MAGNETIC FORCE IN IRELAND. 



173 



a being a constant coefficient whose value is to be determined 

 experimentally for each needle. 



The following observations were made with the cylinders L (r/), 

 L (b), in order to determine the value of the coefficient a for each. 

 The apparatus being inclosed in a large glass bell, the time of 100 

 vibrations of cylinder L (a), commencing with the arc of 10, was 

 observed at the mean temperature of the room, and when the air 

 of the bell was heated artificially from below, by means of a spirit 

 lamp. The final arc varied between 4 and 5. The observations 

 with cylinder L (b), were made in the bell without the apparatus. 

 In this case no means were taken to observe with any accuracy the 

 arc of vibration ; and in order to reduce as much as possible any 

 error arising from this source, the observations were continued in 

 each instance until the arcs were reduced to the smallest appreci- 

 able, and the mean of the last five intervals of 100 vibrations then 

 taken as the result. The chronometer's rate varied from + 8 '6 to 

 + l s> 4 per diem, and had therefore no appreciable influence on the 

 results. 



Cylinder L (a). Cylinder L (b). 



The constant coefficient sought is to be calculated from the 

 formula 



* A series of observations had been made with this cylinder, in the same manner 

 and on the same days as those with cyl. L (a) ; but the results were unsatuf 

 some of them indicating an increase of force with increased temperature. 1 

 dictory results have been noticed by many observers, and are usually at! 

 disturbing effects of current* of air, determined by inequality of 



