ON THE DATUM LEVEL OF THE ORDNANCE SURVEY. 



22'. 



Copies of Letters from Lieut.-Col. Clarke, R.E., Ordnance Survey Department. 



j r i r r,sc / T i; 1 f Ordnance Survey Office, 



JUL Office Of Works, cjv. Southampton, September 22, 1875. 



The tidal observations taken at Liverpool in 1844, by which the mean level 

 of the sea at that port was determined, have never been published. 



The curves shown in the volumo of Plates of the initial levelling for Liver- 

 pool are those of the self-registering tide-gauge for a selected period. 



I do not think that Mr. Shoolbred has noticed page 599 of the volume of 

 levelling*. I do not get mean tide by the formula L(high + low). And I 

 used one month of the self-registering tide-gauge, viz. May 13 to June 14, 1859. 



(Signed) A. K. Clarke, Lt.-Col. 



KM. Office of Works, J* SoSm^^U llh. 



Sir, — In answer to your letter to Colonel Bayley of May 3rd, which he 

 has referred to me, I beg to say that the height of " bottom of 22nd figure on 

 gauge" is not 22-00, but 22-265, which gives 22-265 -17'8= 4-46 (date of 

 Jris determination 1843) against the 4-67 (determined in 1857). 



I cannot explain this discrepancy of 0-21 foot ; but it happens to corre- 

 spond to some extent with the change you have observed in the mean tide. 



Yours truly, 

 (Signed) A. K. Clarke, Lt.-Col. E.E. 



JUL Office of Works, Sfc. 



Ordnance Survey Office, 

 Southampton, May 1G, 1877. 



My Dear Sir, — Perhaps the enclosed diagram may help to clear up the 

 mystery of the Ordnance Datum. The 22-265 feet has been measured 

 (twice, I believe) from the surface of the stone to the lower edge of XXII. 

 (Canning Dock). The 4-67 refers to a different place altogether (George's 

 Ferry). There is no uncertainty whatever about our datum. 



Tours truly, 

 (Signed) A. K. Clarke. 



Perhaps I omitted to notice in my last letter that the 4-40 (your 4-20) and 

 the 4-67 referred to different gauges. 



2iftct marlc 



1877. 



[See preceding. — Secretary Ordnance-Datum Committee.'] 



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