THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 183 
The only remaining- pickets that were directly in the marshes, are from 347 
to 352, the mean of which gives 9.6321 below north standard, and 5.5660 be- 
low spring tide high-water mark. 
The following therefore is a statement of the depression of the marshes, from 
Sheerness towards the source of the Thames. 
The Yantlet Creek marshes are 0.5128 higher than the Sheerness marshes. 
The Allhallows marshes, in a distance of three miles, are 2.1277 higher than 
the Yantlet Creek, and 2.6405 higher than the Sheerness marshes. 
The St. Mary’s marshes are 0.1392 lower than the Yantlet Creek marshes, 
and 2.2669 below the Allhallows marshes, having fallen that quantity in a 
distance of three miles. 
The Higham marshes are lower than those of Yantlet Creek by 0.7832, than 
those of Allhallows by 2.9109, and than those of St. Mary’s by 0.6440, having 
fallen the last quantity in five miles. 
The marshes between Northfleet and Greenhithe are lower than Yantlet 
Creek 1.6365, than Allhallows 3.7642, than St. Mary’s 1.4973, than Higham 
0.8533, being a fall of this last quantity in 6^ miles. 
On the eastern side of Dartford Creek, the marshes are 3.0152 below those 
of Yantlet Creek, 5.1429 of Allhallows, 2.8760 of St. Mary’s, 2.2320 of Higham, 
and 1.3787 below the marshes between Northfleet and Gravesend; being a fall 
of the last quantity in 4^ miles. 
The marshes near Woolwich Arsenal to the eastward of the practising ground 
are 4.2880 below those of Yantlet Creek, 6.4157 of Allhallows, 4.1488 of St. 
Mary’s, 3.5048 of Higham, 2.6515 of the marshes between Northfleet and 
Gravesend, and 1.2728 below the eastern Dartford Creek marshes, being a fall 
of 1.2728 in six miles. 
The marshes at Greenwich, as far as the few observations I had the oppor- 
tunity of making, are 0.5083 higher than those of Woolwich, therefore less 
that sum than the comparison of the Woolwich marshes. 
At picket 131 (page 14), I intersected the Thames and Medway Canal, three 
miles from its mouth at the Thames. 
The above picket was driven into the water’s edge, another was at the same 
minute driven to the water’s edge in the basin close to Gravesend. I then 
levelled along the banks, imagining that from a mean of simultaneous observa- 
