VARIATIONS OF THE HORIZONTAL FORCE. 



EXPERIMENTS FOR ASCERTAINING DIURNAL CHANGES OF THE HORIZONTAL FORCE. 



Prior to the departure of the Expedition from the United States it was proposed by Prof. 

 A. D. Bache, Superintendent U. S. Coast Survey, that a series of observations be made to ascer- 

 tain how far changes of the horizontal force and declination are synchronous in the two hemi- 

 spheres. Those for the horizontal force were to be made as rapidly as possible, from IS 1 ""- to 19*"- 

 mean time Gottingen, on the first day of each month, unless that day should be Sunday, and 

 then on the second day of the month. 



As the Expedition was not supplied with a portable bifilar magnetometer with which the 

 former changes could be directly observed, they may be inferred from variations of the angle of 

 deflection of the unifilar magnet when the time of vibration of the deflector, its magnetic mo- 

 ment, and the distance of the centres of the magnets, are known. The following observations 

 were made to that end ; and such data necessary for the reduction as are not contained in the 

 foot-notes, will be found under corresponding dates of the experiments for absolute measure in 

 the succeeding pages. 



JCI/T 1, 1850. 



