TRANSACTIONS OF THE SECTIONS. 23 



On a New Induction Dip- Circle. By John Allan Broun, F.R.S. 



The idea of determining the earth's magnetic intensity by its inducing action on 

 soft iron was employed by Dr. Lloyd for the purpose of obtaining the magnetic in- 

 clination. A soft iron bar being placed vertically, so that the induced magnetism of 

 one end should act on a freely suspended magnet, the deflection thus produced was 

 observed, and considered proportional to the vertical component of the earth's mag- 

 netic intensity ; the bar was then placed horizontal, and, the same end acting, the 

 deflection was observed, which was in the same way considered proportional to the 

 horizontal component : were there no sources of error, the inclination might be 

 determined from these two angles. The iron bars employed always possess or 

 acquire a certain amount of induced magnetism, the effect of which is eliminated by 

 inverting the bar for the different deflections ; there are, however, still two sources 

 of error which remain. The most important is that due to the different actions of the 

 different parts of the bar in the vertical and horizontal positions. If the whole mag- 

 netism were accumulated in one point at the acting end of the bar, this source of 

 error would not have existed ; but as the magnetism is distributed over the whole 

 length, that part whose action is equal on both ends of the suspended magnet when 

 the bar is in the vertical position, becomes greater on one end of the magnet than 

 the other when the bar is in the horizontal position. It was probably for this rea- 

 son that Dr. Lloyd's method has never been put into practice. 



Last year, while observing with Dr. Lamont's theodolite magnetometer, Mr. Broun 

 employed a method for the determination of the absolute magnetic inclination, to 

 which it is believed there can be no objection in low magnetic latitudes, and which, 

 with the modifications proposed, may probably be used in all latitudes. 



In Dr. Lamont's apparatus the variations of magnetic dip from place to place are 

 determined by means of two soft iron bars clamped to a horizontal ring, the ring 

 surrounding a freely suspended magnet, one bar vertically above the ring, the other 

 vertically below it. By a series of observations of the deflections produced by the 

 bars in different positions, inverted and exchanged from side to side, the effect of per- 

 manent magnetism is eliminated, and the deflection due to the earth's force is 

 obtained ; the sine of this angle, multiplied by a constant, gives the dip for each place; 

 the constant, however, requires the aid of the usual dip apparatus for its determina- 

 tion. It is evident, however, that if we can incline the bars moving in the plane of 

 the magnetic meridian till the observed deflection be zero (should there be no per- 

 manent magnetism), and observe the angle through which the bars have been moved 

 from the vertical, this angle will evidently be that of the magnetic inclination, for 

 the bar will have been moved into the direction at right angles to that of the total 

 force. This method, as thus stated, requires the determination of the vertical posi- 

 tion of the bars ; and it is supposed that there is no permanent magnetism : as far as 

 the latter supposition is concerned, the error is eliminated by reversing the bars ; in 

 order to render the determination of the vertical position unnecessary, it is only re- 

 quired to observe the angular inclination of the bars, which (for each position) 

 diminishes the deflection by an amount equal to the mean deflection previously ob- 

 tained. It will be observed that for low latitudes, where the bars are moved little 

 from the vertical, the objection applying to Dr. Lloyd's method exists to so small a 

 degree as to be negligible. 



This method, which Mr. Broun employed in India, is, however, liable to error in 

 high magnetic latitudes ; and the following is proposed for use in all positions. A 

 small magnet, 2 inches long, is suspended by a silk fibre as with the usual declination 

 magnet ; a small mirror attached to the magnet allows the determination of the 

 magnetic meridian by means of a telescope having a prism near the wire at the eye- 

 piece, as in Dr. Lamont's apparatus. When the wire coincides with its image re- 

 flected by the mirror (no disturbing cause being near), the magnet is in the magnetic 

 meridian. A vertical circle in the magnetic meridian parallel to the magnet, and 

 3 inches distant, centre to centre, has a soft iron bar clamped to the alidade, so that 

 the acting pole of the bar is opposite the centre of the circle and the middle of the 

 magnet. The reading of the circle is first obtained for the bar vertical : the ver- 

 tically of the axis of the bar may be determined in different manners ; the best, 



