Observations on the Direction and Intensity of the Terrestrial 
Magnetic Force in Ireland , made by the Rev. Humphrey 
Lloyd, 31.A ., F.R.S ., Sfc., Professor of Natural Philoso¬ 
phy in the University of Dublin ; by Captain Edward Sa¬ 
bine, R.A ., F.R.S ., fyc.; and by Captain James Clarke 
Ross, R.N , F.R.S., fyc. 
[With a Plate.] 
The observations which form the subject of the present com- 
munication were made during the years 1834 and 1835, in com¬ 
pliance with the recommendation of the British Association urged 
in the first and second Reports of its proceedings. Their main 
object has been to determine the direction of the lines of mag¬ 
netic dip and intensity in Ireland, and to make a small, but it 
was hoped exact, addition to our knowledge of the laws of dis¬ 
tribution of the earth’s magnetism. The observations are three¬ 
fold : first, observations of the horizontal part of the earth’s 
magnetic force, as determined by the time of vibration of a 
needle suspended horizontally, after the method of Professor 
Hansteen; secondly, observations of dip, made in the usual 
manner; and thirdly, observations of dip and intensity at the 
same time, and with the same instrument, according to the 
method adopted by Professor Lloyd, and already submitted to 
the Association # . 
I. Horizontal Intensity. 
The instruments employed in the first series of observations 
were constructed after the model of that of Professor Hansteen. 
The needles are cylinders 2\ inches long, and *13 of an inch in 
diameter, suspended by a few filaments of the silkworm’s thread. 
They are inclosed in a small rectangular box, supported upon 
levelling screws, and having a tubular pillar screwed on at top 
for the silk suspension. At the bottom of the box is a divided 
circle,for the purpose of noting the arc of vibration: the tempe¬ 
rature is observed by means of a small thermometer inclosed 
in such a manner as to avoid contact with the bottom and sides 
of the box. Before the commencement of the observations, the 
bottom of the box is to be rendered truly horizontal by means 
of the levelling screws on which it rests, and of a small spirit 
level with wdiich it is furnished. The needle being then sus- 
* Fourth Report, p. 557. Transactions of the Royal Irish Academy, vol, xviL 
