xxxiv Introduction to the Makerstoun Observations, 1845 and 1846. 



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

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

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



ft,--.:^^:^^":.::^v;:: 



needle are fixed to a marble slab »n, 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*^ 12'' 1844, in 

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

 gilt-paper internally and externally. 



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 € 



differeutiatin 



where 



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



A Y 



= tan e A € — 



A m 

 m 



tan € = cot 6 



'JT2 



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



