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Appendix.] 



TERRESTRIAL MAGKETISM. 



41 



pendent on the direction of her head at the time of observation. The 

 mode of obtaining these corrections will be subsequently explained. 



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B. — Intensity. 



3. W//^ Weights,— The 5n=?trunieTit being on the gimball tabic and 

 levelled, the plane of the circle parallel to the magnetic ir.eridian, with 

 its face to the East, and the needle shewing the magnetic dip, place the 

 silk carrying the hooks on the grooved wheel; attach one of the constant 

 weights to one of the hooks, and take three readings of the division of 

 the limb at which the needle is in equilibrium, using the precautions 

 already directed of altering the points of support of the axle between 

 each observation, and rubbing with the ivory disk whilst reading off. 



If the needle is deflected past the vertical or horizontal, read and enter 

 the angles as already directed under the head of Inclination. 



Change the weight to the other hook, when the needle will be deflected 

 to the opposite side of the apparent dip tr> what it was before, and take 

 three more observations. Half the difference of the mean ;f the arcs 

 with the weight on either hook is the angle of deflection due to the con« 

 stant weight employed; or half their sum, if one of +he arcs was past the 

 horizontal and lias therefore the negative sign prefixed, 



4. With Deflectors, — The instrument being adjusted as already de- 

 scribed (and without using the hooks, which are only designed for tlie 

 observations in which the weights are used), adjust the circle at the back 

 by means of its verniers to the apparent dip, so that the deflectors, when 

 screwed in, may coincide with the line of the dip; the needle will then 

 be repelled to one side; make three observations of the division to which 

 the needle is thus deflected, observing the usual precautions of moving 



the bracket at the back, reading both ends of the needle, and rubbing 

 with the ivory disk. 



Move the needle past the deflector to the other side of the dip by means 

 of the bracket, and take three more observations: if the needle is de- 

 fleeted past the vertical or horizontal, read and enter the angles in the 

 manner already described ; half the difference of the arcs on either side 

 of the apparent dip, or half their sum if one be past the horizontal and 

 have the negative sign, will be the angle of deflection produced by the 

 deflector. Instead of the deflectors a second needle may be used as a 

 deflector, either with the end of the needle-case marked N (containing 

 the north pole of the needle) screwed into the arm marked N, or the end 

 marked S screwed into the arm marked S. 



The thermometer attached to the circle m.ust be observed at the com- 

 mencement and close of the observations of intensity, whether with 

 deflectors or weights. 



^^ 



