488 British Association for the Advancement of Science. 



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 five-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 

 and springs, and quoted by seamen generally as the half-tide 

 mark. Capt. Denham is not as yet prepared to state whether 

 some small constant difference might not be found as to the instant 

 of the half-elapsed time of spring-tide, high and low water, and that 

 of neaps, producing the actual half-range of tide to inches ; but so 

 satisfied is he of a closer approximation than is generally allowed, 

 that, though he would never propose to adjust soundings to that 



