Instrumental Equipment of Parties 3 



which engages into a i)in in the bottom of the stirrup. The suspension material 

 is silk fiber; for the purpose of taking out torsion a brass bar equal in weight to 

 the long magnet is used. In order to eliminate possible efi"ects of impure brass, 

 as also to prevent damping in oscillation work, the magnet houses are made of 

 wood as far as possible. The details regarding these instruments are given in 

 Table 1, page 8; this type is shown on Plate 2, Fig. 1. 



Soon after the inauguration of the work of the Department, magnetometers of 

 the first type (1) and style (a) were designed, combining the best features of the 

 Coast and Geodetic Survey pattern and that of the Magnetic Survey of India. 

 These are also portable instruments, and although somewhat heavier and larger 

 than the Coast and Geodetic Survey instrument, because of increased detail, still 

 between it and the Magnetic Survey of India^ magnetometer in weight. To 

 eliminate as far as possible questions arising because of irregularities in the shape 

 of magnets, they are perfect hollow cylinders of such dimensions as to make the 

 second distribution coefficient theoretically zero; they are inclosed in aluminum 

 sheaths which carry the optical and centering arrangements. The graduated 

 scale for declination work is not put in the focus of the collimating lens of the 

 magnet as for the Coast and Geodetic Survey magnetometer above described, 

 but on a glass diaphragm in the magnetometer telescope. The suspension used 

 is a phosphor-bronze ribbon. The torsion is readily removed by a torsion plummet 

 with graduated rim, read by a secondary lens which may be turned into the optical 

 system of the magnetometer telescope. The deflection bar is of brass in one piece 

 and practically rectangular in cross-section. The details of these instruments are 

 given in Table 1, but it should also be noted that magnetometers Nos. 2, 3, and 4 

 were made some time before Nos. 5, 6, 7, 8, 9, and 10, and that as a result some 

 of the purely mechanical details and dimensions were slightly altered. Plate 3, 

 Fig. 1 shows this type of instrument. 



The earlier land operations of the Department, especially in regions more or 

 less difficult of access, having emphasized the need of more portable and more 

 compact instrumental outfits than had been used, without sacrificing observational 

 accuracy, the second style (b) of the first type (1) was designed.^ The greater part 

 of the work must be done in regions where travel is generally difficult and, accord- 

 ingly, a bulky and heavy equipment constitutes a serious obstacle to safe and 

 rapid transportation. The controlling conditions, therefore, in the designs by the 

 Department, constructed in its instrument shop, were: (a) portability; (b) compact- 

 ness; (c) simplicity; (d) minimum of separate parts; (e) readiness and availability 

 for immediate use, and (/) the attainment of an observational accuracy equal to that 

 of the best field instruments previously in use. 



The magnet system for this instrument consists, as in type 1 (a), of a long 

 and a short magnet, each a true cylinder of the dimensions given in Table 1 ; the 

 ratio of the lengths of the two magnets is such as to eliminate theoretically the 



* Fraser, H. A. D. The unifilar magnetometer of the Magnetic Survey of India. Terrestrial Magnetism, v. 6, 

 1901. (65-69.) See also Hazard, D. L., /. c. (59-60.) 



' For a more detailed description of this type see Fleming, J. A. Two new types of magnetometers made by the 

 Department of Terrestrial Magnetism of the Carnegie Institution of Washington. Terr. Mag., v. 16, 1911. (1-12.) 



