Magnetic Instruments on the Galilee 21 



The horizontal circle is 12 cm. in diameter. The gimbal stand is so placed with respect 

 to the vertical plane passing through the central fore-and-aft line, and the horizontal circle 

 IS so graduated, that its zero and center are approximately in this fore-and-aft vertical 

 plane when a certam footscrew is placed in the proper groove. If, for example, the ship is 

 heading to the magnetic north and the circle is set at zero, the plane of suspension of the 

 needle will be in the magnetic meridian. From the course, then, as shown by the standard 

 compass, with any necessary deviation corrections or index corrections applied, the instru- 

 ment can be readily placed in the magnetic meridian with sufficient accuracy without 

 resortmg to the magnetic-prime-vertical method. In the recent instruments, constructed in 

 accordance with our specifications, the horizontal circle is graduated continuously from 

 0° to 360° in the dii-ection of counting azimuths (S, W, N, E) instead of the usual quadrantal 

 (0 -90°) graduation. This was done especially to facilitate, when required, the determina- 

 tion of the dechnation on land with the aid of the compass attachment mentioned above. 



The gimbal stand used for mounting the dip circle aboard ship is solidly constructed 

 of brass, the upper part consisting of two gimbal rings, of which the inner one has three 

 radiating grooves for receiving the footscrews of the dip cncle; the brass balancing-weight 

 IS adjustable in height by a screw cut m the rod passing through it, so that the period can 

 be regulated to suit the conditions. In the recent stands these gimbal rings move on knife- 

 edges. For readily placing the gimbal stand in the required position on deck, a lubber-line 

 is cut acros.s the outer fixed ring encncling the gimbal rings. When this lubber-line has 

 been placed in the vertical plane passing through the central fore-and-aft line of the ship, the 

 stand is firmly bolted down to the deck. To overcome the ship's vibration somewhat, 

 a solid sheet of rubber about three-fourths inch thick is put imder the base of the stand in 

 each case. The stand is furthennore surrounded by a brass raiUng, against which the 

 observer can lean while obser\ang. (See Plate 4, Fig. 2.) For the improved form of gim- 

 bal stand, constructed in the instrument shop of the Department of Terrestrial Magnetism 

 and used on the Carnegie, see Plate 14, Figure 5, and pages 196-197. 



For shore observations, a special non-magnetic tripod is provided, as seen in Plate 4, 

 P'igure 1. 



Improved Sea Dip-Circle Used on Cruises II and III. 



Besides the minor changes already mentioned above, the following modifications were 

 introduced in the first L. C. dip-cncles as supplied for use aboard the vessels of the United 

 States Coast and Geodetic Survey and on the Galilee, chiefly during the period 1904-06, 

 when L. A. Bauer was in charge of the magnetic work of the said Survey and of the Depart- 

 ment of Terrestrial Magnetism. Both organizations, therefore, share in whatever credit 

 may be due, E. G. Fischer, chief of the instrument division of the Survey, deserving special 

 mention. 



The first improvement resulted from our insistence on perfection in construction of the 

 various parts of the instruments by the English maker, A. W. Dover, of Chariton, Kent, 

 notably of the pivots and jewels. It was impossible at first to get the Kew Observatory 

 to furnish inchnation-corrections on their standard closer than 5'. Thus if, for example, 

 the correction of one needle were actually about —2' and another about —8', the Kew 

 Observatory might give as corrections of the respective needles 0' and —10'. When 

 questioned, it was stated that, as the observational error of these instruments was large, 

 the Observatory was not warranted in giving closer corrections. However, the observers 

 trained by us reached an accuracy not so very far behind that with land dip-circles, and we 

 finally succeeded, owing to Dover's skill, in getting instruments for which the Kew Observa- 

 tory was willing to give the differences on their standard at least to the nearest whole minute. 



Next, as the instrument was designed specially for high values of the total intensity, 

 it was found that the original deflection distance was a trifle short, and, in consequence, 



