ON THE DATUM-LEVEL, ETC., OF GREAT BRITAIN. 219 



Th ird Report of the Committee, consisting of Professor Sir William 

 Thomson, Major-General Strachey, Captain Douglas Galton, 

 Mr. G. F. Deacon, Mr. Kogers Field, Mr. E. Roberts, and 

 Mr. J. N. Shoolbred (Secretary), appointed for the purpose of 

 considering the Datum-level of the Ordna,nce Survey of Great 

 Britain, with a view to its establishment on a surer foundation 

 than hitherto, and for the tabulation and comparison of other 



Datum-marks. 



[Plate XIII.] 



Appointed in 1875 at the Bristol meeting, to inquire into some uncer- 

 tainties as to the exact position of the Datum-level of the Ordnance 

 Survey of Great Britain, the Committee presented in 1877, at the 

 Plymouth meeting of the Association, a Report upon the subject. 



At the conclusion of that report, the Committee requested to be re- 

 appointed ' in order to obtain information as to some of the various local 

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

 the difference of each relatively to the Ordnance datum ; which would 

 thus become a means of comparison between them.' 



The Committee beg to present, as an appendix to this Report, a list 

 of about fifty local datum -marks in Great Britain ; the connection of each 

 of which has been obtained on reliable authority. In Ireland the position 

 of some datum-marks there relatively to the datum of the Ordnance 

 survey of that country has also been ascertained. 



On the assumption that the mean sea-level, as given in the book of 

 Ordnance levels of the respective countries, is uniform across the Irish 

 Sea, the difference between the systems of levels in use in the two coun- 

 tries has been computed. In a similar way also has the difference been 

 ascertained between the Ordnance datum of Great Britain and the official 

 datum in nse in France (Zero du Nivellement general de la France — 

 ligne de Bourdaloue) ; and, through it, with the official levelling in 

 Belgium and in Holland. Several local datum-marks in these countries 

 hp.ve been obtained, each with the connection with the Government 

 levelling of its own country. 



The Committee trust that this list may serve as a basis, which maybe 

 further extended, and become a means of obtaining accurate comparative 

 levels, not merely for engineering and other levelling operations, but also 

 for the connection of tidal observations round our coasts. On the 

 assumption already mentioned, of an uniform sea-level, tidal observations 

 on the adjoining coasts of Ireland and of the Continent may also be 

 included. 



The question of a suitable datum-level as a basis for these international 

 tidal observations, has been considered by the Committee. A level which, 

 while sufficiently low, so as to exclude negative readings, shall bear an 

 easily found relation to the respective datum-marks of the different 

 countries, is requisite for the purpose. 



It is found that, with the differences given in the list appended, a 

 level of • 20 ft. below the Ordnance datum of Great Britain ' coincides 

 (to within O'OT metre, or § in.) with ' 5-50 metres below the French 

 Zero du Nivellement;' and also to '12 ft. 6 in. below the Ordnance 

 datum of Ireland ' (to within 0-04 ft., or \ in.). This level has, more- 

 over, the advantage of being below the range of almost all the tides 



