_ the Trigonometrical Station on Rumbles Moor, Yorkshire. 131 
other, it follows that the mean of two observations ¢arefully 
made with the instrument erect and inverted would give the 
correct angle, and render the adjustment of the level superfluous. 
To ascertain and determine the value of the inclination of the 
sides of the glass tube, two further observations with the spirit- 
level taken off and reversed, immediately succeeded, and the 
mean of the four readings considered as the true angle. The 
level being curved proved so very unmanageable with its con- 
cave side uppermost, that the instrument was shortly laid aside 
for the horizon sector, No. 2, which will be best understood by 
describing the method of verifying its adjustments. 
Having placed the instrument upon an immoveable stand, 
fix the point of intersection of the cross wires of the 20-inch 
brass telescope (c), Plate II. upon some distant well-defined 
object by means of the rack-work (d), and the clamp (e) of the 
brass stand (FG), and the intersection should remain perfect 
during an entire revolution of the telescope in its Ys (hh) (17 
inches asunder). Then throw open the semicircular rings (ab), 
and having placed the left hand index (7) at zero on the limb 
(7) by the rack-work () and the pinion (Z), cause the bubble 
of the level (m) to appear at its mark by altering the inclination 
of the telescope as before. When properly adjusted, the telescope 
will bear being reversed in its Ys without displacing the bubble. 
Lastly, i»vert the telescope and repeat the verifications for the 
right hand limb, index. and level (z). During the operation 
the limbs should -be rendered vertical by their respective cross 
levels (at 2 and o). When the adjustments are not perfect they 
are rendered so, in the usual manner, by the adjusting screws of 
the cross wires and of the levels, 
The divisions (on silver) have a radius of 15 inches and ean 
be read off to five seconds. . ; 
Both the instruments were made and divided by the late Mr. 
James Allan (Fetter-lane) from models sent to him. 
In making observations the instrument is placed upon a tripod 
stand, or, what is preferable, upon a rock or pile of stones, and 
the telescope accurately pointed and clamped upon the object. 
The left hand limb is afterwards (which is a great advantage) 
rendered vertical ; the index levelled, and the angle read off by 
the elevation as well as by the depression side of the zero of the 
yernier (of the index), and a proper mean registered. To verify 
the adjustment of the telescope it is now inverted, again clamped 
upon the object, the right-hand index levelled, and double 
readings repeated. ‘Ihe mean of the four angles is considered 
as the correct one. 
At the observatory a greater degree of accuracy was obtained 
by reversing the telescope at each observation, and reading “ed 
R 2 the 
