180 
MR. LLOYD ON THE DIFFERENCE OF LEVEL BETWEEN 
the standard mark, and the other on the Isle of Grain, about fifty feet from 
the picket. 
Between the high-water mark at the Isle of Grain and the standard at Sheer- 
ness, there is a long ledge or bank of mud, extending nearly half a mile, and 
quite dry at low water. At these times I found it very difficult to make ob- 
servations, and together with the wind occasionally interfering, and the inter- 
ruption from the sun’s rays, which caused a great vibration in the atmosphere, 
I spent day after day uselessly waiting at the instrument. I was, however, 
more successful afterwards, by taking advantage of the first hour or two of 
day-light, which was more favourable for observation, as the wind had gene- 
rally lulled, and the air was in a quiescent state. 
After the instruments were in perfect adjustment, an observation was made 
at each side of the river alternately; first by myself at Sheerness, and then by 
my assistant at Grain. A mean of a considerable number was taken, the differ- 
ence of which, after the curvature was subtracted from each, would give the 
refraction, provided the instruments were in good adjustment, and the obser- 
vations correctly taken. (See page 5 of Levellings.) 
Not being satisfied with the observations made at my first visit to Sheerness, 
I returned at a subsequent period, and made a great number most satisfactorily, 
from which the best were selected, and a mean taken. 
As there was not a fit place for a standard mark near the margin of the river 
Medway, I caused a block of granite, about 900lbs. weight, to be placed on 
the foundation of the old church wall (the church was once much larger) of 
St. James, near the porch-door, into which block was sunk one of the brass 
standards. (See page 6 and 8.) 
The face of this standard is 28.7454 feet above the north standard mark; 
32.81 15 above spring tide high-water mark, and 41.8965 above the mean level 
of the sea. 
From St. James I almost immediately came into the marshes, levelling in a 
direct line towards Yantlet Creek. In consulting the Ordnance map of Kent, 
there appear but two or three ditches in the marshes ; but in a distance of less 
than three miles I passed thirty-three dykes and ditches, from fifteen to twenty- 
five feet broad, and four or five deep, over which there is no mode of passing, 
but a bridge perhaps a mile distant. 
