44 



TERREcjTRIAL MAGNETISM. 



i^Sect IL 



enu. 



and read its vernier, which will be face East, SotUh end; raise the Ys 

 ^ud lower gently ; repeat the reading?;, comixiencing now with the South 



The mean of the four readings iis the incliuation with the face 

 ±.ast^ poles direct, or B,. 



3- Turn the vertical circle 180^ in azimuth, and repeat the process 

 in No. 2, taking again the mean of the four readings./^rhich will be 

 180' — ^^; 0.^ being the inclination -with the fece West, poles direct, 



4. The poles of the needle must now be reversed by means of the bar 



rnagnetb. 



by the following process : — Take the needle off the agates, 

 Luiding it by the end which in the preceding observations was a South 

 pole, arid which is now- to be converted into a North pole; place it with 

 the flat Side (which is lettered) uppermost in the wooden frame dcsi--^ned 

 to prevent any injury occurring to the axle, being careful that the end to 

 be made a North pole is placed towards that part of the v. uoden frame which 

 is marked accordingly ; secure the needle by the brass centre-piece, and 

 place the frame \vith one end towards the I'ight hand and the other 

 towards tb( left, and let a;; assistant keep it in that position. Now take 

 tbn bar-magnets, one in each hand, and let the North pole of the bar- 

 magnet be lowermost in the hand which is towards the end of ^i\< ^'^ame 

 in wliich that end of the needle is placed which is to be made a Soutk 

 pole; and let th^ South pole of the bar-magnet in the other hnnd be 

 lowermost. Draw^ the magnet about ten limes along the flat side of the 

 needle : the North pole of one bar-magnet being drawn along the end of 

 the npedlt; which is to be made a South pole ; and the South pole of the 

 Other bar-magnet being drawn along the end of the needle which is to be 

 made a North pole. The needle must then be turned over in the wooden 

 frame, so that its other flat side may become uppermost, which must also 

 be rubbed by the m .^nets ten times in the manner already described. 



The bar-magnets should be held one in eacls hand, nearly in a vertical 

 position, the lower ends resting on the needle ; and must be drawn al 



o 



the grooves in the wooden frame from near the centre to beyond the 



ends of the needle. When the process thus described has been guue 

 through, it w^ill be found, on replacing the needle on the agates, that the 

 end which previously dipped below^ the horizontal line is now inclined 

 hove it. 



a 



5. The observations in Nos= 2 and 3 must now be repeated, which will 

 S^ive two other mean readings with the face of the circle first East and 



thei 



W'--- These readings will be B^ (the inclination with the face 

 East^ poles reversed) ; and 180^-04; B^ being the inclination with the 

 face Wcsty poles rever^^ed^ Then 



6y f ^2 + ^S + ^4 ^.^ 



4 



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ihQ- true magnetic inclination at the place of observation. 



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6. '^^' ' 



' - results ' 



repeat the o 



7. On 

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ifht: 



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direct: ai3^ 



Dr. Lloyd 



for irteasiu"- 



approac' 



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 in winch thp 



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