, due to the fact that a part of the great tidal 
wave, generated in southern latitudes, enters St. George’s Channel round 
by the south of Ireland, and thence moves forward simultaneously in one 
vast current throughout, to a position in the Irish Sea abreast of the Isle 
of Man, where it meets that part of the ocean tide which passes by the 
north of Ireland and turns southwardly with great velocity through the 
North Channel by the Mull of Cantire. This meeting causes an up- 
heaval of the tidal volume, which is transmitted laterally to such parts of 
the adjoining coasts as are within its influence, the Bay of Liverpool 
coming in for its share, and thus enabling it to project a tidal wave far up 
the river Mersey to Woolston Weir, 33 miles from the mouth of the 
river, and to Frodsham Bridge on the Weaver, 19 miles distant from the 
same point. At these points the tidal flow is barred by weirs on both 
rivers. 
The gross volume thus sent into the estuary has been calculated at 
10,000,000 cubic yards on springs, and 281,000,000 cubic yards on neaps. 
It now remains to describe the share which man has taken to complete 
the benefits which Nature has so lavishly bestowed, and this may best 
be done by a brief and necessarily very general description of the works 
‘and docks which have brought Liverpool into such prominence and active 
ouch with the outside world. 
The major portion of the space upon which the Liverpool docks have 
%een constructed has been gained from time to time by inclosing the 
foreshore of the river. Its width varies from 2,300 feet, where back 
‘land was low lying, at the mouth of the river, to 700 feet in the centre 
of the river frontage of the city, opposite the narrows of the river 
