ON THE DATUM LEVEL OF THE ORDNANCE SURVEY. 223 



while the zero of the inner gauge at the Canning Dock is only 4*20 feet below 

 the Ordnance Datum. 



3. Published statement of the Mersey Docks and Harbour Board. 



In an annual printed statement issued by this body there is given, 

 among other matters, information as to the " Levels of Tides at Liverpool " *. 

 This has been issued regularly for some years past. At the commencement, 

 and for some time after, the " mean-tide level " was there given as " 4 ft. 9 in. 

 above the Old Dock sill." Towards the end of 1853 the self-registering 

 tide-gauge was established. In 1864 the information derived from the ten 

 completed years' observations (1854-63) of the self-registering gauge was 

 tabulated, and the " Levels of Tides " were henceforth given as " derived " 

 from that information. In this amended form the " mean-tide level " became 

 " 5 feet above 0. D. S " (in lieu of the previous 4 ft. 9 in.). This informa- 

 tion continued exactly in this form for some years, until in 1871 the words 

 " (Ordnance- Datum) " were inserted after the " mean-tide level " and before 

 the "5 feet"f. 



But the Ordnance Datum (as stated at p. 599 of the 'Abstract of Level- 

 ling,' see Appendix) depends upon tidal observations taken by the Ordnance 

 Department in March 1844 ; and it has therefore nothing whatever to do 

 with the 1854-63 observations of the Mersey Docks and Harbour Board. 



Another very erroneous view of the Ordnance Datum, but one which hardly 

 needs consideration in order to dispel it, is, that it continues to represent the 

 mean tidal level at Liverpool. The mere mention of the facts that in 1844 

 the Ordnance Department considered the mean sea-level at that port to be 

 4-67 feet above the Old Dock sill, while the records of the self- registering tide- 

 gauge from 1854-63 give it as 5-01 feetj above that datum, and that the 

 records of the succeeding decade, 1864-73, show it to have stood during 

 that period at 5*24 feet over the same datum, clearly points out that the 

 moan sea-level, at Liverpool at least, is a varying one, and therefore cannot 

 now represent the Ordnance Datum. 



In conclusion, the Committee are of opinion : — 



1st. That of the two tide-gauges at Liverpool, now purporting to be 

 referred to the level of the Old Dock sill, the zero of that fixed at the 

 S.E. corner of the Canning Dock is about 5-54 inches above the zero of that 

 on the river-face of the Canning Island, Liverpool. 



2nd. That in order to reconcile the statement in the Ordnance Book 

 of Levelling, that "the Datum Level for Great Britain is -f^ of an inch 

 above the mean tidal level obtained from the records of the self-recording 

 tide-gauge on the St. George's Pier, Liverpool," with the actual facts which 

 the Committee have collected, it is necessary to bear in mind that the records 

 of the self-recording gauge referred to were the observations of one month 

 only taken in the year 1859, and that the mean tidal level of that month 

 was 6-26 inches below the mean of the period from 1854 to 1873, taken by 

 the same self-recording gauge. 



3rd. That the difference of level between the Old Dock sill and the Ord- 

 nance Datum, given in the Ordnance Book of Levelling as 4-67 feet, is 

 correct on the assumption that the zero of the gauge on the river-face of tho 



* See Appendix. 



t [Since this report was presented to the Association, the engineer to the Mersey 

 Docks and Harbour Board, on the matter being explained to him, lias directed that the 

 words " (Ordnance Datum) " be in future omitted, thus restoring the information as to 

 the tides at Liverpool to its original form. — Secretary of the Ordnance- Datum Committee.] 



t Sec Report of the British Association for 1875, Bristol, p. 164. 



