THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
181 
It was desirable therefore to find some means of conveying my instruments, 
men and apparatus over, without losing so much time by going round. The 
men first used leaping-poles ; and a case for the instrument was constructed 
water-tight, so as to swim over the ditches. But from the sides of the ditches 
being steep, the instrument was constantly subject to heavy blows in being 
taken out of the water; I therefore adopted another mode. On our arrival at 
a broad ditch, two leaping-poles, about eighteen feet in length, with cross pieces 
at the bottom to prevent their sliding too far in the mud, were connected to- 
gether at the top by passing the eye of a rope just over the ends of them, where 
it was confined by two thmnb-cleets ; to this rope was attached, at any height 
that the depth of the ditch might require, a hook having four tails, and like- 
wise hooks to them ; these fixed into the four arms of the box, through which 
slid the poles for carrying it. The ends of the two leaping-poles were merely 
placed in the centre of the ditch, forming a pair of sheers, the apex of which 
was inclined on one side or the other by guys. The instrument was in this 
manner taken from one bank and landed on the other with the greatest gentle- 
ness; it was quickly unhooked, and men and apparatus passed over in the same 
manner. In this mode 263 ditches were passed between Sheerness and 
Greenwich. 
On my arrival at the marshes, I found, from the nature of the soil, the great- 
est difficulty in adjusting the instruments ; the movement of any of the men at 
a considerable distance caused a motion in the bubble, and the least alteration 
in my position whilst standing at the instrument shifted the bubble. In order 
to avoid moving, which heretofore was necessary to examine the level, I 
caused a square mirror, about eight inches by five, to be mounted on a maho- 
gany frame, which permitted it to stand on the upper limb with the face placed 
at an angle of about 60° from the telescope, by which I could, without moving 
my head from the eye end of the telescope, read off the length of the bubble. 
This precaution was not however sufficient in some parts of the marshes, where 
the ground was of hardly more consistence than a bog, but invariably most 
unsubstantial where the most dry. 
At these places, large pickets, from four to six feet in length, were driven in, 
having a groove on the iron head of each. On three of these, the iron-shod 
points of the stand rested ; and after the observations were made, the pickets 
were drawn. 
