Instrumental Equipment of Parties 9 



dip and the latter in regions of low magnetic dip. An instrument of recent con- 

 struction by Dover is shown on Plate 4, Fig. 2. The diameters of the horizontal 

 and vertical circles are each 12.5 cm., and the least count of the verniers is one 

 minute of arc. As the result of our field experience in total intensity determinations, 

 continuous graduation for both vertical and horizontal circles from zero through 

 360 instead of, as formerly, in four quadrants of 90 each, has been introduced 

 in the newer instruments of the Department. This modification obviates the 

 confusion which may arise when readings occur in various quadrants, as they 

 actually do in the total intensity work. The length of needle used in the later 

 instruments is 8.9 cm.; in some of the earlier instruments the needles are somewhat 

 longer. The knife-edge agate supports are so adjusted that the axis of the axle 

 of the suspended needle is as nearly as possible in the prolongation of the axis of 

 the vertical circle bearing and center. Settings are made by sighting the ends of 

 the needle with opposite microscopes so adjusted that their plane of collimation 

 passes through the center of the vertical circle bearing, and may be read by two 

 verniers 180 apart. 



In the later instruments, a compass attachment, shown on Plate 4, Fig. 2, has 

 been added both for obtaining approximate declinations and for use in setting the 

 plane of the dip circle in the magnetic meridian. This attachment consists of a 

 deep but thin needle with pivot suspension, mounted in a closed case with glass 

 cover, at the ends of which are short graduated arcs, the zeros of these arcs being 

 placed, as nearly as possible, in line with two sighting vanes and the pivot. The 

 whole is arranged so that it may be quickly mounted centrally on the dip circle, 

 the housing being clamped in place by two screw clamps. Settings are made on 

 the thin, tapering ends of the needle by means of an ordinary lens, the observer 

 being assisted in the avoidance of parallax by the depth of the needle.^ 



The modified Lloyd-Creak form of ship dip circle,^ owing to lack of sufficient 

 equipment, also had to be used in the earlier work. The order of accuracy attain- 

 able with it is somewhat less than with the regular land form. As this circle is 

 designed for use on board ship, the knife edges of the land pattern are replaced by 

 jewel cup bearings in which the pivots of the needle rest. The increased friction 

 resulting from this form of mounting is overcome by tapping or rubbing, with an 

 ivory scraper, a brass point on the top of the circle, thus imparting sufficient 

 vibration to the needle and making it drop to the lowest point of the bearing. 

 The needle, 11.35 cm. long, swings in the plane of the vertical circle, which is 11.4 

 cm. in diameter and is graduated to 10-minute intervals; the microscopes are 

 similar to those of the land pattern except that with their aid the angular readings 

 are made directly on the circle, the nearest minute of arc being estimated. In the 

 later instruments the graduations of both the horizontal circle (diameter 12 cm.) 

 and the vertical circle are numbered continuously through 360 for the same reasons 

 as stated above for the land circle. This circle is also provided with a compass 



' Cf. Bauer, L. A. Results of magnetic observations made by the Coast and Geodetic Survey between July 1, 

 1903, and June 30, 1904. Report 1904, U. S. Coast and Geodetic Survey, Washington, D. C. 1904. (196-197.) 



' For detailed description see Bauer, L. A. Results of magnetic observations, &c., /. c. (192-194); also Terrestrial 

 Magnetism, v. 11, 1906 (77-78), and v. 14, 1909 (164-169). 



