TERRESTRIAL MAGNETIC FORCE IN IRELAND. 171 



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 temperature 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 which it 

 is furnished. The needle being then suspended so as to hang 

 near the bottom, its deviation, if any, from the horizontal position 

 will be detected by its inclination to the surface. It is then to be 

 slightly moved to one side or other in the brass stirrup by which 

 it is supported, until it hangs truly parallel to the lower surface of 

 the box ; and when this adjustment is once accurately made, no 

 further alteration will be required, unless the change of dip be 

 considerable. 



When an observation is to be made, the needle is raised or 

 lowered by a small roller to which the silk suspension is attached, 

 so that it may hang about midway between the upper and lower 

 surfaces of the box. It is then drawn aside from the magnetic 

 meridian through an arc of 25 or 30, by a piece of brass wire 

 inserted in the side of the box, and is allowed to oscillate. The 

 registry of the oscillations is commenced when the amplitude of 

 the vibration on either side of the meridian is reduced to 20, and 

 it is continued during 360 vibrations ; 'the moment of the comple- 

 tion of every 10th vibration during that interval being noted by a 

 chronometer. The amplitude of the final arc, or of the arc of the 

 360th vibration, is also observed ; and the temperature of the air 

 in the box, as indicated by the interior thermometer, is noted at 

 the beginning and end of the observation. 



It is obvious that in this manner seven intervals of time are 

 obtained, each corresponding to 300 vibrations, viz. the interval 

 between the Oth and 300th vibration, between the 10th and 

 310th, &o., and between the 60th and 360th ; and the mean of 

 these is taken as the result. But to this result several corrections 

 must be applied. 



1. The time as shown by the chronometer is to be corrected for 

 rate; and accordingly the chronometer's rate must be determined 

 from time to time by comparison with a good timekeeper, or by 



