64 
BRITISH ASSOCIATION.- 1835 . 
On the Survey of the Mersey and the Dee. By Captain Henry 
Mangles Denham, R. N., Resident Marine Surveyor of the Port 
of Liverpool. 
Captain Denham exhibited his trigonometrical survey of the 
Mersey and Dee, including the extensive sand-banks and channels 
of Liverpool bay, which, being delineated on the scale of four inches 
to the mile, afforded a detailed development of the submarine undu¬ 
lation, illustrative of his remarks on the action of the tidal stream in 
connexion with those differently shaped estuaries. The self-choking 
effects of the Dee, with its expansive mouth and gradual contraction, 
resembling a lateral section of a cone, were contrasted with the 
scouring effects of the Mersey, its contracted mouth and attenuated 
throat resembling a lateral section of a bottle with its neck pointed 
seaward. To this figure of the estuary of the Mersey, Capt. Den¬ 
ham ascribed the impetus of its expansive back-water, which has re¬ 
cently forced a channel of half a mile wide, and two miles long, and 
twelve and thirty feet below the low-water level, through sands, 
situated eight miles outside its coast-line confines, at a tangent to its 
regular course. Thus a most valuable and unexpected channel has 
been produced for navigation, and a compensating escape provided 
for its waters at a time when an injurious deposit was taking place 
across its usual path, where the efforts of the ebb become evanescent. 
The position was ascertained by Captain Denham to be fourteen miles 
below the docks, or tidal straits, where the first impulse amounts, (and 
continues so five hours out of six) to five miles per hour on spring- 
tides. The form of this channel corresponds to the contour of in¬ 
cidence and reflection throughout its whole course, and indicates the 
exhaustion of the velocity of the water by expansion in the propor¬ 
tion of 14 to 25. It proves also the certain power of the Mersey to 
command a navigable avenue to the ocean, so long as its guardians 
preserve the high-water boundaries from artificial contraction. 
In the course of his professional duties, Capt. Denham proposes 
to himself a further investigation of the proportions of silt, &c. held 
in suspension and gradually deposited, as well as a determination of 
certain peculiarities in the vertical range of the tides with reference 
to atmospheric elasticity. He has already, by the liberal arrange¬ 
ments of the dock-trustees, been enabled to connect a series of ob¬ 
servations, even to Jive-minute grades , during the twenty-four hours. 
From these, by extensive tabulary interpolations, the half-hourly rise 
and fall upon every stage of the moon was determined, and the ma¬ 
riner enabled at a glance to know what water existed in excess of 
his chart, and hence when certain subsidiary channels were passable, 
or the several banks might be crossed. He had thus ascertained 
the tidal establishment , or the time of high-water upon full and 
change of the moon, and determined another constant proportion as 
a standard—for graduating future tide-gauge operations, for testing 
soundings hereafter, for fixing a point of departure for engineers 
when levelling eminences, canals, railroads, &c.,—viz. the oscillating 
point, or mean centre which every six hours is common to neaps 
