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knowledge of the cause of such disturbances for its safe applica- 

 tion is as essential as that of the principle itself; and still more 

 so, as it will protect its possessor from being misled by inven- 

 tions which pretend to neutralize the effects of local attraction. 

 The law of magnetic action will show what is within the limits 

 of possibility and what is not ; and that the less the natural cur- 

 rent is disturbed, the more correct must be the indication of the 

 needle. 



We are to consider magnetism as a stream of subtle fluid con- 

 stantly moving from south to north ; should it be disturbed from 

 its natural course, nothing short of the removal of such disturb- 

 ances can give its true direction. The author made several ex- 

 periments on this subject during his voyage from South America 

 to England ; therefore he speaks from experience, as well as from 

 deductions founded on experiments. The most important points 

 to be attended to are the following : to make the needles in the 

 best form for retaining their magnetism, and in such a manner 

 as to allow the currents to enter and issue out at two points dia- 

 metrically opposite, and that the current may pass through with- 

 out being interrupted by a dissimilar metal ; and to place such 

 needles in situations sufficiently distant from local disturbances. 



Variations. 



The charts of magnetic curves of variation are drawn at pre- 

 sent in a manner that tends to mislead persons. Instead of lay- 

 ing down the direction of the needles as they would naturally 

 appear when left to the sole control of the magnetic stream in 

 their undulating meridional lines, they are marked in the most 

 confused manner with irregular inflections of curves, indicating 

 a total w T ant of symmetry, and thus complicating the question 

 of variation by lines which have no necessary connexion, pro- 

 ductive of no advantage, but of much real injury to the problem 

 itself. On reference to a chart, it will be observed that it con- 

 tains two distinct lines, which are called lines of no variation ; 

 the western passing in a north-west direction from the South At- 

 lantic Ocean to a point called the magnetic pole, round which a 

 series of lines are drawn, having the appearance of a general con- 

 vergence towards it, which on a superficial inspection leads to 

 the idea that they represent the actual direction of the horizontal 



