MAGNETICAL OBSERVATIONS IN IRELAND. 
137 
II. Dip and Intensity . 
All the observations with dipping needles are comprised in the 
two Tables which follow. The first (Tab. III.) contains the 
results obtained with needles of the ordinary construction, and 
used exclusively for the determination of the dip. In the first, 
second, and third columns are given the place , day of the month, 
and hour of observation. The fourth column contains the observed 
inclination (the mean of the usual 8 readings) when the marked 
end of the needle is a north pole ; the fifth contains the similar 
result of observation with the poles reversed and the sixth is 
the mean of these angles, or the resulting dip. The needles em¬ 
ployed are Needle L(l) constructed by Robinson, and NeedleS(l) 
made by Dollond; the latter of these is 11| inches in length, the 
former 4-J- inches. 
Table IV. contains the observations made for the purpose of 
determining the dip and intensity at the same time; the latter ele¬ 
ment being deduced from the direction in which the needle rests 
under the combined influence of magnetism and gravity, while 
the former is inferred from the position assumed under the in¬ 
fluence of the earth’s magnetism alone. Each of these angles of 
direction is deduced from the usual eight readings, all the re¬ 
versals being made just as in the ordinary mode of observing the 
dip, the reversal of the poles of the needle excepted. These 
angles are given in the fifth and sixth columns of the table; £ 
is the angle which the needle makes with the horizon when un¬ 
loaded, and 0 the inclination when a small weight is attached 
to the southern arm at a fixed distance from the centre. The 
temperature is noted at the commencement and end of each ob¬ 
servation, with the view of correcting the value of the force ; and 
the mean temperature is set down in the fourth column of the 
table. The needles employed in these observations are of the 
same dimensions as those used for the determination of the dip 
alone, and are adapted to the same divided circles. Three small 
holes are drilled close to each other on each arm, at a distance 
from the centre about two thirds of its length; and much care 
has been bestowed to make them coincide accurately with the 
axis of form of the needle. The weight is a small cylinder of 
brass, which is inserted in one of the holes on the southern arm, 
the diameter of the cylinder corresponding accurately to that of 
the hole. This weight is so adjusted as to bring the needle into 
a position nearly at right angles to the line of the dip, that being 
the position in which the resulting value of the force will be least 
affected by the friction of the axle on its supports. 
