242 



in the subsequent observations of Gilpin and Bowditch, it has, 

 nevertheless, escaped the attention of more recent observers. 

 There is but a faint indication of its existence in the Gottin- 

 gen observations, which were made at the hours of 8 a. m. and 

 1 P.M. ; and Professor Gauss finds, in the mean results de- 

 duced from these hours, no " important fluctuation dependent 

 on season." A similar negative result is deduced by Dr. 

 Lamont from the Munich observations, which were made 

 twelve times in the day. 



It will be easily understood, that the determination of 

 the annual variation is much more difficult than that of the 

 diurnal change ; both on account of the much smaller fre- 

 quency of the period itself, and the difficulty of preserving 

 the instrument in the same unchanged condition during the 

 much longer time, or of determining and allowing for its 

 changes when they do occur. The Dublin observations 

 appear to possess a peculiar value for this determination. 

 Since the spring of 1841, the magnet of the declinometer has 

 remained absolutely untouched ; and the suspension thread 

 (which elsewhere has frequently broken) has continued per- 

 fect since the instrument was first mounted. 



The course of the annual variation in Dublin, as deduced 

 from the results of the years 1841-4, is as follows: 



1. During the first three months of the year, the mean 

 daily declination is nearly constant. 



2. In the month of April the declination begins to in- 

 crease; and it continues to increase until the beginning of 

 August, when it attains its maximum. 



3. From the beginning of August it decreases ; and the 

 decrease continues, with rapidity, until the end of the year, 

 when the declination is about five minutes less than at the 

 commencement. 



4. The increase, from the beginning of April to the be- 

 ginning of August, is four minutes. The decrease, from the 

 beginning of August to the beginning of January, is nine mi- 



