THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
173 
is therefore added, having a box and ring with double compass gimbles, and a 
horizontal motion ; into this the staff slides. 
There is also a small brass tripod with iron legs, having a hole in the centre 
plate, over which another small plate slides, fixing to the tripod by two clamping 
screws; this is used to confine the bottom of the station-staff over the picket. 
Adjustment. 
The only adjustment required to this instrument, is to enable it to be placed 
vertically over the picket. 
Slide the staff into the piece carrying the gimbles ; suspend it with the 
gimbles as nearly as possible at right angles, and as distant as conveniently 
may be from the ground. When the staff does not oscillate, observe if the two 
bubbles on the vane are correct ; if not, by means of the small screws at either 
end raise or depress them, until the bubble remains in the centre. 
Having now the means of placing the staff immoveably and vertically over 
a spot, it is to be accomplished as follows : Fix the support that carries the 
staff, as nearly as possible over the required spot ; pushing the legs into the 
ground, place the small tripod as shown in the Plate, unclamp the milled 
heads, and pass the staff through the gimbles of the support into the plate of 
the tripod. As this plate will move in any direction horizontally, the staff is 
to be adjusted, until perfectly upright, by means of the levels on the vane ; 
then clamp the plate firmly, and push the staff down to the head of the picket, 
and turn it until it is directly in front of the level. 
In March 1830, having every thing prepared, I departed for Sheerness, de- 
termining to commence my observations at that point. 
As part of the main object of my commission was to ascertain the height of 
different places above the level of the sea, it was necessary to endeavour to de- 
termine that point with accuracy. From the observations made from time to 
time, at the caisson at Sheerness Dock Yard, of high and low water, I could 
only obtain this point within a certain degree of accuracy. I therefore deter- 
mined to take advantage of the permission granted me by the Admiralty, to 
commence the erection of a tide-gauge at the Dock Yard. Accordingly, after 
having a model made on a principle that I hoped would be the most simple 
