22 Ocean Magnetic Observations, 1905-16 



deflections were impossible when within about 30° or 40° of the magnetic equator. In this 

 region the total intensity of the Earth's magnetic field was too small in comparison with 

 that exerted by the deflecting needle, and so the suspended needle would not come to rest 

 perpendicular to the deflecting needle. Thus, on the 1905 cruise of the Galilee, the L. C. 

 dip-circle became unavailable for total-intensity observations before the vessel reached 

 Honolulu. Similar experiences were encountered in 1904 by the Coast and Geodetic Survey 

 steamers Bache on a trip to Jamaica and Colon and by the Patterson on a trip to Honolulu. 

 Accordingly the original deflection distance was increased from 7.3 cm. to 7.9 cm. and at 

 the same tune a second deflection distance (9.4 cm.) was introduced, making the instrument 

 everywhere available, at least for the latter distance. It only requked a change in the deflec- 

 tion distance from 7.3 cm. to 7.9 cm. to make it possible to use the instrument and method 

 over the entire Pacific Ocean, instead of for the Umited region above mentioned. A brass 

 case (see Plate 4, Fig. 3) for the deflecting needle was made, so as to avoid handhng the 

 needle during a set of observations, the change from short distance to long distance being 

 effected by a simple inversion of the case, in which the needle is mounted eccentrically. 



Next, the milled heads of the footscrews were graduated and means provided for insur- 

 ing that the instrument when mounted on the gimbal stand should actually be level. The 

 heights of the footscrews were repeatedly detennined and controlled for an invariable and 

 level position of the circle whenever the vessel was in port, and from these detenninations 

 it was possible to set the instrument level at any time. We do not recall seeing described 

 in any book on ocean magnetic work in what way the dip circle was actually set level on 

 the gimbal stand, although the full error of level may go into the inclination. Upon one 

 occasion the accidental setting of footscrew B in the place intended for footscrew C pro- 

 duced an error of about 1?5 in the inclination. In this connection, special attention was 

 also paid to the accurate balancing and leveling of the instrument, with the aid of counter- 

 poise, when mounted on board ship on the gimbal stand. 



From the method of observation mvariably followed, four determinations of inclina- 

 tion were secured, two of these being with the regular dip-needles accordmg to the absolute 

 method, inclusive of reversal of polarity of needle, and two being "deflected dips," i. e., 

 those resultmg from the deflection observations at two distances for getting total intensity, 

 hence not involving any additional time. The scheme of observation was such that each dip 

 applied practically to the same moment of time and to the same geographic position of ship, 

 which of course was movmg throughout the observations. In 1905 the agreement between 

 the values of the inclination obtained from the deflection observations and from the regular 

 dip-needles was not always satisfactory. Upon mvestigation it was found that this was 

 chiefly due to the lateral play of the suspended intensity-needle, No. 3, in the jewels. In the 

 first L. C. dip-circles, the pivots of the various needles were not always precisely of the same 

 length; hence, m order not to have the jewels so close as to bind on the pivots of any one 

 needle, they were put far enough apart to prevent this. It thus occurred that some lateral 

 play resulted for the needle with the shortest distance between the ends of the pivots, which, 

 in the case considered, happened to be intensity needle No. 3. The rubbing of the brass 

 knob with the ivory scraper, or the motion of the ship, doubtless caused the suspended 

 needle to move so as to change its distance from the fixed deflecting-needle (No. 4), by a 

 fraction of a millimeter — sufficient to produce an appreciable en-or in the observations. 

 To overcome this difficulty the jewels were adjusted so as to fit needle No. 3, and other 

 needles were substituted for those that were found to bmd for this position of the jewels. 



For certain shore work there were also provided, for use when necessary, a compass 

 attachment and an astronomical telescope, so as to make the sea dip-cncle a universal 

 instrument — a theodoUte, dip circle, and magnetometer combined. The compass attach- 

 ment served ashore for settmg the plane of the dip circle in the magnetic meridian when it 

 was not desired, or not possible, to use the magnetic-prune-vertical method, and also for 



