When it is desired to place one of the telescopes in a given position (its position of zero, for example), without acting on the alidade, it may be done by acting directly on the telescope itself without the intermedium of the winch. For such purpose it is necessary to interrupt communication with the mechanism by pressing on the button, q. If the telescope be turned to one side or the other of its normal position, in making it describe an angle of 90°, it will abut against stops, and these two positions will permit of determining the direction of the base.
The alidades themselves are provided with a button which disengages the toothed sector from the endless screw, and permits of their being turned to a mark made on the table. A regulating screw permits of this operation being performed very accurately. In what precedes, we have supposed a case in which the movable point is viewed by two observers, and in which the table, T T, is stationed at a place distant from them. In certain cases only two stations are employed.

License information: nan
MPAA: G
Go to source: http://www.gutenberg.org/files/8452/8452-h/8452-h.htm