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40 



TERRESTRIAL MAGNETISM 



[Sect. II, 



division nearest to the needle to coincide with the corresponding division 

 of the second graduated circle iminediately behind it, by which means 



parallax is avoided. 



The mean of the three observations or six readings above described is 

 the apparent inclination by direct observation with the facf East. 



2. Observation -with Deflectors, — Having made the preceding observa- 

 tion, screw in the deflector N (or the north pole of a second needle 

 used as a deflector), and adjust the circle at the back by means of its 

 verniers, so that the deflector may be 40^ on one side of the division which 

 in the preceding procesb (§ 1.) was read off as the direct observation 

 with the face East. The needle will then be repelled, and will settle on 

 the opposite side of the dip ; read off (always whilst rubbing with tlic 

 ivory disk) the divisions indicated by the two ends of the needle. Re- 

 peat tho observation three times, altering the beariiigs of the ends of the 

 axle between each observation as before directed. Turn the back circle 

 through SO^^, so that the deflector may Be 40° on the other side of the 

 apparent dip. Move the needle ])y the bi'acket, so that it may be de* 

 fleeted on the opposite bide of the apparent dip to what it was before, 

 and make three observations. The mean of the six observations or 

 twelve readings is the apparent inclination with a deflector, face East, 



Instead of placing the deflector -m 40^, another angle, as 45° or 50°, 

 may be taken ; or a second angle may be used for the purpose of varying 

 the observations when it may be desired to repeat them ; the only essential 

 point being, that the angle at which the deflector is placed should be the 

 same on each side the apparent dip. 



' Instead of deflector N (or the North pole of a second needle used as 

 a deflector), deflector S (or the South pole of the second needle) may be 

 screwed into the opposite point of the back circle, and six observations 

 taken with it will give as before the apparent inclination with a deflector, 

 face East. 



In writing down the obsei'vations the following directions must be 

 attended to; if the needle be deflected past the vertical the division cf 

 the limb should be read off according to the graduation and noted 

 accordingly, but the mean of the readings must be taken from 180% in 

 order to give the true arc corresponding to the position of the needle: if 

 it be deflected past the horizontal, the readings must be entered as 

 marked on the limb, but with the negative sign preflxed, in which case 

 the mean result will be half the difference of the means of the negative 



and positive readings. 



The apparent inclination obtained as above directed, whet'^ r by the 

 direct method or with deflectors, requires two corrections to give the 

 true inclination, viz.— 1'^ the index-correction of the particular needle 

 employed, and 2° a correction for the influence of the ship's iron de- 



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