266 



REPORT — 1861. 



Table VIII.— Conclusions from Table VI. and VII. 



b. Variations of the Total Force determined hy the Statical Method. — This 

 method is described in the ' Manual of Terrestrial Magnetism,' 3rd edition, 

 pp. 27 & 28, section B. The Dip Circle No. 30 was furnished with two addi- 

 tional needles, Nos. 3 and 4, the poles of which were at no time reversed or 

 disturbed. No. 3 was an ordinary ^dipping-needle, and No. 4 a similar needle 

 loaded with a small fixed and constant weight, deflecting it from its natural 

 position in the magnetic direction. The frame carrying the microscopes was 

 fitted to receive and retain No. 4 securely in a constant position when used to 

 deflect No. 3. 



The experiment consists of two processes : the first being the observation 

 of the position of equilibrium of No. 3 between the action of the earth's 

 magnetism and that of No. 4 used as a deflector ; the north pole of No. 4 

 being directed alternately towards the (magnetic) north and south : and the 

 second process being the observation of the position of equilibrium of No. 4 

 between the action of the earth's magnetism and that of the small fixed and 

 constant weight with whicli it is loaded. 



By the first process we obtain the inclination to the horizon of No. 3 when 

 deflected by No. 4 =Mi= half the difference between the readings (in a single 

 position of the circle and needle) with the north pole of No. 4 directed alter- 

 nately north and south ; and by the second process we obtain the inclination 

 to the horizon of the loaded needle observed in the four positions of the 

 circle and needle =-q. Then d—rj=u is the deviation of the loaded needle 

 from the position due to the earth's magnetism alone, being the mean dip 

 observed with needles 1 and 2 at the same time and place. 



We have then the following expression for the total force (f) at each 

 station, 



* 



=a/ 



cos jy 



sm u sm u. 



