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X. An Account of Operations carried on for ascertaining the Difference of Level 
between the River Thames at London Bridge and the Sea ; and also for deter- 
mining the Height above the Level of the Sea, 8$c. of intermediate Points passed 
over between Sheerness and London Bridge. By John Augustus Lloyd, Esq. 
F.R.S. F.R.G.S. 8s F.S.A. 
Read March 3 , 1831. 
Jn February 1830, at the suggestion of the Royal Society, I had the honour 
to receive directions from the Lords Commissioners of the Admiralty to make 
such observations as I might consider necessary, to ascertain the difference, if 
any, between the level of the waters at certain points on the river Thames, and 
the mean level of the sea near Sheerness, as well as the height of different inter- 
mediate points above the sea, such as Gravesend, Greenwich Observatory, &c. 
Having found, while employed in the Isthmus of Darien, how inadequate 
the present levelling instruments were to obtain very accurate results, and 
being desirous of conducting the interesting observations, I now had orders to 
make, with the most scrupulous exactness, I thought it necessary, in the first 
instance, to bestow some attention to the improvement of the instruments 
required to be used, endeavouring to combine superior steadiness and motion 
in azimuth, more delicacy in the level itself, more permanency in its position, 
and greater power in the telescope. 
After several trials, and by the assistance of Mr. Cary, (to whom I am 
indebted for many valuable suggestions,) I determined upon having an instru- 
ment made exactly after the accompanying Plate. 
Fig. 1 . is a perspective view of the instrument. It is supported by three 
foot-screws, similar to those used in the best altitude and azimuth circles, 
with thirty threads to an inch, and serving to place the instrument horizontal. 
The stand is formed by two plates of brass, which are firmly connected 
together by three pillars. To the limb that carries the telescope, in very sub- 
