242 Mr. Michael Donovan on certain Improvements in 



coincide with it. The resuhants will represent the intensity of the directive 

 forces of the compound needle, and in reasoning on the subject may be substi- 

 tuted for them. In proportion as the bars of the needle are opened, and the 

 vertical angles which include the magnetic meridian are enlarged in conse- 

 quence, the resultants or directive forces diminish in energy, until at length 

 the bars are entirely opened and lie with their dissimilar poles contiguous, 

 exactly iu the same vertical plane : tlie resultants then vanish ; there is no 

 longer any dii-ective force ; and consequently the needle will remain in any 

 position. 



This state of indifference depends on the condition laid down, that the four 

 poles possess exactly the same intensity and distribution of magnetism, and that 

 the bars lie precisely in the same vertical plane with the synonymous poles con- 

 tiguous to each other. If this position of the bars be ever so little disturbed, 

 by turning one of them on the common axis away from the other, even to the 

 amount of one or two degrees, the resultants begin to exert their influence, 

 weakly, it is true, but suiBciently to cause the compound needle, previously indif- 

 ferent, to take up a position at right angles with the magnetic meridian, because 

 the resultants coincide with it. In that position the needle will permanently 

 remain, and if disturbed, will, after a few oscillations, return to it. Thus the 

 apparently anomalous phenomenon, which surprised me because it had never 

 been previously observed, is very easily understood. 



If instead of equal distribution of magnetism, the power of one bar of the 

 compound needle exceed that of the other, the needle, instead of becoming in- 

 different when both bars are brought into the same vertical plane, will obey the 

 predominant power of the stronger bar, and pass at once into the magnetic me- 

 ridian. Indeed the tendency to do so may be exhibited before the two bars are 

 brought into the same vertical plane ; the resultants, still feebly in operation, 

 may more or less antagonize the predominant power of the stronger bar ; a ba- 

 lance of the two forces will take place ; the compound needle will take up a 

 position nearer to or farther from the magnetic meridian, according to the de- 

 gree of resistance which the resultants offer : but if the bars be brought into 

 the same vertical plane, the resultants will be eventually overpowered. 



In order to bring the compound needle to a right angle with the magnetic 

 meridian, the deviation of its bars from the vertical plane must be more or less 



