184 
MR. LLOYD ON THE DIFFERENCE OF LEVEL BETWEEN 
tions made at the two pickets at the water’s edge, I should obtain the best 
proof to my levellings ; the results (page 6 of Proof Levels) will show how little 
confidence is to be placed on water, as a true level, under such circumstances. 
At the Lock Gate of the canal, close to the Thames (see page 15), I made 
some observations of the tides, and found, June 7th, that high-water mark at 
Gravesend was 1.1018 higher than at Sheerness, and June 8th, was 0.8367 
higher ; but these two observations are not to be depended on as giving a 
mean difference between the two places, as the height of the tides at Gravesend 
are much affected by any winds. 
On the new pier at Gravesend, I caused one of the brass standards to be sunk 
in the granite on the eastern side, the face of which is 0.1828 below the north 
standard mark,and3.8833 above mean spring tide high-water mark at Sheerness. 
On my arrival at Greenwich Hospital, I commenced a set of branch levels, 
from thence to the Royal Observatory at Greenwich, in order to determine 
the height of that place above the level of the sea. From the abruptness of 
the ascent, the operation was very tedious ; and I here found the advantage of 
the extra lens to the telescope, as there was seldom a distance of more than 
twelve or twenty feet between the pickets. 
I levelled up to a small brass standard already placed for me by the direction 
of the Astronomer Royal in the block of stone immediately under the eye-end 
of the transit instrument pointing southward. 
This standard (page 3 1 of Levels) is 
140.6806 above the north standard at Sheerness. 
153.8317 above the mean level of the sea. 
144.7467 above the mean spring tide high-water mark. 
162.361 1 above the mean spring tide low-water mark. 
These observations being completed, it occurred to the Astronomer Royal, 
after minutely examining my instrument, that it might be used as a proof in 
ascertaining the correctness of the horizontal point of the two mural circles. 
Ry his directions I placed my level upon the high window-frame in front of 
and about eight feet from the object-end of the mural circle, pointed towards 
the north, and at such a height that (from a known principle in optics that all 
rays are parallel *,) I could intercept some of the rays from the object-glass. 
* If any object be placed at the focus of a lens (viz. the wires), the emergent rays are parallel. 
