THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
179 
Having now completed this part of my observations, I levelled from the 
Dock Yard to Queenborough : — the result of the operation will be seen in pages 
1 and 2 of the Levelling Book. And it will be there observed, that between 
the levels and proofs there is a difference (in a distance of nearly three miles) 
of 0.0269 about fths of an inch. In the commencement of my commission I 
had a new instrument, and many little difficulties to overcome. In part of the 
Queenborough observations, I made use of another level which I had from 
Troughton, on a different principle to the one by Carey ; and in page 3 of the 
rough Levelling Book, an observation is there made on the difficulty I had in 
getting the level to adjust, from the fault I have mentioned in my remarks, of 
the bubble having been placed in its tube with only a little paper, and be- 
coming loose. 
I used also my large instrument with a level adapted to it, made by Riechen- 
bach, which was lent me by Captain Sabine. It was of a large diameter, and 
ground most accurately, but not hermetically sealed, having two plates of glass 
ground in to the ends. It was exceedingly delicate, but at any considerable 
motion, the bubble would break into many globules, requiring a long time to 
collect again. The second day I used it in the marshes the day was hot, and 
I had not at that time the means of shading it from the sun ; the bubble, while 
observing, suddenly contracted, and almost immediately disappeared. I threw 
some water on the instrument, and the bubble gradually appeared again, but 
much lengthened. The next day I placed the level in the sun with a thermo- 
meter; when at 68° the end was forced out, and I found it contained ether, 
which appears to be quite unfit for a level in a high temperature. For future 
observations I replaced it by one of Cary’s make. 
The mean of the observations give as follows : — Queenborough standard is 
higher than southern standard 9.9981, and 10.6029 higher than northern stand- 
ard; and 14.6690 above spring tide high-water mark, and 23.7540 above the 
mean level of the sea. 
Having driven a large picket into the embankment at the Isle of Grain, at 
a point the shortest distance I could obtain from the north standard (viz. 
about 5000 feet), across the Medway, I now commenced the observations 
necessary to ascertain the difference of level between the zero or north stand- 
ard, and the picket at the Isle of Grain. One instrument was placed near 
2 a 2 
