20 Ocean Magnetic Observations, 1905-16 



Description of Sea Dip-Circle Used on Cruise I. 



Plate 4, Figure 1, shows the original form of L. C. dip-circle mounted on a tripod 

 for shore observations. The counterpoise, seen attached to the base, is only used for bal- 

 ancing the instrument when mounted on board ship on the gimbal stand. The illustration 

 also shows a compass attachment, added by us for shore-work, as described later. 



The graduated back-circle and other parts of the Fox dip-circle are omitted in the L. C. 

 instrument and replaced by thick ground glass. There is, therefore, only the one grad- 

 uated vertical-circle for reading the incUnation of the needle; it is 11.4 cm. inside diameter 

 and is graduated every 10 minutes. At our request the recent instruments have the vertical 

 circle nmnbered every 2° instead of every 5° as formerly, counting continuously from 0° 

 to 360° instead of from 0° to 90° for each quadrant. 



The needles are 11.35 cm. long and have cone-shaped axles terminating in small 

 cylindrical ends, about 0.5 mm. long, rounded off at the extremity and highly polished. 

 The needle, when mounted, swings in the plane of the vertical circle. The ends, or points, 

 of the needle come very near the graduated arc and the readings are made with sufficient 

 accuracy directly on the circle with the aid of the microscopes, there being no verniers, such 

 as used in land instruments. 



As the sea dip-circle is designed for use on board ship, the agate knife-edges of the 

 land dip-cu'cle are replaced here by jewel cup-bearings in which the pivots of the needle 

 rest or turn. The jewels, fixed to the cross-bars of the circle, are highly polished sapphires 

 in which conical cavities, sUghtly larger than the axles of the needles, have been drilled and 

 polished. The upper half of the jewel is removed, thus leaving a cup into which the axles 

 of the needle can be lowered by the lifter provided. By this arrangement the needles can 

 be retained in place even when the gimbal stand, described below, upon which the instru- 

 ment is placed, is subject to iiTegular motions, due to those of the ship. 



Each microscope for reading the position of the needle is faced with ivory to Hght the 

 circle and contains a single central thread; in making observations the microscope-thread is 

 set on the point of the needle, whereupon the degrees and minutes (by estunation into tenths 

 of a 10-muiute space) are read directly upon the circle. It is not absolutely necessary to 

 set the thread directly on the point of the needle, for the reading can be taken by noting 

 the position of the point directly on the circle ; in the deflection observations of the intensity 

 determinations, however, in order to secure perpendicularity of the two needles to one 

 another, it is essential to set the thread on the point of the suspended needle. 



Holes are drilled in the weighted needle for inserting the weight in either end according 

 to sign of the inclination. A small box of spare weights is provided. 



As in the case of the Fox dip-circle, an ivory scraper is provided, to be used in rubbing 

 and slightly tapping the vertical brass knob on the top of the instrument (below the compass 

 attachment; see Plate 4, Fig. 1). With this ivory scraper, sufficient vibration is imparted 

 to the pivots of the suspended needle to overcome the friction between them and the sides 

 of the jewels, so as to cause the needle to settle down to the lowest point of the bearing. 



The brass case shown at right angles to the microscopes is for the purpose of protecting 

 the deflecting needle from injury to the pivots and from sudden changes of temperature 

 during intensity observations. 



The chief improvement of the Lloyd-Creak dip-circle over the Fox dip-circle consists, 

 accordingly, in the construction of the needles and in the removal of the upper halves of 

 the jewels in which the pivots of the needle work. This form of bearing permits making 

 observations in all positions employed for securing absolute results on land, and also permits 

 ready removal and replacing of the needle. The dip needles used in the sea work can, 

 therefore, have their polarities reversed for elimination of error due to eccentricity of center 

 of gravity, just as for land work. 



