10 Land Magnetic Observations, 1905-10 



attachment very similar to that for the land circle; see Plate 5, Fig. 2, for a photo- 

 graph of the instrument as modified. 



In both patterns of dip circles provision is made for the determination of total 

 intensity by Lloyd's method. To make the ship form available for the determi- 

 nation of total intensity over all parts of the earth, the original deflection distance 

 was slightly increased and also a second, and longer, deflection distance was intro- 

 duced. In some of the instruments a small astronomical telescope was added, so 

 that, with the compass attachment, it could be used as a universal instrument for 

 reconnaissance work when other instruments were not available. 



Circles of pattern (c) are of the land type but difi"er in many respects from the 

 Kew pattern. They are smaller, the horizontal and vertical circles being about 

 8 cm. in diameter; the least count of vernier is 1 minute of arc. The settings of the 

 vertical circle are made by the use of reading lenses and concave mirrors mounted 

 on the vertical circle, which rotates while the vernier is fixed. The needles are 

 about 7 cm. long and swing on agate knife edges, as in the Kew circle. The housing 

 is circular and of brass with glass sides, the upper half being removable to facilitate 

 handling of the needles.^ 



Circle No. 2025, by Tesdorpf after Eschenhagen's design and made as a part of 

 the universal magnetometer No. 2025 already described, was used by the Depart- 

 ment for a short time during 1906. The diameter of the vertical circle is 11.4 cm.; 

 the least graduation is 20' of arc. Readings are made by estimation to the nearest 

 minute of arc. The length of the needle is 11.5 cm. A complete and illustrated 

 description of this pattern is given by Prof. K. Haussmann in the January 1906 

 number of the Zeitschrijt fiir Instnimentenkunde. 



A list of the various circles used, together with the needles and their desig- 

 nations, will be found in the notes under the heading of "Dip Circle Corrections to 

 Standard," pp. 44-50. 



Earth Inductors. 



But two earth inductors have been used thus far by the Department, pending 

 the completion of a design more suitable for field use than is the present one. 

 These two inductors are designated as No. 48 and Department No. 2, the first 

 being constructed by Schulze and the second by Toepfer & Son. They are both 

 substantially of the design originated by Wild^ and as modified by Eschenhagen. 

 Plate 5, Fig. 1, shows No. 2. After the instrument is leveled the reading of the 

 plane of the magnetic meridian on the horizontal circle may be determined by 

 mounting a compass on the axles of the ring bearing the coil, the axis of coil being 

 then clamped in this plane. The mean diameter of the coil is about 9.5 cm. and 

 it is rotated by means of a flexible shaft as shown in the figure. Angular readings 

 of the vertical circle may be estimated to 0.1 minute of arc by means of glass 

 diaphragms divided into ten 1-minute graduations and mounted in the reading 



' For an illustrated description of this instrument see Mascart, E., TraitS de Magnelisme Terrestre, Paris, 1901. 

 (214-215.) 



' Wild, H. Inductions-Inclinatorium neuer Construction und Bestimmung der Absoluten Inclination mit 

 demselben in Pawlowsk. St. Petersburg, Mem. Ac. Sc, Ser. 7, v. 38, No. 3, 1891. 



