224 
Proceedings of the Royal Irish Academy. 
level. The soil is peat, over a stratum of clay and shale, there being 
110 rocks in the immediate vicinity. 
Care has been taken, in erecting the observing-house, that it is 
sufficiently removed from any iron fittings likely to affect the results 
obtained, and nothing but copper and brass have been used in the 
fastenings and fittings. 
The observations have been made during the first and third weeks 
of each month ; two complete sets of horizontal force measurements 
on the first day, one before and the other after noon ; one determina- 
tion of declination before noon, and one of dip after noon, on the 
second day ; and the same again on the third day ; making four com- 
plete sets of observations, the mean of which has been taken for the 
month's record. 
The instrument used in determining the horizontal force is the 
unifilar, which is in two parts. Por the vibrations a mahogany box, 
with a suspension tube, is attached to the deflection apparatus. A 
tubular magnet, 10 cms. long by 1 cm. in diameter, of hardened steel, 
in one end of which is fitted a lens, and at the other end, which is at 
the chief focus of this lens, a plain glass engraved with a scale. This 
scale is observed by the aid of a small telescope ; and the time of the 
passage of its centre across the wire of the telescope is observed by 
counting the beats of a chronometer. The magnet is suspended by 
two fibres of unspun silk from the top of the glass tube, about 30 cms. 
in length, and made to vibrate in the horizontal plane on either side 
of its position of rest. The square of the number of vibrations in a 
given time is a measure of the intensity of the horizontal component 
of the magnetic force of the Earth, a centimetre of space, a gramme of 
mass, and a second of time, being the units employed. 
The vibrating magnet is dismounted from its suspension, and with 
the wooden box in which it is swung is removed from the apparatus. 
The instrument is fitted with a horizontal circle capable of rotation 
round its axis, which also carries a copper frame, acting as a damper 
when the magnet to be deflected is set in motion, and tends to bring 
it to rest. This magnet is also a hollow cylinder, but of smaller dimen- 
sions than the one previously described, and is provided with a mirror 
placed vertical and normal to its axis. A telescope is screwed to the 
revolving base, and fitted with an ivory scale, the divisions of which 
are seen as reflected in the mirror of the magnet, and read by the tele- 
scope. A brass bar is fixed, its centre coinciding with the centre of 
the instrument, divided to centimetres, upon which the magnet pre- 
viously vibrated is placed, first at 30 cms. on either side, and then at 
