THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
189 
Therefore xxm — 21.333 — 1.667. 
And 3.9099 — 1.667 = 2.2429, the height of mean high-water mark at the 
London Docks above the same at Sheerness. 
And xxiii — 22.812 = 0.188, therefore 2.0549 — 0.188 = 2.0361, the height 
of spring tide high-water mark at London Docks, above spring tide high-water 
mark at Sheemess. 
And xxm — 19.604 = 3.396, and 5.7539 — 3.396 = 2.3579, the height of 
neap tide high water at London Docks above the same at Sheerness. 
No observations have been made on low-water mark ; but from the Trinity 
mark it appears the spring tide low- water mark is considered to be 17-833 
below Trinity mark, or rather below the high-water mark. 
Therefore 22.812 — 17-833 = 4.979, and xxm — 4.979 = 18.021, the height 
of xxm above spring tide low- water mark. 
And 19.6699 — 18.021 = 1.6679, the height of springtide low water at 
London Docks, above spring tide low water at Sheerness. 
Taking 22.812 and 4.979, the mean level of the sea is 13.896. 
Therefore xxm — 13.896 = 9.104, the height of xxm above the mean level. 
Then 11.1399 — 9.104 = 2.0359 gives the mean level at London Docks 
above the mean level of the sea. 
The following is a Summary of the different heights. 
Spring tide H. W. at London Docks, above the same at Sheerness 2.0361 
Mean H. W. mark 
ditto 
ditto 
ditto 
2.2429 
Neap tide Ditto 
ditto 
ditto 
ditto 
2.3579 
Spring tide low water 
ditto 
ditto 
ditto 
1.6679 
Mean level of the tides 
ditto 
ditto 
ditto 
2.0359 
Or taking more correctly the ^ difference between spring - '! 
high and low water at Sheerness, the mean spring level y . . 1.7249 
is 10.8289 below xxxiii, therefore 10.8289 — 9.104 = J 
0.2068 
0.1150 
0.6900 
0.3680 
Note . — It seems by the above summary that as the water decreases in height, 
so the height of the water’s surface at London Docks, above the same at Sheer- 
ness also decreases, with the exception of spring tide at London Docks and 
the neap tide ; these are means, not of the highest tides, but of tides at a par- 
ticular time of the moon’s southing. 
