172 
MR. LLOYD ON THE DIFFERENCE OF LEVEL BETWEEN 
ination, I permitted the wires to be some distance from the axis of the telescope; 
and, in levelling, a mean of four or six observations were taken, with the tele- 
scope turned half round in its collars at each observation. 
Note . — I take this opportunity of mentioning a substitute I have occasionally 
used for wire or cobweb with success, viz. asbestus ; very fine fibres of which 
can be obtained by being thrown into hot water, when it easily divides. These 
fibres are tough enough to be placed with ease on the diaphragm, and have the 
advantage of being opaque. 
Being now in possession of an instrument equal to perform the most delicate 
observations, my next object was to make some improvement in the station- 
staves, so that they might point out as minute a quantity as the instrument 
could detect a difference of. This was something difficult, without encroaching 
on the portability and quick application of the staff to its use. — The following 
is a description of the staves I used, a Plate of which is given. 
The staff itself is a rod of six feet six inches in length, of solid seasoned maho- 
gany on the face of which is let-in a slip of brass, riveted at intervals from end to 
end ; by the side of this is also a slip of holly, fixed in the same manner. The 
divisions are laid down in feet and tenths, on the holly ; and in feet, tenths and 
hundredths, on the brass. The lower part of the staff is fitted into a square 
tube of brass about eight inches long, four inches of which are occupied by the 
staff, and the remainder filled up with lead. 
The vane is a plane of seasoned holly, with two semicircles of stained ivory 
let into the face of it (see Plate). It is fixed by screws to a brass box with 
tightening springs on the staff itself: there is another small slide on the staff, 
having two clamping screws, and a long tangent screw, which is attached to 
the box of the vane by a female screw ; the last gives a slow vertical motion to 
the vane. 
On the top of this vane, and at right angles to one another, are two small 
spirit-levelSj mounted in brass ; affixed to the vane, in a square hole in the 
centre, and levelled on one side, is a small brass vernier, the edge of which 
slides on the divisions of the staff reading to the lOOOdth, and, by practice, to 
the 10,000dth of a foot. 
The station-staff thus described is in itself complete ; but for accurate ob- 
servation it requires to be immoveable on the picket : a three-legged support 
