EARTH'S LOCAL HORIZONTAL MAGNET POWER FOUND. 71 

 Multiplying together the two equations 



E 1 47T 8 



and extracting the square root, 



*? I M - 

 T\' k' 



a result which depends upon no property whatever of 

 the magnets employed, except M the moment of inertia 

 of A. To the determination of the numerical value 

 of M we will now give our attention. 



If, as is usually the case with large magnets, the 

 form of the magnet be a parallelepiped, and its structure 



, ,, , f ,.. .,, , mass in grains 



homogeneous, the value of M will be - x 



\.2i 



{(length in feet) 2 + (breadth in feet) 2 }. This calcula- 

 tion is easily made with accuracy. It is necessary to 

 add the moment of inertia of the stirrup or other hook 

 which supports the magnet and vibrates with it : this 

 in general is a very small quantity, and can be obtained 

 with sufficient accuracy by weighing that apparatus 

 and estimating its radius of gyration. 



If the form of the magnet be not so simple, or if 

 there be any grounds for suspicion of the accuracy of 

 this process, the device proposed by Weber may be 

 adopted. The time of vibration of the magnet having 

 been observed, as above mentioned, the two ends of the 

 magnet are then loaded with brass weights, very care- 

 fully weighed, which rest upon the magnet by sharp 

 points, so that the weights do not partake of the cir- 



