By this arrangement as the tube assumes a horizontal position and becomes, so to speak, heavier, the counterpoises do the same, while when the tube becomes perpendicular, and most of its weight falls upon the trunnions, the counterpoises are carried mostly by their axle. A continual condition of equilibrium is thus reached, the tube being easily raised or depressed to any altitude desired. It is necessary, however, to constrain the wire rope b b′ b″, to move in the arc of the circle described by the end of the tube and ends of the levers and hence the twelve rollers or guide pulleys g g′ g″. Over some of the same pulleys a thin wire rope h h′ runs, but while its ends are fastened to the lower part of the tube at b, the central parts go twice around a roller connected with the winch i, near the eye-piece, thus enabling the observer to move the telescope in altitude, without taking the eye from the eye-piece.
Fig. 28.
Sectional View of Observatory.
The iron wire rope required to be carefully made, so as to avoid rigidity. It contains 2 1/3 miles of wire, 1/100 of an inch in diameter, and has 300 strands. Each single wire will support 7 pounds. It is, however, more flexible than a hempen rope of the same size, owing to its loose twisting.
Fig. 29.
The Mirror Support.