ON TIDES IN THE KIVER MERSEY. 168 



yet proved), tlierc still exists sufficient irregulaiity, shown sometimes in one 

 direction and sometimes in another, at times also when no new alterations 

 in the form of the channel were taking place, to disprove the accuracy of 

 Capt. Dcuham's theory of a permanent oscillating point as far as regards the 

 tides in the Mersey. 



As to his second assertion, " that this half-tide level line is passed hy the 

 oscillation of each tide at three hours before and three hours after high-water 

 time, and not at the half-elapsed time of high and low water," its value can 

 only be ascertained by a close inspection and analysis of the tidal diagrams 

 themselves. 



The writer can, however, state the opinion of the present Marine Surveyor 

 of Liverpool (who has had charge of these tidal observations almost ever 

 since their commencement, and therefore is intimately acquainted -with 

 them), who considers that this assertion of Capt. Denham's is quite un- 

 tenable. 



The selection of the half-tide level at Liverpool as the datum for the 

 Ordnance Survey of Great Britain was made after a series of tidal observa- 

 tions carried on in 1859 at a number of ports round the coast of England*. 



In the Ordnance book, ' The abstract of Levelling in England and Wales,' 

 1861, it is defined in the following terms: — "The datum level for Great 

 Britain is the level of mean tide at Liverpool, as determined by our own 

 obsei-vations ; it is ^ of an inch above the mean tidal level obtained from 

 the records of the self-recording tide-gauge on St. George's Pier, Liverpool." 

 These records, as will be seen by the annexed Table, give the mean half- 

 tide level for the five years preceding 1859 as 4-948 feet above the Old Dock 

 Sill. If to this be added the j\ of an inch (-066 foot) referred to in the 

 Ordnance book, a total of 5-014 feet above Old Dock Sill (the zero of the tide- 

 gauge at the St. George's Pier) is obtained as the level of the Ordnance 

 datum, a difference which is quite borne out by the actual levelling of 

 several engineers. 



The Ordnance book of levels already referred to gives, in another portion 

 of the book, in the column of levels only 4-67 feet as the difference between 

 the Ordnance datum and the zero of the tide-gauge. While, to render the 

 discrepancy stUl more intelligible, no doubt, the printer has omitted the 

 minus sign before the last-named level, so placing the zero of the tide- 

 gauge above the half-tide level — thus introducing a possibility of error (to 

 strangers to the locality) of over nine feet in the comparison of these two 

 important systems of levels (the Ordnance and the Old Dock Sill). 



When it is considered of what importance to the country are those most 

 carefully prepared maps of the Ordnance Sui-vey, and the system of levels 

 which they introduce, it will be readily seen that it should be a matter of 

 paramount importance to dispel any discrepancy or uncertainty which may 

 exist as to the very fons et origo of that system of levels. 



In conclusion, it must not be supposed from the above remarks that the 

 writer, while adducing the irregularity of the half-tide level at Liverpool, as 

 evidenced by the result of twenty years' observations, wishes to argue against 

 the practical uniformity of the mean level of the open sea all the world over 

 — a fact which is being each day more fully admitted. Liverpool is not on 



* Since writing the above, it appears, from ii commuiiieatiou received by the author 

 from the Ordnance Office at Southampton, that "the assumed mean water at Liverpool 

 depends upon tidal observations taken by this Department in March 1844, " and that 

 the 1859 observations published in the Ordnan ce book are those of the self -registering 

 tide-gauge. 



ii2 



