ON TIDES IN THE RIVER MERSEY. IGl 



The following thermometers have also been issued : — • 



To Mr. J. A, Bosworth, a protected Ncgretti, to be used in a deep boring 

 in Shropshire. 



To Mr. Atkinson, of Newcastle, a similar instrument, to replace one broken 

 in his previous observations. 



To Mr. Pengelly, a protected Phillips, for a boring at Torquaj'. 



Tides in the River Mersey. Half-tide Level at Liverpool. 

 By James N. Shoolbred, C.E. 



[A communication ordered by the General Committee to be printed in exiemo.'] 



[Plate IV.] 



In the year 1835, at the Meeting of the British Association in Dublin, 

 Captain (now Admiral) Henry Mangles Donham, R.N., Resident Marine 

 Surveyor of the Port of Liverpool, announced " that, from observations which 

 he had been enabled to take on the rise and fall of a number of tides in the 

 lliver Mersey, he had ascertained that there was an oscillating point or mean 

 centre which every six hours is common to all tides, whether spring or neap, 

 and called the half-tide mark or level." 



In 1837 Captain Denham further announced to the Association, at its 

 Meeting in Liverpool, " that the oscillation of each tide, whether spring or 

 neap, passed this line, viz. the half-tide level, at three hours before and three 

 hours after every high-water time, and not at the balf-elapsed time of high 

 and low water." 



Captain Denham considered this half-tide mark, though not a suitable 

 one to adjust soundings to, to be a most valuable datum for tide-gauge 

 operations, or as a point of departure for engineering levelling-operations — 

 a remark which was subsequently fully confirmed by the selection of the 

 "level of mean tide at Liverpool" as the datum level for Great Britain for 

 the Ordnance maps. 



It will be seen, therefore, that tliis subject of the half-tide level is one that 

 has already received some attention, and has had a practical and important 

 ajiplication. 



It appears, therefore, not out of place to make a few remarks on this half- 

 tide level, and as to its real nature, especially as much information as to the 

 action of the tide in the Mersey has been obtained during the interval of 

 forty years that has elapsed since Capt. Denham first broached the subject. 



In the latter part of 1853 a self-recording tide-gauge was established at 

 Liverpool, near to the St. George's Pier-head, advantage being taken of one 

 of the bridges which connected the floating landing-stage to the shore, the 

 rising and falling of the bridge with the tide being communicated by a chain 

 arrangement to a self-recording drum driven by clockwork and there suit- 

 ably registered. 



These observations had been continued without interruption, save for short 

 repairs, since the above date until twelve months ago, when they were neces- 

 sarily suspended, owing to the burning of the landing-stage and the removal 

 of the connecting-bridge during its reconstruction. 



As, however, the whole of the stage is expected to be again open for use 

 in the course of a short time, and as the connecting-bridges are now in posi- 



1875. M 



