one at either end of the rectangular axis. These floats carry 

 about 98 per cent of the moving parts of the telescope, or some 

 90 tons, the remaining two per cent being carried by two large 

 spherical defining bearings. The instrument is controlled . by 

 electric motors which provide for three rates of speed in both 

 north and south and east and west directions. 



The driving clock which moves the telescope at a uniform 

 rate corresponding to the rotation of the earth is placed within 

 the concrete pier which supports the instrument and near the 

 soutli end. The driving shaft extends from the clock and 

 meshes with a worm wheel 17 feet in diameter which is at- 

 tached to the telescope axis. The cutting of the gear of this 

 large wheel is now nearly completed after continuous work for 

 several months. vSince the successful operation of the telescope 

 depends mainly upon the smoothness and uniformity of the 

 motion communicated to it by the clock, this operation has been 

 a task requirmg great care and accuracy. After the cutting is 

 finished the shaft and wheel will be ground together for several 

 days in a bath of oil and pumice stone to insure accurate mesh- 

 ing and the removal of all rough surfaces. 



The telescope is mounted on a concrete pier 33 feet high 

 and about 40 feet long. Within this pier are located a room for 

 silvering operations, an electric elevator for carrying the mirror 

 when removed for silvering, and the apparatus for maintaining 

 a circulation of water through coils of pipe around the mirror. 

 In this way it is hoped to keep the glass at a relatively uniform 

 temperature. 



The building and dome which enclose the telescope form a 

 steel structure 100 feet high and 95 feet in diameter. The 

 walls and roof are double throughout to admit of the free cir- 

 culation of air, and thus help to equalize the temperature within 

 the building. The shutter is of the double section type, divided 

 in the center, and when fully open provides an aperture 20 feet 

 wide. Like the observing platform, the crane hoist and the 

 dome mechanism, it is operated by electric motors. The dome 

 is mounted on 24 four-wheeled trucks running on specially 

 ground rails, and power is applied by two driving trucks at 

 opposite sides. When rotated the motion of the dome has been 

 found to be remarkably smooth and free from vibration in spite 

 of the great weight involved which is approximately 600 tons. 



At the present time the dome and buildings are completed 

 and the erection of the mounting has reached the point at which 

 the sections of the tube are placed in position. After the com- 

 pletion of the tube and the adjustment of the mechanical at- 

 tachments which control the movement of the telescope the 

 mirror will be transported to Mount Wilson and placed in its 

 cell. It is expected that the telescope will be ready for use by 

 the middle of the coming summer. 



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