MAGNETICAL INSTRUCTIONS. 19 



" Another advantage offered by the instrument is the substitution of the statical for the vibration 

 method, in observations on the Intensity. With the delicate suspension required in vibration experi- 

 ments, such an observation, except in very rare circumstances, would be hopelessly impossible." 



In the Antarctic Magnetic Survey, performed in H.M. Ships " Erebus " and " Terror," Fox's circle 

 was always used in the one selected spot in each ship. The face of the circle always towards the 

 east (unless expressly mentioned otherwise), and the marked side of the needle towards the observer. 

 The poles of the needle were at no time reversed, and great care was taken in mounti ig and dis- 

 mounting it to avoid injury either to the axle or the pivot. 



The index error, occasioned by the force of the needle being always directed towards the east, 

 was examined by comparison with results obtained with needles whose poles were reversed. 



The Dips were observed either " direct " or by the aid of " deflectors," the intensities of the force 

 occasionally by weights and occasionally by deflectors ; en land always by both methods, and at sea 

 occasionally so. 



In the " Erebus " and " Terror," in which the whole or nearly the whole of the disturbance 

 arising from the ship's iron was caused by induced magnetism, the deviation of the Declination in the 

 southern hemisphere was a maximum to the east when the ship's head was to the west, and to the 

 west when the head was to the east, passing through its zero when the ship's head was either north 

 or south, or nearly so.* 



In the Inclination and Total Force, on the other hand, the deviation {in the southern hemisphere) 

 was a maximum when the ship's head was approximately either north or south, and passed through 

 its zero as the ship's head was directed towards the east or towards the west The Dip of the south 

 end of the magnet was least, and the south polar force greatest when the ship's head was to the south ; 

 and the south Dip greatest and the south polar force least when the head was to the north. 



Precautions to be observed in the Use of Fox's Inclination and Intensity Circle. 



1. The needles designed for observations of Intensity are never to have their poles inverted, and 

 care must be taken not to place them inadvertently near other magnets or iron. 



2. In replacing the needles and deflectors in the travelling box, care should always be taken that 

 the poles of each occupy the places marked for them in the box. 



3. When changing the needles at a land station, care should always be taken not to injure the 

 jewels, or the terminations of the axle of the needles. When a needle is changed, it is desirable to 

 hold it only by the grooved wheel, and not to touch the steel ; and the pivot should first be put 



from the circle at high rates of speed, in order to prevent the great jar on the axle and jewels from the screw motion. 

 It is, however, believed that the employment of cushions of india rubber, or an equivalent elastic substance, under the legs 

 of the instrument will obviate this presumable source of injury to the needles. Experiment will speedily determine the 

 several points involved. 



* From the large amount of sub-permanent magnetism which is known to exist in the steam machinery of the 

 " Challenger," this reversal of the compass deviations from east to west in a general sense will probably not be experi- 

 enced, unless indeed the ship attains a very high latitude on her southern exploration between the Cape of Good Hope 

 and Australia. 



