2 Land Magnetic Observations, 1905-10 



the greater part of the globe are to be published on a uniform basis. Thus, in view 

 of the differences exhibited among themselves by even well-constructed instru- 

 ments, one of the problems was the determination of the corrections necessary 

 for the reduction of the results to a common standard. The Department has 

 now completed a variety of investigations on this subject and has others in progress. 

 In the course of its work it has had opportunity to compare its instrumental stand- 

 ards with the chief ones of nearly every country. The preliminary results of 

 these extensive intercomparisons will be found in the journal Terrestrial Magnetism 

 and Atmospheric Electricity, vol. 16, pp. 61-84, pp. 137-162. 



INSTRUMENTAL EQUIPMENT OF PARTIES. 



Magnetometers. 



The greater part of the present data has been obtained with magnetometers 

 of the following types: 



1. The so-called theodolite-magnetometer type in three designs, viz.: (a) that of the Department, 



similar to magnetometer No. 3, the particulars of which are given in Table I ; (b) that of 

 the Department, similar to No. 13, and (c) that of the United States Coast and Geodetic 

 Survey, similar to No. 20. 



2. The Kew type of magnetometer in two designs, with auxiliary theodolites for astronomical 



work, viz.: (a) the regular design as constructed by Elliott Brothers, similar to No. 73, and 

 (b) the Magnetic Survey of India design, similar to No. 36. 



3. The light and portable type used in the Magnetic Survey of France, similar to No. Ii. 



4. The universal magnetometer type of the style designed by Eschenhagen and constructed with 



modifications by Tesdorpf, similar to No. 2025. 



The theodolite-magnetometers of the first type (1), although of three patterns, 

 are in many particulars similar in detail. The essential units of this type are four, 

 viz.: (o) the base, center, and horizontal circle; (b) telescope standard frame; 

 (c) telescope and vertical circle, and (d) magnetometer. In the first year's work of 

 the Department, use was made, by the courtesy of the United States Coast and 

 Geodetic Survey, of the standard magnetometers of that Bureau;^ these are of the 

 pattern (c). They were made in the instrument shop of the Sur\'ey and embodied 

 the results of the extensive experience gained by its observers. The same base 

 and horizontal circle serve for both the magnetometer and the theodolite attach- 

 ments, thus making the instrument compact and portable without sacrifice in 

 requisite accuracy. The brass deflection bar, while in two pieces, is so mounted 

 as to insure invariability of the deflecting distances. The magnets are octagonal 

 bars with the interior hollow and cylindrical; on the south end of the magnet is 

 mounted a lens with a graduated scale, and on the north end is a collimating lens. 

 The octagonal form of magnet, as also of stirrup, was adopted for convenience in 

 inversion of magnet when determining the reading of the magnetic axis; for centering 

 of magnet in the stirrup, a shallow groove is cut at the middle of the magnet, 



' Hazard, D. L. Directions for magnetic measurements. United States Coast and Geodetic Survey. Washington, 

 D. C. 19U. (53-55.) 



