LEVEL. 
possible : indeed, it is always best to con- 
fine them to distances not exceeding three- 
hnndred yards ; because the dilFerence be- 
tween the rectilinear and the horizontal 
levels are tlieu greatly diminished, and the 
whole survey will prove far more correct. 
This will be easily seen from the following- 
sketch. (Plate VIII. Miscellanies, fig. 2). 
LetAB represent a sight taken at 2100 
yards, and let AC be an equal distance 
measured on the surface of the segment 
AD, but broken off at every three-hun- 
dred yards, i. e. into seven portions : it 
must be obvious that the line B C will give 
a greater length than would result from 
the proportion already stated, the perpen- 
dicular falling so much beyond D ; and that 
such difference would increase in its dis- 
proportions according as the range of sight 
might be enlarged. To prove this still 
more clearly, let us state that the quad- 
rant O P (fig 3), of the earth’s surface 
stands on a radius PS of four-thousand 
miles. Now the first taken from the sum- 
mit O of that quadrant would be a parallel 
to that horizontal radius, and a tangent to 
the arc at its summit, as from O to X. It 
is evident that if a sight of four-thousand 
and one miles could be taken in the direc- 
tion O X, a perpendicular falling from X, 
would not even touch the point P, from 
which it w'ould be a mile distant. It is 
true, that our sights are not to be com- 
pared with the foregoing extents ; but it is 
equally true, that we verge towards the 
error above shown when we take too long 
sights. 
Perhaps nothing can be more deceptive 
than the common mode of estimating 
levels : more than once we have witnessed 
the opinions of smatterers in this branch 
of surveying, who having levelled the in- 
strument with great exactness, and direct- 
ed the telescope to a very distant hill, on 
seeing the point of intersection cut near 
its summit, have concluded the spot so 
indicated to be on a level with that where 
the instrument stood. This mis-conception 
arises purely from a long rectilinear sight, 
without considering that the base of a re- 
mote hill is a plane whose surface stands 
at a very great angle from that on which 
the level is placed ; as shown by the dotted 
lines representing a hill R intercepted by 
the line of sight O X. 
A very good kind of level is made on a 
portable plan, by several mechanics in this 
branch. It consists of a small tube of glass 
let into a plate with which it is exactly 
VOL, IV. 
parallel. This is the surtace, shewing the 
bubble in the tube, as before described : 
the under part of the box, which may be 
about eight inches long, two broad, and 
two deep, has a spring and screw that cause 
the box to change its direction from either 
above or below the horizon, to an exact 
rectilinear level, as indicated by the air 
left in the glass tube. At the centre of the 
bottom of the box, is a brass stud, serving 
to fix into a hole made in the top of a walk- 
ing stick, &c. This kind of level, in the 
hands of a skilful surveyor, may be used 
to great advantage where very great pre- 
cision is not required; but as the sight is 
taken only from the upper edges of the 
box’s ends, it does not admit of that great 
nicety which is indispensable in many opera- 
tions, and where the smallek deviation 
from a true level might occasion immense 
expence and inconvenience. 
The level represented in Plate Level, 
was made by the justly celebrated Jesse 
Ramsden, F. R. S, and considered by him 
as the most complete. It stands on three 
legsj which fold up into the size and ap- 
pearance of a moderately thick round staff, 
three sliding rings, or leather bands, suffice 
to keep them compact and firm. The upper 
parts of the legs fit into a brass plate, as in 
theodolites, on the surface of which is a 
strong male screw, serving to fasten the 
working part of the instrument at pleasure, 
to its centre. The female screw is cut 
withinside a projection 6 ft of a brass plate 
a a : this projection has a hole through its 
top, and contains a brass ball d, screwed 
into another similar plate ee; by which 
means the two plates are connected toge- 
ther, and the upper one can be turned 
about in any direction, while the lower 
one remains fixed. Four screws n passing 
through the lower plate, being worked 
in until they touch the bottom of the 
upper plate, serve either to fix the latter 
firmly; or, by unthreading pne or two, 
and working in the opposite ones, to 
change the inclination of the upper plate 
and to bring it to an exact level. These 
are called the parallel plates, though they 
often stand at an angle, the one with the 
other, when the direction of the upper one 
requires changing as above shown. 
The ball d is perforated with a conical 
hole, to receive an axis I, that is screwed 
to the bottom of the compass box, f, on 
which are two square brass arms P C, pro- 
jecting from it diametrically opposite to 
each other. H is a small brass angle, or 
