.] 



t-:* 



42 



TERRESTiUAL MAG^^ETISM, 



[Sect II. 



A convenient routine of the observations at sea may be stated as 



er- 



follo^vs : 



1. Take three observations of the apparent dip by the direct ob^ 

 vatlon, 



2. Screw in the deflectors N and S, and adjust the back circle to the 

 dip. Make three observations of the angle of deflection produced on either 

 side of the apparent dip ; this furnisher one result for the intensity of 

 the magnetic force. 



3. Repeat No. 2 -^ith a second needle used as deflector N, ^vhich will 

 ;ive a second result for the intensity of the force. 



4. Repeat No. 2 with the second needle used as deflector S, which will 

 give a third result for the intensity of the force. 



5. Renjove the deflector and repeat No. Ij which will give a second 

 result for the apparent inclination. 



On days when the weather permits, observe the intensity also by the 

 constant v^eights. 



111.— Observations on Shore. 



1. The instrument being adjusted with the plane of the circle coinciding 

 with the n-agnetic meridian^ and the face East, make a complete series 

 of observations of the Inclination with and without deflectors, and of 

 the Intensity with the deflectors and weights, similar in all respects to 

 the observations which have been or which are intended to be made at 

 sea ; the needle, deflectors, and weights to be those employed, or to be 

 cirnployed, in the sea-observations. 



unfurnished with a separate apparatus for determining the true 

 inclination, substitute in Mr, Fox's apparatus the needle which admits of 



2. 





its poles being reversed (viz. that needle which is not intended to be used 

 in observations of intensity), and obtain the true inclination from the 

 mean of the angles read in four different positions of the instrument, viz. 

 with the face of the circle East and West, and repeating the process with 

 the poles reversed, following the directions for reversing the poles given 

 in Appendix 3. The difference between the true inclination thus 

 obtained, and the apparent inclination with the face East observed with 

 the needle used at sea, ascertained at the several shore stations, furnish 

 one of the data from which the index correction to be applied to 

 the observations made at sea ia to be computed. 



3. When Mr. Fox's appar-atus is furnished with more than one needle 

 for the observations of intensity, -ach needle must be successively sub- 

 stituted in the shore observations for the needle used at sea, and ihe 

 inclination as well as the ang-les of deflection with constant weight? 

 observed with at. 



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