248 SEC. 9. MAGNETISM. 



Intensity Observations. For this purpose the box which carries the dip 

 circle is fitted with two apertures filled with glass, and a torsion circle on the 

 top. The two flat needles, one of gun-metal for eliminating tension, and the 

 other for horizontal vibrations, have metal pins screwed into the centres by 

 means of which they are attached to the stirrup suspended by silk fibres from 

 the torsion circle. The vibrations are observed through the glass sides, and 

 the magnetic meridian by the edge-bar horizontal needle before described. 



This apparatus closely resembles that used by David Douglas on the north- 

 west coast of America and the Sandwich Islands in 1829-34. 



1147. Dip Circle. By Robinson; date, 1830-75. 



In this circle the needles are 6 inches long, flat and pointed. They move 

 on agate planes in the centre of a graduated circle, and observations are read 

 off by means of lenses fixed in the ends of a moveable arm centred on one of 

 the glass sides of the instrument. 



The advantages of this form of dip circle are: 1. That both the needles 

 can be read off for nearly every angle of dip. 2. Portability, from compact- 

 ness of stowage in the box, as the vertical circle is fitted so as to be readily 

 detached from the horizontal. 



An instrument of this kind was used by Major Estcourt during the survey 

 of the river Euphrates in 1836. 



1148. Dip and Intensity Circle invented by R. W. Fox, 

 F.R.S. By Mr. George, of Falmouth ; date, 1834-75. 



The principal object of this instrument is the observation of Dip and In- 

 tensity at sea, and when placed on a properly constructed gimballed table 

 this can be accomplished, except in very bad weather. 



The needles are flat, tapering from the axis to a point, and 6 9 inches long. 

 The axles are finely pointed, and work in the jewelled holes fitted to the 

 bracket and centre of a concentric disc in the back of the instrument, which 

 also carries the bracket. The grooved wheel on the axis is used for carrying 

 the hooks and deflecting weights in intensity observations. In the holes in 

 the cross arms of the verniers at the back of the circle, the deflectors, N and 

 S, are screwed for dip and intensity observations, and are set at any required 

 angle by means of the graduated circle. Of the two large thumb screws in 

 the back of the moveable disc, one works the bracket when mounting the 

 needle and vice versa- ; the other works the clamp. At other times they are 

 used in conjunction for the purpose of moving the disc when altering the bear- 

 ings of the needles in the jewels. The pointed projection between these 

 screws, when rubbed by the ivory disc, opposes the effect of friction in the 

 needle and jewels. 



The needles are packed in metal cases with screw ends, and may thus be 

 used as deflectors. 



Instruments of this construction have been largely and successfully used in 

 the various magnetic surveys made at sea in H.M.'s ships. 



1149. Hansteen's Intensity Apparatus ; date, 1819-50, 

 This form of intensity apparatus is that first adopted by M. Hansteen in 



his magnetic survey of Norway and the Baltic shores in 1819-24, and since 

 largely used by various observers. The vibrating needle is cylindrical, 

 pointed at the ends, 2 65 inches long and 0*15 inches in thickness. II is 

 suspended from the moveable pulley at the end of the brass tube by a fibre or 

 silk secured to a brass strap and loop in its centre. By means of the pulley 

 the needle can be adjusted to the required height in the vibrating box. 



The value of the observations depends on the permanency of the magnetic 

 condition of the needle. 



