.^oo 



NATURE 



[December 20, 19 17 



poses, when proper care is exercised in stowing the 

 carg-o, she is non-magnetic. This enables obser-, 

 rations to be taken in less time and with higher 

 accuracy than on the Galilee. Between them the 

 Galilee and the Carnegie have traversed 224,000 

 miles of ocean, and declination observations have 

 been taken once for each 109 miles on the average, 

 but, owing to the improved facilities, the average 

 distance apart of the Carnegie's stations has been 

 less than half that of the Galilee's. 



The experience of sea conditions has led to 

 modifications of the instruments available in 1905 

 and to the development of new ones. Much work 

 has been done with the Lloyd-Creak dip-circle, or. 



as the present volume calls it,. the " sea dip-circle." 

 This was devised by Capt. Creak as an improAe- 

 ment of the Fox circle. When provided with deflec- 

 tion needles and weights, after the method devised 

 for land circles by Humphry Lloyd, it supplies the 

 total force as well as the dip (I), and so indirectly 

 the horizontal force (H). Ry adding a compass 

 needle and a simple contrivance which enables the 

 distance of the deflecting needle to be varied, the 

 Carnegie Institution has made the instrument also 

 g-ive the declination (D), rendering it at the same 

 time more serviceable for its original purpose. 

 While the dip-circle can supply values for D and 

 NO. 2512, VOL. 100] 



H, these are not quite so accurate as those given 

 by special D and H instruments. The primary 

 declination instrument as used on the Carnegie 

 is a somewhat elaborate modification of the Ritchie 

 liquid compass. For measuring H a new instru- 

 ment termed a " sea-deflector " has been invented. 

 It employs a deflection method analogous to that 

 adopted with the ordinary land magnetometer. 

 The deflected needle is the magnet system of a 

 liquid compass; the deflecting magnet, is hori- 

 zontal, but with its centre in the same vertical as 

 the centre of the deflected needle. It is attached 

 to a sighting arrangement. When the compass 

 needle is sighted it is kn6wn that it and the deflect- 

 ing magnet are at right angles 

 to one another. If when this 

 occurs u is the inclination of the 

 compass needle to the magnetic 

 meridian, 



H = mC/sin u, 



where C may be regarded as a 

 constant, and m is the mag- 

 netic moment of the deflecting 

 magnet. Allowance may be 

 made for the variation of m 

 with temperature ; and compari- 

 sons made, when opportunity 

 offers, with ordinary magneto- 

 meters on land supply the neces- 

 sary information as to the 

 decay of ni with time. Another 

 new departure, known as the 

 "marine earth-inductor," is a 

 form of dip-inductor suitable for 

 use at sea. It has a moving- 

 coil galvanometer, the sensibility 

 of which with a scale distance 

 of I metre is i mm. = io~^ am- 

 pere, the period being 2*4 seconds. 

 An absolutely null method is not 

 feasible, but this does not prove a 



I serious drawback when care is 

 .taken to secure a nearly uniform 

 speed of rotation of the coil. 

 Under favourable conditions the 

 inductor appears an instrument 

 of higher precision than the dip- 

 circle, but it requires at least two, 

 and preferably four, observers. A 

 guiding principle seems to have 

 been to have at least two independent ways of 

 measuring D, I, and H, and to use the less exact 

 instrument as- a check on the more exact. 



The magnetic sea observations taken on each 

 cruise are numbered and tabulated separately. 

 Each table gives the date, the geographical co- 

 ordinates, and the values of D, H, and I. Except 

 in the case of the two last cruises of the Carnegie, 

 the results for which appear only to be preliminary, 

 the tables also include particulars of the hours of 

 observation, the instruments usedj the ship's 

 course, the angle of roll, the state of the sea and 

 the weather. Observations were often taken with 



