224 report — 1877. 



Canning Island, and not that of the gauge in the Canning Dock, be taken as the 

 correct level of the Old Dock sill ; and that, as is stated in the Ordnance 

 Book of Levelling, the Ordnance Datum be taken at T 8 ^ of an inch above the 

 mean tidal level of the month of May 13 to June 14, 1859, as ascertained 

 by the self-recording tide-gauge of the Mersey Docks and Harbour Board. 



4th. It is thus apparent that the Ordnance Datum is an entirely arbitrary 

 level, which could not be again obtained from tidal observations. 



The Committee have further thought it advisable to take advantage of 

 the present inquiry in order to obtain information as to some of the various 

 local datum-marks in use in the British Isles, and to endeavour to ascer- 

 tain the difference of each relatively to the Ordnance Datum, which would 

 thus become a means of comparison between them. In order to enable 

 the Committee to carry out this work, they request to be reappointed. 



APPENDIX. 



Extract from '■Abstract of Levelling in England and Wales,'' 1861 (p. 599). 



Tidal Observations. 



This assumed mean water at Liverpool is the same imaginary plane to 

 which all the heights in the preceding pages are referred. It depends upon 

 tidal observations taken by this Department in March 1844. The error, 

 however, is very small, as appears from the results shown by the self- 

 registering tide-gauge at that Port. 



The Tide-gauge at Liverpool in connection with the self-registering 

 gauge is divided from zero in both directions. The zero corresponds with 

 the level of the Old Dock sill. 



By taking the annual means of H.W. and L.W. of self-registering gauge, 



{1854, 15-424 above zero. f 1854, 5-544 below zero. 



1855,15-425 „ M L w I 1855, 5-570 „ 



1856,15-515 „ ^^^'l 1856,5-449 



1857,15-478 „ [1857,5-532 



These results are remarkably close, and give + 15-460 as the mean of 

 H.W. for four years and — 5-524 as that of L.W. for the same period. If 

 we assume the mean of these to represent mean water, we should have 

 4-4*968 as the reading of mean water. Mean water is, however, strictly 

 speaking, not the mean of H.W. and L.W., but the mean of all heights re- 

 corded at indefinitely small intervals, and for as long a period as possible. 



An examination of the curves traced by self-registering gauges, even 

 for one month or less, affords an accurate means of determining the mean 

 height, inasmuch as we can measuro the heights at any intervals of time as 

 small as we please. By this means we find from the curves traced between 

 May 13th, 1859, to June 14th, 1859, that the true mean water at Liverpool 

 reads 4-602 above the zero of the gauge. Now, by levelling, it appears that 

 this zero is 4-670 feet (see page 2) below our assumed plane of difference. 

 Consequently the true mean water at Liverpool is 0-068 foot below our as- 

 sumed plane of reference. 



If, therefore, we woidd strictly refer our heights to mean water at Liver- 

 pool, we should increase every quantity in the preceding pages by 0*068 

 foot, 



