ON THE DATUM LEVEL OF THE ORDNANCE SURVEY. 221 



George's Pier are referred. Being consequently the lovel to which the 

 Ordnanco Datum is referred, it is therefore of the greatest importance 

 that its exact position should be clearly determined ; and the primary object 

 of this Committee is to set at rest the doubts which have hitherto existed on 

 the subject. 



The uncertainties appear to have arisen from the following causes : — 



(1) The difference between the levels given in the Ordnance Book of Levels 

 ('Abstract of Levelling in England and Wales,' with plates, 1861) and in 

 the tracings of original levelling in Liverpool in 1843-44, sheet 29, as sup- 

 plied by the Ordnance Department to the Borough Engineer's Office, 

 Liverpool. 



(2) The existence in Liverpool of two gauges, each purporting to be a re- 

 production of the Old Dock sill. 



(3) A published statement under the authority of the Mersey Docks and 

 Harbour Board as to the position of the Ordnance Datum with reference to 

 the Old Dock sill. 



1. («) 'Abstract of Levelling in England and Wales.' 



At page V of this book occurs the above-quoted definition of the Ordnance 

 Datum ; and at page 2, in the list of levels, we find " Zero of the Tide-Gauge 

 at George's Ferry Basin, near George's Baths, Liverpool. Altitude in feet 

 above Mean Level of the Sea at Liverpool 4-670 ;" a minus sign ( — ) should 

 evidently have been prefixed to this level, as the zero of the tide-gauge was 

 below, not above, the mean sea-level. The zero of the tide-gauge was re- 

 garded as identical with that of the Old Dock sill ; and, judging from the 

 remarks on page 599 of the same volume, it was assumed to be so by the 

 Ordnance Department. 



These remarks* begin by saying that the mean-water datum plane depends 

 upon the observations taken by the Ordnance Department in 1844. Then 

 follows a statement showing that the Department were in possession of 

 the records of the self-registering gauge for four years, from 1854 to 

 1857, and that the mean water during that period was 4-968 feet above the 

 level of the Old Dock sill. Notwithstanding this fact, recourse was had (as 

 is stated further on) to the tidal curves traced by the self-registering gauge 

 between May 13th and June 14th, 1859 (one month) ; the mean water of 

 which period is there announced as the true mean water at Liverpool, 

 and that it differs but 0-068 foot (the -fa of an inch of the definition) from 

 the assumed Orduance Datum plane. 



The statement in the definition, " the mean tidal level obtained from the 

 records of the self-recording tide-gauge," without any qualification as to the 

 time during which the records were taken, might therefore be misappre- 

 hended, as it only refers to the mean obtained from one month's observa- 

 tions, and is not borne out by the records taken over longer periods. 



The tidal curves, moreover, shown under the head of " Liverpool " in the 

 volume of Plates issued with the Book of Levelling above named, would natu- 

 rally be supposed to be those taken by the Ordnance Department for deter- 

 mining the mean level of the sea. The letter of Lieut.-Col. Clarke, B.E., 

 Ordnance Department, Southampton, sent to the Secretary of this Committee, 

 shows, however, that such is not the caset. 



(b) Tracings of original levelling in Liverpool, 1843-44, sheet 29 (as supplied 

 to tho Borough Engineer's Office, Liverpool, by the Ordnance Department). 



On this sheet, at a point near to the S.E. corner of the Canning Dock (the 



* See Appendix. 



t See Appendix for this and other letters from Lt.-Col. Clarle, E.E. 



