TRANSACTIONS OF THE SECTIONS. 21 
_ mote from all disturbing influences. The building is, forty feet in 
length by thirty in depth. It is constructed of the dark-coloured 
_argillaceous limestone, which abounds in the valley of Dublin, and 
_ which has been ascertained to be perfectly devoid of any influence on 
_ the needle. This is faced with Portland stone ; and within, the walls 
are to be studded, to protect from cold and damp. No iron whatever 
will be used throughout the building. With reference to the materials, 
_ Professor Lloyd mentioned, that in the course of the arrangements 
- now making for the erection of a Magnetical Observatory at ”Green- 
_ wich, Mr. Airy had rejected bricks in the construction of the building, 
_ finding that they were in all cases magnetic, and sometimes even polar. 
Mr. Lloyd has since confirmed this observation, by the examination of 
specimens of bricks from various localities ; and though there appeared 
_ to be great diversity in the amount of their action on the needle, he 
_ met with none entirely free from such influence. 
- The building consists of one principal room, anc two smaller rooms, 
one of which serves as a vestibule. The principal room is thirty-six 
_ feet in length by sixteen in breadth, and has projections in its longer 
sides, which increase the breadth of the central part to twenty feet. 
_ This room will contain four principal instruments, suitably supported 
on stone pillars ; viz. a transit instrument, a theodolite, a variation in- 
strument, and a dip apparatus. The transit instrument (four feet in 
_ focal length,) will be stationed close to the southern window of the 
room. In this position it will serve for the determination of the time ; 
_ and a small trap-door in the roof will enable the observer to adjust 
_ it to the meridian. The theodolite will be situated towards the other 
_ end of the room, and its centre will be on the meridian line of the 
_ transit. The limb of the theodolite is twelve inches in diameter, and 
_ is read off by three verniers to ten seconds. Its telescope has a focal 
length of eighteen inches, and is furnished with a micrometer for the 
_ purpose of observing the diurnal variation. 
The variation instrument will be placed in the magnetic meridian, 
_ with respect to the theodolite, the distance between these instruments 
_ being about five feet. The needle is a rectangular bar, twelve inches 
_ long, suspended by parallel silk fibres, and inclosed in a box to protect 
it from the agitation of the air. The magnetic bar is furnished with 
an achromatic lens at one end, and a cross of wires at the other, after 
_ the principle of the collimator. This will be observed with the tele- 
_ scope of the theodolite, in the usual manner ; and the deviation of the 
line of collimation of the collimator from the magnetic axis will be 
certained by reversal. The direction of the magnetic meridian being 
_ thus found, that of the true meridian will be given by the transit. It 
a only necessary to turn over the transit telescope, and, using it also 
a collimator, to make a similar reading of its central wire, by the 
w “scope of the theodolite. The angle read off on the limb of the 
q theodolite i is obviously the supplement of the variation. This use of 
1e transit has been suggested by Dr. Robinson; and it is anticipated 
that much advantage will result from the circumstance, that the two 
eeereuritics of the arc are observed by precisely the same instrumental 
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