3T0 Dr. PiTTON on the Strata below the Chalk* [App. C. 



ll'320feet; his mark {called the " North Standard"), heing 4*0661 feet above the mean level 

 of high water, and 21*6811 feet above that of low water at spring tides. He then ascertained 

 by very careful levelling, the heights above the standard, of several points in the vicinity of 

 the Thames, between Sheerness and London * : whence it appears that what is called the " Trinity 



TRINITY 



" high-water mark," at the London Docks, *^;^^- (" answering to XXIII. of the indices 



A 



" marked on the south-west side of the entrance",) is 2'0112 feet below his "North standard," 

 and consequently 19*6699 feet above low-water mark at Slieerness. The last-mentioned quan- 

 tity, therefore, must be added to the height of any station above the Trinity mark, at the London 

 Docks, in order to obtain the height of the same station above the sea at low-water at Sheerness. 



The places in the South-East of England mentioned in the subjoined lists, are grouped in the 

 order of the counties referred to in the preceding pages, beginning on tlie north. The names 

 are disposed alphabetically. The heights are expressed in feet and decimals of a foot. The au- 

 thorities and references to them are the following : 



1. " O." — An alphabetical list of all the heights ascertained during the progress of the Ord- 

 nance Survey, given in the official account of that operation, published in 1811 f, calculated 

 from the level of the sea at low water. The places of the stations are mentioned in the Phil. 

 Trans, for 1800, pp. 576 to 583 ; and in the " Account," above mentioned, pp. 70 to 81. 



2. "C" — Bradshaw's maps of the canals and navigable rivers in the midland coimties of En- 

 gland, in three parts. The heights are taken from low water at Liverpool, (at a point 6 feet 

 10 inches below the sill of the Old Dock); which point is stated to be 21 feet below the Trinity 

 House high-water mark at the London Docks, and consequently I'SSl feet beneath low-water 

 mark, mean spring tides, at Sheerness, according to Mr. Lloyd. 



8. " M." — A manuscript list of heights, principally in Cambridgeshire, ascertained by 

 Mr. M'Lauchlan, of the Ordnance Survey, and Mr. Glaisher of Cambridge, and for which I am 

 indebted to the former gentleman. In this list (which is not an official document,) the heights 

 are counted from the level of the ?ea at low water, in Lynn Deeps. 



4. " B." — A MS. list of heights above the sea at low water, ascertained by the late Mr. Bevan 

 of Leighton, for which 1 am indebted to his son, Mr. Bevan of Wellingborough, and to Mr. Mat- 

 thews of Leighton. 



5. " W." — A short list of heights in the Isle of Wight, subjoined to Mr. Webster's'letter to Sir H. 

 Englefield. These are chiefly barometrical, and from their being connected with some of the mea- 

 surements of the Ordnance survey, it is probable that the zero was low-water mark. 



* Phil. Trans. 1831; p. 167 et seq. Mr. Lloyd (p. 190,) "concluded his leveliings at a 

 " standard mark at the landing-place on the north-east side of the New London Bridge ; "' 

 which " mark was 2*3967 feet below the North Standard mark at Sheerness ; " and conse- 

 quently 0*3855 foot, (= 4*626 inches), below the Trinity mark at the London Docks. The 

 mark here referred to is a flat piece of brass, let into a cavity in one of the two large flags or 

 slabs of granite, which form the landing-place at the bottom of the second flight of steps, de- 

 scending from the footway on the north-east side of the bridge. The upper flight consists of 29 

 steps ; the second, at the foot of which is the mark, of 26 ; the lowest flight is commonly more or 

 less concealed by the water. The cavity in which the mark is lodged, is about 3 inches square, 

 with rounded angles ; and is two feet from the eastern wall or side of the bridge, and 2 feet 

 8 inches from the southern edge of the stone. The surface of the brass is about half an inch 

 beneath that of the stone, which is itself a few inches below the level of the water at high spring 

 tides. 



t "An account of tlie Trigonometrical Survey, &c. 1800, to 1809; by Lieut.-Col. Mudge, and 

 "Capt. (now Lieut.-Col.) Thomas Colby, R.E."; 4to, 1811, vol. i. p. 299— 31 1. This work refers 

 to the previous publications in the Pliil. Trans, for 1795, 1797, 1800, and 1803, and carries on 

 the account of the operations to the end of 1811. 



