xxxiv_ [INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 AnD 1846. 
/ is indicated in the figure); those on the other, for viewing them and determining 
their position, which is done accurately by the microscopes 6 b carrying micrometers ; 
the micrometer heads a a@ are divided into 50 divisions. The supports g of the 
Na a mo we ln tl md 
1 : 
needle are fixed to a marble slab m, cemented to the stone pillar s ; the horizontality 
of the slab is indicated by a level m, the lower edge of the rectangular box is covered 
with velvet, and it is screwed hard to the slab by the screws i7. 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 € 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 € 
differentiating this equation, dividing by it, and having regard to the sign of a ¢ 
A Am 
—— = tan € ae — — 
Ys m 
where 
T 
tan € = cot 6 75 
where @ is the magnetic dip, T’ is the time of one vibration of the needle in a horizontal 
