Catalog, Vol. 1 (Classic Reprint) - Hardcover

Inhaltsangabe

Excerpt from Catalog, Vol. 1

The Tubes are of brass, attached to a central cube and axis of gun-metal. The Pivots are hollow cylinders of hard steel, resting on curved supports of bell-metal, the upper edges of which are bevelled, and fit so closely to the pivots that it is practically impossible for dust or grit to get between.

The pressure on the pivots is lightened, as far as stability will allow, by friction rollers resting on hollow cylindrical steel bars, which are supported on the shorter arms of levers, the fulcra of which are in openings cut through the piers, sliding counterpoises of lead being on the longer arms.

The pivots were carefully examined by powerful microscopes provided for that purpose, and were found to be so nearly cylindrical that correction was considered unnecessary.

The Clamp acts on the West side of the Axis, between the piers, and is accessible to both the transit and circle observers. The tangent screw is within the reach of the transit observer only.

Stars sufficiently near the Equator are observed at some or all of seven wires, placed at equatorial intervals of about seconds of time. Six additional wires were originally used for stars near the Pole; but two of these, nearest the centre wire on each side, gave way in the course of the work, and were not replaced: so that, throughout the greater part of the time, the total number of vertical wires, in the focus of the eye-piece, has been eleven. There are two horizontal wires at an interval of seconds of arc.

Both the vertical and horizontal wires are movable by micrometer screws. The head of the screw which moves the R. A. Wires is protected by a covering which is locked at all times, unless when observations are made to ascertain the inclination of the Axis and the line of Collimation. The R. A. Micrometer head is lefi at a fixed reading, thus leaving a small error of collimation to be corrected for in the reductions.

To obtain the Inclination of the Axis and the Nadir Point, a circular trough of mercury, somewhat larger than the object glass, rests permanently under the flooring, on the block of stone which supports the piers, and is used two or three times each week. There are no screws for adjusting the instrument in Level or Azimuth, and the Axis remains nearly in the same position year after year, with a slight increase of error in both directions.

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