MOUNT WILSON SOLAR OBSERVATORY. 283 



of the mountain road. For about one-half of the distance the road-bed 

 has been widened from 3 to 8 feet, with a corresponding gain in safety 

 and ease in making the numerous turns with the loaded trucks. The 

 work was begun on January 1 and continued until April 26, when it 

 was interrupted in order that transportation to the mountain might not 

 be obstructed. Owing to unusually heavj^ rains occurring after the 

 work was under way, the labor was considerably increased. 



Notwithstanding the loss of the 3-ton truck by accident, this means 

 of transportation has proved efficient and expeditious. The 1-ton truck 

 has made daily trips with supplies and members of the staff, excepting 

 during the interval of work upon the road and an occasional day of bad 

 weather, A new 3-ton Mack truck, of the same make and type as the 

 old one but better adapted to our purposes, has been purchased and 

 was in daily operation during the construction season on the mountain. 

 With it about 275 tons of material, mainly for the 100-inch telescope, 

 the reconstructed 60-foot tower telescope, and the 10- and 6-inch tele- 

 scopes, have been hauled to the summit. 



CONSTRUCTION ON MOUNT WILSON. 



The work on the 100-inch telescope ended last year with the comple- 

 tion of the pier and the footings for the dome. During the present 

 season the steel framework and the inner sheet-metal covering of the 

 circular building, 100 feet in diameter, which incloses the instru- 

 ment, has been put in position. The structure is complete up to the 

 rails which are to carry the dome, and these have been accurately 

 adjusted and are being ground. The concrete floor has also been laid. 

 The circular form for the rails has been assured by mounting on a pivot 

 support at the center of the building a skeleton steel arm, 50 feet in 

 length, which can be rotated throughout the entire circumference. For 

 the finishing of the rails a motor-driven grinder was attached to the 

 end of the arm, which will later serve as a boom for the erection of the 

 dome. The powder required for the riveting of the building and dome 

 is supplied by an 8 by 8 inch air-compressor, temporarily installed in 

 the base of the pier. 



The next most important piece of work is the reconstruction of the 

 60-foot tower telescope, which is now nearing completion. The outer 

 tower for the support of the 20-foot dome that will shelter the ccelostat 

 is finished, the tube is in position, and the reinforced-concrete observing 

 building is practically finished. The dome is not yet in place, but will 

 be provided before the winter season begins. In the meantime, the 

 coelostat and driving clock are being thoroughly overhauled and im- 

 proved in the Pasadena shops. 



Good progress has also been made upon the buildings for the 10-inch 

 photographic and the 6-inch visual telescopes. Reinforced-concrete 

 buildings have been constructed for both instruments, and the 20-foot 

 domes are now being erected. 



