30 REPORT — 1901. 



board of her arose. The only instrument hitherto used for this purpose 

 has been Mr. R. W. Fox's dip and intensity apparatus invented in 1835, 

 and little or no advance made in its construction since then. It certainly 

 did valuable work in the Antarctic Magnetic Survey carried out in the 

 ' Erebus ' and ' Terror ' under Sir James Ross, and also in the ' Challenger ' 

 expedition of 1872-76. An examination of the work done in the 

 ' Challenger ' under most favourable circumstances disclosed certain 

 defects of a character which are quite inconsistent with the precision now 

 required. 



For example the needles could not be reversed, and hence there was 

 constant necessity for frequent comparisons with an absolute instrument 

 on land to obtain index errors. The magnetic moments of the needles 

 were liable to change with no accurate means of knowing when the 

 change took place, thus vitiating the sea observations of total force 

 made by the method of a constant deflecting weight. Again the deflect- 

 ing magnets used for a second method of obtaining the total force were 

 liable to changes with no means for ascertaining the period of such 

 change at sea. The Fox instrument was therefore not suited for the 

 purposes in view. 



Previous experience having shown me the excellent values of the 

 absolute horizontal force to be obtained with the Barrow's Dip Circle 

 fitted with Lloyd's needles, especially in high latitudes, I arranged for a 

 series of experiments to ascertain the best methods of applying the 

 principles of Lloyd's method to an instrument which could be used on a 

 gimbal table on board ship. The use of needles with cylindrical axles 

 resting on agate planes, either for dip or force, was impossible, and trials 

 with various forms of needles and jewels resulted in my adopting the 

 forms for both in the instrument exhibited. All the needles have axles 

 terminating in a cone with the sharp point rounded off and highly 

 polished. The jewels are highly polished sapphires fixed to the cross 

 bars of the circle in which conical cavities, slightly 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 table, upon which the 

 instrument is placed, is subject to irregular motions, due to those of the 

 ship. 



With the circle thus fitted the absolute dip and total force can be 

 observed agreeably with the usual methods described in the Admiralty 

 Manual of Scientific Enquiry. 



As there might be a slight oscillation of the needle at times when the 

 ship is unsteady in a seaway, I have arranged that the ends of the 

 needles shall come so near the graduated arc that the readings may be 

 made directly by the microscopes without the use of verniers, as in the 

 land instruments. 



To obviate friction between the axles of the needles and the jewels I 

 have fitted a knob on the top of the circle, which should be gently rubbed 

 with a circular motion of the ivory rubber provided. 



The readings of the circle may be accurately made at night by placing 

 a candle at the back of the circle when the light will be reflected by the 

 ivory faces of the microscopes to the graduated arc. 



The zero of the graduations on the base plate is so placed tliat when- 

 ever the magnetic direction' of the ship's head is known by a compass 



