the Trigonometrical Station on Rumbles Moor, &¥c. 191 
nature, the following instrument was prepared ; but, from the 
marshy nature of the summit of Rumbles Moor, could not be 
rendered serviceable. 
An inflexible deal rod, carrying at each end a thin steel plate 
about six inches square, and exactly ten feet asunder, being 
placed upon a perfectly horizontal plane, the brass plate of the 
large sector was attached to the rod in a vertical position, and 
the bubble of the index-level (at zero) adjusted to its mark. The 
instrument being placed upon two stands erected upon the de- 
clivity of the moor, with their upper surfaces truly horizontal, the 
index when levelled would mark the angle of inclination, which 
with the constant radius of ten feet, and a table of natural sines, 
would give the fall from one stand to the other, from the sum- 
mit to the base of the mountain. The brass plate with the di- 
visions being perpendicular to the steel ends, and the stands on 
which they rest perfectly level, it follows that the former would 
be always in a vertical position, and that the bubble when once 
at its mark could not be displaced, however the position of the 
instrument might be varied. 
To avoid the expense of a levelling instrument, the fall from 
Rumbles Moor to the canal at its base was determined with a 
four-inch theodolite, two staves about twenty inches in length, 
and a 100-feet tape. 
When the reaches of the road would admit of it, the staves, 
commencing at the station, were placed nearly 200 feet asunder, 
and the theodolite erected and adjusted at the middle distance. 
The cross wires being fixed upon the centre of the white circles 
in the upper part of the dark-coloured staves (which to avoid 
parallax were described on thin paper, and had one common cen- 
tre), the angles of elevation and depression were carefully read 
off to half minutes on the two-inch semicircle. The distances 
from the centres of the circles to the axis of the divided arch 
were next measured with the tape, and affixed to their respective 
angles.—With these data and a table of logarithmic ses, the 
difference of altitude of the staves is easily calculated. . 
A base trigonometrically determined (with favourable angles) 
to be 6719 feet, being measured with the tape, served to ascer- 
tain its error, as well as the trifling one of the scale with which 
in future it was from time to time compared. 
This novel method of levelling is, 1 firmly believe, little in- 
ferior to the usual one in point of accuracy, and evidently pre- 
ferable as far as regards convenience and dispatch, The staves 
may be placed (as is frequently required) in an oblique position, 
and the steeper the descent the greater the accuracy of the 
operation. It is scarcely necessary to remark, that the constant 
error of the instrument, the refraction, and the allowance for 
curvature, 
