Descriptions of Instkuments 7 



somewhat heavier grade is used. At Washington, where the horizontal intensity is about 

 0.20 C.G.S., the torsion for the hghter ribbon, for the suspension-length used, amounts 

 to about 5 minutes of arc for 90 degrees of twist. The total length of suspension, that is, 

 the distance from the bottom of the ribbon clamp at the suspension head to the center of 

 the magnet when in place, is from 326 to 375 mm. for magnetometers of type 4(6), and 

 from 314 to 364 nmi. for the combined magnetometer and earth-inductor type 4(c). Ordi- 

 narily the total length is about 340 mm. although in some cases longer, depending on the 

 adjustment of the elevating screw of the upper suspension head when the magnet is in the 

 center of the field of the magnetometer telescope. 



The scale by means of which the position of the collimation line of the magnet is 

 referred to the horizontal-circle setting consists, as in the previous instruments made by 

 the Department, of sixty divisions engraved on the piano-parallel glass diaphragm of the 

 magnetometer telescope. As before, particular attention has been paid to the engraving 

 of these scales to insure accurate graduations and such suitable width of the engraved lines 

 as to permit of sharp and definite readings and estimations. The deflection bar for each 

 of the two new styles of instrument is of rectangular cross-section, 5 nmi. thick and 15 mm. 

 deep, mounted just below the magnet house, being centered and securely held in place by 

 two slightly tapered pins, one at each side of the house in the case of type 4(c). In the 

 universal magnetometers of type 4(6), because of the greater weight of the casting sup- 

 porting the magnet house than is the case with the other instruments and the consequently 

 much broader and better bearings, the bar is held in place by one tapered pin mounted in 

 the center of the instrument and operated by a milled head. For centering and protecting 

 the magnet against sudden changes of temperature during deflection observations, it is 

 mounted in a specially constructed wooden box. The lower end of the magnet-centering 

 pin of the deflection box is flnished so as to fit snugly in rectangular grooves of the deflection 

 bar at three distances, 20, 25, and 28 cm., thus insuring invariability of the deflection dis- 

 tance. For maintaining the vertical plane of the center, two broad arms extend down to 

 the metal base of the box the full depth of the deflection bar, against which a heavy spring 

 on the rear side presses them; a suitable counterweight is suspended on the opposite side of 

 the deflection bar to maintain the level of instrument during deflections. 



COMBINED MAGNETOMETER AND DIP CIRCLE. 



This universal magnetometer, of type 4(6), is completely assembled in one unit, com- 

 prising astronomical telescope and magnetometer for the determination of magnetic decli- 

 nation and horizontal intensity, and dip circle for determination of inclination and total 

 intensitj-; when mounted on its tripod, it is therefore always ready for inmaediate use. 

 The advantage of being able to proceed with any one of the desired observations, without 

 first assembling a number of parts, is evident and permits one to embrace every available 

 opportunity for work. 



The base, horizontal circle, and center are practically identical, as regards graduation 

 and verniers, with those of the theodolite-magnetometer, except a heavy top-plate attached 

 to the base to provide means for the securing of the heavier upper works of this intrument. 

 The level attached to the base has a sensitiveness of 30 seconds of arc. 



The astronomical telescope, of magnifying power IS, is mounted eccentrically, the 

 distance between the center of the instrument and its vertical plane of collimation being 63 

 mm. for No. 14 and 82 mm. for the others of this tj^pe listed in Table 1. (See Plate 2, Fig. 1.) 

 It is provided with a swiveled prismatic eye-piece and suitable shade-glasses for solar 

 observations. The single, strong telescope-.standard is securely mounted on the base casting 

 supporting the magnet house; the bearing for the axis of the telescope is 18 mm. long for 

 No. 14 and 37 mm. long for Nos. 19, 20, 21, and 22. This standard also carries the various 



