MOUNT WILSON SOLAR OBSERVATORY. 239 



CONSTRUCTION WORK ON MOUNT WILSON. 



The greater part of the construction work on Mount Wilson during 

 the past year has centered about the 100-inch telescope pier and the 

 foundations for the building. The site, which had been roughly 

 graded the previous year, has been lowered an additional 3 feet and 

 a concrete retaining-wall built around the south and west portions 

 to prevent washing by the winter rains. The foundations for the 

 building, consisting of two concentric rings of concrete piers, each 

 6 feet square at the base and 6 feet high, and amounting to 40 in 

 number, were completed early in July and work was then commenced 

 on the telescope pier. This is now nearly finished. 



The pier proper measures 20 by 45 feet at the ground-level and 

 is 32 feet 11 inches in height. On the south side is a long extension 

 pier with a top sloping at an angle corresponding to the latitude of 

 the observatory. This pier is designed to carry large fixed spectro- 

 graphs and other apparatus intended for use with the telescope in the 

 coude form. In order, however, to avoid the necessity of construct- 

 ing a pier with a very large upper surface the plan was adopted of 

 mounting the instruments on brackets attached to its vertical face. 

 The optical axis of the telescope passes about 2 feet outside of this face, 

 a distance sufficient to allow for the use of the largest spectrographs. 

 The height of the pier is such that a concave-grating spectrograph 

 of 21 feet radius or a plane-grating spectrograph of 30 feet focal length 

 may be employed if desired. The whole pier is inclosed by an outer 

 concrete wall and roof to assist in maintaining a constant temperature. 



The telescope pier itself is hollow in construction, with three 

 heavily reinforced floors running across it at different elevations. 

 The walls are also strongly reinforced and further strengthened by 

 four buttresses on the outside, two each on the east and west sides. 

 The first floor, at a distance of 16 feet from the ground, is designed to 

 carry a large water-tank, which will form the reservoir for the water- 

 circulation system enveloping the 100-inch mirror. At a height of 

 25 feet is the floor on which the driving-clock, worm-gear, and quick- 

 motion right-ascension mechanism will stand. At the northern end 

 on this floor is the room planned for the silvering of the large mirror. 

 The electric elevator used for handling the mirror moves up and 

 down through an opening 14 feet in diameter near the center of the 

 pier. 



The top of the pier consists of a circular concrete floor 11 inches 

 thick and 53 feet 10 inches in diameter. On the east and west sides 

 it is supported by massi/e reinforced concrete brackets extending 

 outward from the pier. A metal wall about 8 feet high reaches 

 from the edge of this floor to the level of the main steel floor of the 

 building and the joint between the two may be made air-tight by 

 means of a water seal, if found desirable. 



