MAGNETIC OBSERVATIONS. 



203 



of the table give the place where, and the date when, each observation was made. 

 The third and fourth columns give respectively the observed deflections when the 

 north ends of the deflecting magnets were directed towards the west and towards 

 the cast; the distance of their centres from the centre of the compass needle being 

 in both cases eleven inches. The fifth column gives the mean of the four observed 

 deflections recorded in the third and fourth columns. The sixth, seventh, and 

 eighth columns contain, in precisely the same manner, the observed deflections, and 

 their mean, when the centres of the deflecting magnets were at a distance of fifteen 

 inches from the centre of the compass needle. Now, let r be the distance, 

 expressed in feet, between the centres of the deflecting magnets and the centre 

 of the compass needle ; u, the observed angle of deflection given for each value of 

 r in the column headed "mean"; 777, the combined magnetic moment of the two 

 deflecting magnets; and //, the earth's horizontal force at the place where the 

 observation was made, taken from the table on page 61. Then we shall have 



771 



jr tan u = -=. 

 and the ninth column contains the mean of the two values of log. computed 



respectively from the angles of deflection observed with r= 11 inches =0.917 

 foot, and r = 15 inches = 1.250 feet. The tenth column contains the value of 



log. m, found by adding to log. the known value of log. H. 



Jl 



* In this observation n=i2 inches. 



