SOr,AR OBSERVATORY. 15I 



of the mountain, but with considerable difficulty, owing to its great weight, 

 the heavy grades, and the softness of the roadbed at many points after weeks 

 of exposure to the sun. As the expense of operating the truck is necessarily 

 large, it has proved to be cheaper to haul the materials for the steel dome and 

 building (aggregating about 150 tons) to the summit by mule teams. The 

 limiting load that can now be taken up in this way is about 2 tons, but a 

 four-mule team driven by a good teamster accomplishes relatively more than 

 the truck run by three skilled men. The use of the truck will therefore be 

 confined to the transportation of the 5-ton pieces of the 60-inch mounting, 

 and other large parts. 



100-inch Reflector. — The 4.5-ton glass disk for the 100-inch reflector was 

 ordered from the French Plate Glass Companies, of St. Gobain, France, in 

 September, 1906. In view of Mr. Hooker's desire that the work be pushed 

 forward as rapidly as possible, the Hooker building, in which the grinding, 

 figuring, and testing of the mirror is to be done, was constructed during the 

 winter. A large room 34 feet square and 20 feet high, opening into a long 

 testing-hall, has been provided for the work. The walls of this room are 

 very heavy, made of brick laid in cement, and covered with a roof of rein- 

 forced concrete. This portion of the building, which is separated from the 

 other part by iron doors, may therefore be regarded as fire and earthquake 

 proof — a consideration of no small importance, in view of the nature of the 

 work to be done within it. A heavy steel I-beam, supported over the door- 

 way, carries a traveling crane, by means of which the mirror-disk and heavy 

 parts of the grinding-machine, one of which weighs 3.5 tons, can be brought 

 into the room and handled when necessary during the progress of the work. 

 The testing-hall, 100 feet long and 10 feet wide, will permit the mirror to 

 be tested from the center of curvature or with parallel light. For the latter 

 purpose a 60-inch plane mirror will be constructed on the grinding-machine 

 previously used for the mirror of the 60-inch reflector. On one side of the 

 testing-hall are rooms for the grinding and polishing tools and for the 

 apparatus required to maintain the polishing-room at a uniform temperature. 

 There is also a large fireproof vault, for the storage of astronomical photo- 

 graphs. On the other side of the hall a series of computing offices is pro- 

 vided, affording, with the rooms already available in the adjoining building, 

 suitable accommodations for a large staff of computers. It is hardly neces- 

 sary to say that the Hooker building forms a most valuable addition to our 

 resources in Pasadena. 



Work on the grinding-machine for the 100-inch mirror is well advanced 

 and will probably be completed before the glass reaches us from France. 



I visited the Paris offices of the French Plate Glass Companies in June 

 and discussed with the director, M. Delloye, various questions involved in 

 the manufacture of the 100-inch disk. At that time the special appliances 

 needed for the work at St. Gobain had been completed, and the first attempt 



