70 



MK BROUN ON THE BALANCE MAGNETOMETER, 



commencing with a large arc, tlian if it belong to a series commencing with a 



small one. 



These conclusions I had arrived at nearly two years ago, and accordingly 

 only small arcs Avere used in determining the time of vibration, seldom above 5'.0 

 commencing. 



The following series of observations was made in January 1844, before re- 

 moving the needle for the purpose of determining its temperature correction. 

 Many other series made previously give the same result ; but the following will 

 be sufficient to prove the facts stated above. 



Table II. 



Obsei-vations for the Time of Vibration of the Balance Needle in the Vertical 



Plane, for different Arcs. 



The semi-arcs were observed by my assistant Mr Welsh, at one microscope, 

 while the times of each vibration were observed by myself at the other. 



It is not my intention, in the present communication, to enter into any 

 examination of the i auses of these peculiarities ; my object is simply to point 

 them out as sources of error. I shall therefore merely state my conclusions, with 

 their evidences. 



6. 4th, The time of vibration in a vertical plane depends, to a considerable 

 extent, on the temperature of the needle. 



