VI. TERRESTRIAL MAGNETISM, 247 



VI. APPARATUS FOR THE OBSERVATION AND RE- 

 GISTRATION OF THE TERRESTRIAL MAGNETIC 

 ELEMENTS. 



DIP AND INTENSITY INSTRUMENTS USED IN MAGNETIC 

 SURVEYS ON SHORE AND AT SEA. 



EXHIBITED BY THE ADMIRALTY HYDROGRAPHIC DEPARTMENT. 



1145. Dip Circle for observations at sea, fitted with special 

 arrangements for finding the magnetic meridian. By Nairn and 

 Blunt ; date, 1772-1834. 



This instrument may be considered as intermediate in construction between 

 that made by Nairn for Captain Phipps, in his voyage towards the North Pole 

 in 1773, and the Fox circle introduced by Mr. R. W. Fox in 1834, hereafter 

 described. 



It is suspended by an universal joint, from a wooden stand carrying one 

 adjusting screw. The needle, 9 inches long, with steel axles, vibrates within 

 a circle graduated to 20', and the ends of the axis are fitted to work in the 

 agate holes of two adjustable screws in the vertical bars supporting the circle, 

 and otherwise strengthening the instrument. The sliding pointers on the 

 graduated circle are intended to be adjusted to the mean position of the needle 

 when the motion of the vessel causes it to vibrate on either side of the dip. 

 The screw on the under side of the circle works the metal supports on which 

 the needle is placed until adjusted in the agate holes. A thermometer 

 graduated to 38 is placed inside the instrument. 



The peculiar arrangement for ascertaining the magnetic meridian consists 

 of a small compass gimballed at the end of a wooden arm. The other end of 

 this arm has a brass fitting to fix on pins in the graduated circle on the top of 

 the frame. The motion of the arm in azimuth causes the whole apparatus 

 to move in the wooden stand until the Dip Circle is in the magnetic meridian, 

 as indicated by the compass. 



1146. Dip Circle and Intensity Apparatus. Fitted with 

 arrangements for ascertaining the magnetic meridian by three 

 methods. By Dollond ; probable date, 1776-1834. 



The Dip Circle is made after the pattern described by the Hon. Henry 

 Cavendish in the Phil. Trans., vol. xlvi., in which the dipping needle rolls 

 upon horizontal agate planes, and a contrivance is applied for lifting it off 

 and on to the agates at pleasure. A milled headed screw works this lift, and 

 an adjacent butterfly screw, an arrangement for causing the needle to vibrate. 

 The vertical circle is graduated to 20' ; the outer circle of the base plate is 

 also graduated to every 45. 



The direction of the magnetic meridian may be ascertained by two methods 

 other than that usually adopted: 1. An edge bar horizontal needle fitted with 

 an agate cup may be placed on the steel point fixed to a balanced axis pro- 

 vided for placing on the agates like the dipping needles. The coincidence of 

 this needle with the plane of the vertical circle shows the latter to be in the 

 magnetic meridian. 2. The same edge-bar needle can be placed on a pivot 

 screwed in the centre of the graduated circle at the bottom of the travelling 

 box. 



Of the three dipping needles, two are flat and one cylindrical and sharply- 

 pointed. The axes are made of gun metal, and one of the flat needles is fitted 

 with a brass cone on Mayer's principle. 



