Balance or Vertical Force Magnetometer. xxxv 



The needle is placed at right angles to the plane of the magnetic meridian, it is ac- 

 curately adjusted to horizontality by a screw e which balances the needle, another 

 screw d working vertically, regulates its sensibility. The apparatus, and a ther- 

 mometer h which gives the temperature of the needle, is covered by a rectangular 

 box k having glazed openings on both sides opposite the spider crosses ; those on one 

 side allowing light to be thrown on the crosses from two small mirrors, (one of which 

 I is indicated in the figure) ; those on the other, for viewing them and determining 

 their position, which is done accurately by the microscopes b b carrying micrometers ; 

 the micrometer heads a a are divided into 50 divisions. The supports g of the 

 needle are fixed to a marble slab m, cemented to the stone pillar s ; the horizontality 

 of the slab is indicated by a level n, the lower edge of the rectangular box is covered 

 with velvet, and it is screwed hard to the slab by the screws i i. A four-fold cover 

 of thick cotton cloth was placed over the rectangular box, July 18 d , 12 b 1844, in 

 order to keep the temperature as uniform as possible ; the box itself is covered with 

 gilt-paper internally and externally. The large copper stove which was heated for 

 the last time, January 3 d , 1844, and which occupied a position about 7 feet from the 

 balance magnetometer (see Plate I.), was removed from the Observatory, Nov. 4 d 23 h , 

 1844 ; it was found to have no effect upon the balance needle. 



47. If m be the moment of free magnetism of the needle, Y the vertical com- 

 ponent of the earth's magnetic force, G the weight of the needle into the distance of 

 its centre of gravity from its centre of motion, and e the angle contained by the line 

 joining these two centres and the magnetic axis of the needle, the latter being hori- 

 zontal, the equation of equilibrium is 



m Y = G cos 6 

 differentiating this equation, dividing by it, and having regard to the sign of a f 



a Y a m 



= tan € A e — — 

 m 



Y 



where 



rr/2 



tan e = cot 6 — 



where is the magnetic dip, T' is the time of one vibration of the needle in a horizontal 

 plane, and T is the time of one vibration in a vertical plane.* a e is obtained from 

 the observations in micrometer divisions, one division being = 0' - 1003.f 



The time of one vibration in the horizontal plane, T' = 12 s 00.| 



Time of vibration in the vertical plane. 



The needle being in its usual position on the agate planes, the moveable wire 

 of the left micrometer is made to bisect the spider-cross ; the needle is then vibrated 

 by means of a small piece of steel, through an angle of about 40 micrometer divi- 



* See Dr Lloyd's Account of the Magnetical Observatory of Dublin, p. 38. 



t Introduction, 1843, p. xxxviii. J Introduction, 1841-2, Table 15, p. xxxv. 



