20 



LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 



By means of the observations in the preceding Table, we obtain the ratio of the 

 horizontal intensity at each station to that at each of the others specified in the Table. 

 The absolute horizontal intensity was nowhere observed, because Captain Belcher 

 was not furnished with an instrument for the purpose, and no such instrument has 

 yet been carried to any of the stations which he visited. For the purpose of express- 

 ing the ratio determined by the observations, we may select any one of the stations 

 as a base-station, and assign an arbitrary value for the horizontal intensity at that 

 station. I have chosen Panama, and have made the horizontal intensity there 

 = 1000, because that is the value which it bears at Panama in M. Gauss's theo- 

 retical map of this element*, and those who may desire it will thus be enabled to 



* Atlas des Erdmagnetismus nach den elementen der theorie entworfen, Plate XI. ; and Taylor's Scientific 

 Memoirs, vol. ii. Plate XXII. 



