130 



I have spoken of the changes that take place with a greater or 

 lesser degree of regularity ; there are others, sometimes very material, 

 that cannot be accounted for and which require' the constant watchful- 

 ness of the observer to detect. The greatest change of this unaccount- 

 able character, that has come under my personal observation, was a 

 deflection of about forty-four minutes in eight or ten minutes of time. 

 This was probably due to an electrical storm, which could not other- 

 wise have been noticed. 



The glass cover of the compass sometimes become charged with 

 electricity, which causes the needle to apparently stick to the glass. 

 This is of rather frequent occurrence. Wetting the glass immediately 

 dispels the electricity. 



Any state of the atmosphere in which electricity is an element, 

 greatly effects the needle ; electricity and magnetism being, it would 

 seem, almost the same ; the power of an electrical motor for mechanical 

 purposes, being dependent on the magnetic force induced in iron by an 

 electric coil surrounding it. 



In many places a purely local attraction causes the needle to swerve 

 from its general course, from five minutes to fourteen degrees, as noticed 

 by myself during the twelve years I was actively engaged in surveying; 

 and instances have been recorded where this local swerving exceeded 

 twenty-five degrees. 



These considerable deflections of the Magnetic Needle in certain 

 localities are doubtless due to large deposits of magnetic substances. 

 In the vicinity of Thetford and Coleraine the iron ore that is dissemin- 

 ated through the serpentine and so-called asbestos, attracts the needle 

 very sensibly. 



Navigators have to contend with another perplexing source of error 

 in compass reading, which is not easily overcome, particularly in these 

 days when iron enters so largely into the construction of ships and that 

 iron so used sometimes effects the needle to a serious extent, and from 

 causes that are not always apparent. 



It is a well known fact that iron, remaining long in one position, 

 sometimes becomes magnetic, and it has been found that portions of 

 iron ships become magnetic. Now, the action of unmagnetised iron, 



