Trigonometrical Survey. 43c* 
tlie original pipe : but upon considering that this was a mat- 
ter not absolutely essential to the ascertaining of the actual 
length of the base by any future measurement, provided the 
axes of the guns were made to coincide with those of the 
pipes, it was determined to fix the cannon, without digging 
the pit to a greater depth than that of ten feet. In this posi- 
tion, however, it was evident, that the muzzle of the gun 
would rise higher than the surface of the pickets, which had 
been put into the ground for finding the centre ; which ren- 
dered it necessary to drive in and adjust four outer pickets, of a 
proper height, to determine the centre of the bore of the gun, 
by the intersection of another set of wires. The tops of the 
first set of pickets were therefore cleared, and the silver wires 
extended along the fine lines which had been made on the 
lead. A plummet was then suspended from above, and moved 
till it fell on the intersection of the wires. Being fixed in this 
position, another set of wires was stretched across the tops of 
the four outer pickets, till their intersection also coincided 
with the vertical wire of the plummet, in which position, fine 
lines were drawn under the wires on the top of each of the 
outer pickets. The truth of the operation now depending on 
these last pickets, they were carefully guarded by another set 
which surrounded each of them, and these last were again 
bound round with ropes, to preserve the centre pickets from 
any possible accident. These precautions being taken, and 
the pit cleared, a large stone of 2^- feet square, and 15 inches 
deep, containing a circular cavity in its upper surface to re- 
ceive the cascabel of the gun, was placed in the bottom of it, 
the centre of the hole being nearly under the intersection of 
the wires, as determined by a plummet. The gun was then 
3 L 2 
