MAGNETIC FORCE IN IRELAND. 191 



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 ex- 

 clusively 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 ob- 

 servation with the poles reversed ; and the sixth is the mean of these 

 angles, or the resulting dip. The needles employed are Needle L (1) 

 constructed by Robinson, and Needle S (1) made by Dollond ; the 

 latter of these is 11 J inches in length, the former 4^ 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 influence 

 of the earth's magnetism alone. Each of these angles of direction 

 is deduced from the usual eight readings, all the reversals being 

 made just as in the ordinary mode of observing the dip, the re- 

 versal 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 unloaded, and the in- 

 clination 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 observation, 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 

 determinaiion 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 neajrly at right angles to the line of the dip, that being 

 the position in which the resulting value of the force will be leasf 

 affected by the friction of the axle on its supports. 



