18 REPORT OF THE SECRETARY. 



quency of vibrations of a magnetic bar suspended by an untwisted 

 thread, so as to be horizontal, gives the horizontal component of the 

 force of the earth, while the vibrations of a similar bar placed in the 

 plane of the dip, and poised horizontally like a scale-beam on two 

 knite-edges, gives the variations in the vertical force. These vibra- 

 tions, however, will be affected not only by the changes in the mag- 

 netism of the earth, but by that in the bar itself ; and as the latter 

 is affected by the temperature of the place, a series of observations 

 and discussions was necessary to ascertain the corrections due to this 

 cause. For this purpose the room was artificially heated and cooled ; 

 but the value of the correction was finally deduced from an investi- 

 gation of the whole series of regular observations compared with the 

 changes of temperature indicated by the hourly register of the ther- 

 mometer. 



The corrections for temperature were afterwards applied to all 

 the observations. The larger disturbances were then separated from 

 the body of the series in the same manner as had been done with 

 regard to the horizontal force, by which means the effect of the 

 monthly and yearty disturbance of the sun is exhibited analytically and 

 graphically. From the results it appears that the number and aggre- 

 gate amount of disturbances were least in 1844 ; that in each year 

 the greatest number of disturbances occurs in March and September, 

 and the least number in June, or, in other'words, the maximum about 

 the equinoxes, and the minimum about the solstices. 



In an appendix to this paper the connexion of the appearance of 

 the aurora borealis with the disturbances of the direction and force 

 of the earth's magnetism is discussed. From the result of this dis- 

 cussion it appears that there is a periodicity of about eleven years in 

 the recurrence of the frequency of the aurora, as well as in that, of 

 the great disturbances of the needle, and that these are coincident 

 with each other and with the appearance of the spots on the sun. 



The eighth part of the series gives the discussion of the daily and 

 yearly variations due to the action of the sun on the vertical compo- 

 nent of the magnetic force. The mean variation of the force is deter- 

 mined for each hour during each month and for the whole year, and 

 also for the summer and the winter separately. These are expressed 

 analytically and graphically, and an examination of the curve shows 

 a principal maximum about 1 p. m., and a principal minimum about 

 9 a. m. There is an indication of a secondary maximum about 2 a.m., 

 and a secondary minimum about 4 a. m. 



