1858.] Geodetic Operations of the Ordnance Survey, 617 



object on the ground must, under this arrangement, be accurately 

 represented in its true relative position to all others, however dis- 

 tant they may be. Thus the houses represented on a plan of 

 a parish in the centre of the kingdom are not only in their 

 correct relative position to the houses in their neighbourhood, but 

 also to every other house, whether in Caithness or Cornwall. 



The levels on the plans are all given with reference to the 

 mean tide level at Liverpool, or the line above and below which 

 the tide rises and falls. Lines of levels from this datum have 

 been carried through all parts of the country ; and thus the levels 

 also are in correct relation to each other, however distant the points 

 may be which are compared. 



The curious circumstance was adverted to, that the levelling 

 taken in Ireland, connecting the mean tide level at a series of 

 stations all round the coast, seemed to establish the fact, that the 

 plane of the mean tide level was inclined from N.W. to S.E., and 

 was three feet higher on the coast of Donegal than on the coast of 

 Wexford. 



The speaker was not able to offer any other possible explana- 

 tion for this, than that of the impinging of the warm water of the 

 Gulf Stream upon the north-west coast of Ireland, and he offered 

 this as a mere conjecture. 



A set of the Ordnance Plans as now produced were exhibited at 

 the meeting. They consist of 



1 . Plans of Towns, on the ^-^^th scale, or 42 feet to an inch. 



2. Plans of Parishes, on the arVo*^ scale, or 25 • inches to 



a mile ; or one square inch to one acre. 



3. Plans of Counties, on the scale of 6 inches to a mile. 



4. Map of the Kingdom, on the scale of 1 inch to a mile. 



All the plans which are drawn on the larger scales, are reduced 

 by photography to the smaller, and at a very trifling cost, and, as 

 compared with all former methods of reduction, with marvellous 

 rapidity. This method of accurately reducing plans was first 

 introduced by Col. James, and will effect a very great saving in the 

 cost of the survey. The plans of the parishes are zincographed, 

 but all the others are engraved on copper. 



The methods employed for conducting the geodetic operations 

 were then described. 



The first consideration is the obtaining an accurate standard of 

 length ; — the Ordance standard of length is a bar of iron, on which 

 the length of 10 feet as derived from the old Parliamentary standard 

 yard was set off. But this standard yard having been destroyed in 

 the fire which consumed the Houses of Parliament, a new standard 

 has been constructed, by a commission, of which the Astronomer- 

 Royal, and Mr. Sheepshanks and Mr. Baily were members. The 

 superintendent of the survey had therefore to ascertain the length 



