DEPARTMENT OF TERRESTRIAL MAGNETISM. 241 



and one Gambey sextant. It should be noted that the overhauHng 

 was so thorough in each case as to make the instrument equivalent 

 to a new one. Minor instrument repairs were made as necessary. 



In connection with the development of the optical system of the 

 compass variometers it was necessary to go into the subject of casting 

 and finishing speculum metal. Instrument-maker E. K. Skonberg, 

 foreman of the instrument shop, made a careful study of the methods 

 of finishing speculum metal and submitted a report thereon. The 

 difficulty in working the metal is of course largely due to its extreme 

 brittleness, and for the work in hand this difficulty was enhanced 

 because the mirrors were in the form of circular and conical segments 

 with sharp edges. It was found that a lead lap loaded with a medium 

 emery and lubricated with kerosene was best suited for the initial 

 grinding. To obtain the mirror finish, a lap was made of pure bees- 

 wax mixed with clean rosin and covered with a very fine, clean rouge. 

 The proper rnixture depends of course upon the weather conditions at 

 the time of use, and is roughly about one part by weight of rosin to 

 ten parts of beeswax. The mirrors obtained show particularly good 

 reflecting surfaces. 



Instrument-maker C. Huff continued in charge of the brass foundry, 

 which was in operation whenever necessary. Over 1,500 pounds of 

 castings in brass, gun bronze, copper, aluminum, lead, bell metal, 

 speculum metal, and various alloys were made. Some difficulty was 

 experienced in obtaining satisfactory speculum-metal castings of the 

 shapes required, but methods were developed to overcome this. The 

 alloy found most satisfactory was the following: 65.4 per cent copper, 

 30.0 per cent tin, 0.6 per cent zinc, 2.0 per cent silver, and 2,0 per cent 

 arsenic. This speculum gives a very good reflecting surface. Coke 

 being unobtainable, it was found necessary to use anthracite coal for 

 heating the brass-foundry furnaces. Coal was found satisfactory, 

 but it requires greater draft than necessary with coke; a mechanical 

 blower was therefore added to the foundry equipment. 



Besides the members of the instrument shop already mentioned in 

 the preceding paragraphs, Messrs. G. H. Jung, W. F. Steiner, and J. G. 

 Lorz deserve mention for their excellent instrumental work during 

 the year, while Mr. A. Smith, carpenter, has rendered valuable service, 

 particularly in the construction of motion tables and paraphernalia 

 for experimental work concerned with the behavior of sea instruments 

 and in the construction of instrument cases and other improvements 

 about the laboratory. The shop work has been carried out, as hereto- 

 fore, under the direction of the observatory division (J. A. Fleming, 

 chief; S. J. Mauchly, assistant chief). Upon the resignation of J. A. 

 Widmer, at the end of 1917, to become experimental engineer with 

 the Sperry Gyroscope Company, Mr. Skonberg was made foreman of 

 the shop. 



