THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
185 
Having adjusted my level most exactly, I directed my telescope into the 
tube of the great telescope of the mural circle ; and adjusting as for infinite 
distance, by placing a disc of white paper about an inch from the eye-end of 
the great telescope, I observed all the wires most distinctly. I then adjusted 
my horizontal wire for collimation. 
The mean horizontal point was then taken, and the circle adjusted to that 
by the micrometer ; and after again observing my instrument to be in perfect 
adjustment, I sought for the horizontal wire of the circle, and I was astonished 
and delighted to see so perfect a coincidence of the horizontal wires of the two 
instruments, that, until I slightly depressed the eye-end of my telescope, I 
could not see the horizontal wire of the circle separate from my own. The 
circle was then altered, and the wires were again made to coincide ; the quan- 
tity was then read off, and found to agree within a very few hundredths of a 
second to the horizontal point. 
The Astronomer Royal was present at these experiments, and expressed 
himself much pleased at the proof given of the coincidence of the two instru- 
ments. 
From the stairs of Greenwich Hospital I crossed the river in order to level 
up to the different places where tide-registers had been kept. 
After crossing the Isle of Dogs, I arrived at a spot on the south side of the 
lock of the City Canal at Limehouse Reach (see No. 381, page 26), which was 
1.9008 above the north standard at Sheerness ; and this spot was 3.8446 above 
TRINITY 
the HW marked on the face of the masonry. Therefore 3.8446— 1.9008 
1800 J 
A 
= 1.9438 is the height of Trinity mark at the canal below northern standard 
at Sheerness, and 4.0661 (height of north standard above mean spring tide 
high-water mark) — 1.9438 = 2.1223, the height of Trinity mark above mean 
spring tide high-water mark at Sheerness. 
This spot (picket No. 381) is also 0.5202 above a particularly high tide 2 1 /ft. 
1 1 in., 1827, marked on the masonry; but upon referring to the tide-register at 
Sheerness, of the 20th and 22nd of November 1827, no particular rise in the 
tide is to be remarked. It must therefore have been caused by land floods, 
which are the occasion of most of the extraordinary tides near London. 
2 B 
MDCCCXXXI. 
