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Method of adjusting the Dipping Needle. 277 



Art, XIIL — Method of adjusting the Dipping Needle; by 

 Thomas H. Perry, Professor of Mathematics United States 

 Navy. 



TO THE EDITORS. 



Gentlemen^ — Finding it necessary, some months since; to re- 

 adjust a needle belonging to my instrument for measuring the 

 magnetic dip^ — I adopted a method which, from its simplicityj I 

 am induced to communicatej in the hope that it may be service- 

 able to others in similar circumstances. 



The instrument being firmly fixed, and accurately leveled, the 

 direction of the magnetic parallel of latitude, and meridian, and the 

 true dip, were approximately ascertained by properly reversing its 

 faces, axis, and poles. The plane of its face was then made to 

 coincide with the parallel of magnetic latitude, and the substance 

 of the needle carefully ground away, ^rom the sides perpendicu- 

 lar to its plane of motion, until it assumed the same position 

 (the vertical) upon reversing its axis. The plane of the face was 

 then brought into the magnetic meridian, and the needle again 

 ground upon the sides parallel to the plane of motion so as not to 

 affect the previous adjustment much, until it indicated nearly the 

 true dip- These processes were successively repeated, until the 

 errors, saving such as result from the imperfection of the circles, 

 were found, upon making all possible reversions, to be less than 

 the probable errors of observation. 



This method may be advantageously employed in the final ad- 

 justment of new needles. I have employed it successfully in 

 one instance. Two small screws at right angles with each other, 

 might also be added, which would render grinding unnecessary ; 

 but their weight would prove some incumbrance, and they would 

 increase the liability of the adjustments to derangement. 



The value of the process results from the difficulty of render- 



pered 



Its 



correctness of principle is obvious from the impossibility of cor- 

 rect indications in two different positions of the needle, except 



when the centre of gravity coinciding with the axis of motion. 



the influence of this force becomes nothing in all cases. 



be 



In making these adjustments, it is better that the magnetism 



