Trigonometrical Survey . 477 
by which the heads of the pickets were placed in the plane of 
the base, which frequently was the cause of the planes of the 
register-heads being out of the direction of the hypotenuses. 
In this operation, however, the bottoms, as well as the tops of 
them, were placed in the true vertical by means of the transit- 
instrument, and therefore it was not difficult to bring the 
planes of their tops into the required position. 
For the purpose of using the transit as a boning telescope, 
as well as an instrument for taking the angles of elevation or 
depression, Mr. Ramsden provided two mahogany boards, one 
of which was fastened to the register-head, and the other (fur- 
nished with levelling screws) rested upon it, the transit-in- 
strument being placed on the latter. 
The level belonging to the transit was then hung on the 
arms ; and if the axis proved to be horizontal, which it would 
be if the brass heads were rightly placed, the instrument re- 
quired no farther adjustment ; but if that did not prove to be 
the case, the axis was made parallel to the horizon by the 
screws of the levelling-board, which were turned in contrary 
directions, having in the first instance been worked till within 
half the limits of their adjustment. By this means the axis 
was kept at a constant height from the brass'heads. 
A board with a cross piece, whose upper edge from the bot- 
tom of it was equal to the distance of the axis of the instrument 
from the head of the picket, was placed on another picket 
which had been driven till its head was at a convenient height 
in the plane of the base, and the transit moved in the vertical 
till the edge of the wire in the centre of the glass, coincided 
with that of the cross piece. The rest of the pickets in that 
hypotenuse were then driven into the ground, till their tops 
MDCCXCV. 3 Q 
