MAGNETIC FORCE IN IRELAND. 



175 



zontal force are, like those in its direction, not local phenomena, 

 but occur at the same time at places widely separated. To elimi- 

 nate them from our results, therefore, it would suffice to have a 

 regular series of observations made at some fixed station, con- 

 temporaneous with those made at the different stations ; and, if 

 these be not very remote, we may assume that the variation of the 

 observed force at each from its mean amount is the same as that 

 observed at the same time at the fixed station. Unhappily these 

 means of freeing the results from the admixture of what may be 

 called accidental phenomena have not been attended to in the 

 following, or indeed in any similar series of observations, and 

 there is reason to believe that the errors due to this cause are the 

 largest in amount of any by which the present series is affected. 



The amount of these fluctuations, from day to day, may be 

 judged of from the following specimen of a series of observations 

 such as that alluded to, commenced by Captain Sabine in the 

 month of June, 1835. The apparatus in which the needle was 

 vibrated was unmoved during the continuation of the series, and 

 the needle remained permanently suspended. The height of the 

 barometer was noted, as well as the temperature ; the hour of 

 observation was, nearly, 10 A. M. 



Time 0/200 Vibrations of a Standard Needle. 



