250 
SURVEYING INSTRUMENTS. 
bars B. Each leg is divided vertically, and contains a 
long, gun-metal, square-threaded screw C which is made 
to rotate by means of a worm-wheel and endless screw 
worked by a winch handle D, and capable of being firmly 
clamped after adjustment at points about 15 inches 
apart E. The upper ends of these screws are conical, 
and fit into three inverted radial grooves formed in the 
lower side of a cast-iron circle or table, which is thus 
supported by the three screws without being attached 
to them, and is therefore free to accommodate itself to 
expansional changes without restraint. The upper surface 
of the cast-iron circle is turned flat and true to receive 
the tribrach of the instrument. The three screws F 
which pass through the side of this circle are intended 
to adjust the centre of the instrument over the station 
mark. A lever G also passes through the side of the 
circle and actuates three rollers, which when in action, 
support the greater part of the weight of the instrument, 
and enables the horizontal zero to be set without difficulty. 
As the instrument weighs over 400 lbs. it will be seen 
that some such arrangement is absolutely necessary to 
enable it to be moved on the cast-iron circle. When 
the correct position has been obtained, the lever is 
thrown out of action and the instrument remains im 
movably seated upon the circular frame. 
365.—The Foot Screws are tapped through the ends ot 
the tribrach arms in the usual way, but have a range of 
motion not exceeding y 1 ^ inch. This range may appear 
small, but it is really much more than is required, as 
the upper surface of the cast-iron circle can be levelled 
by the long screws in the mahogany legs before the 
instrument is placed on it, so that not more than about 
inch of motion is required. The foot screws do not 
rest directly on the cast-iron circle, but on the extremities 
of an intermediate three-armed plate which is securely 
bolted to the centre of the instrument, the distance