Magnetic Needle as observed on Ship-loard. 287 



every alteration in the dip of the needle, in the position of the 

 compass, or in the direction of the ship's head. 



If the intensity of the terrestrial magnetism be not equal in all 

 parts of the globe, then the anomaly in the variation of tlie com- 

 pass will be also liable to change with every alteration in the 

 magnetic influence of the earth. This is a point of such impor- 

 tance, I conceive, in the science of magnetism, that I was very 

 anxious to procure a dipping needle on my last voyage to Green- 

 land, to ascertain whether the magnetism of the earth, by which 

 the dipping needle is influenced, be not greater near the mag- 

 netic pole than it is in England. If it be equal, the oscillations 

 of the same dipping needle would be performed, circumstances 

 as to temperature and " local attraction " being the same, in 

 equal spaces of time in both places ; but if the magnetic power 

 in either place be greater, the oscillations of the needle would 

 there be quicker. The number of vibrations of a horizontal 

 needle, performed in a certain space of time in Greenland, is to 

 the number performed in an equal space of time in England as 

 5 to 6, each longer vibration in England being performed in five 

 seconds, and in Greenland in six. No alteration was observed 

 in the time required for each vibration, whether the temperature 

 was high or low, but I tliink in a low temperature the vibrations 

 performed by the needle before it stopped were fewer. 



5. The anomaly of variation bears a certain proportion to the 

 dip of the needle, being greatest where the dip is greatest, di- 

 minishing as the dip decreases, and disappearing altogether on 

 the magnetic equator. 



Captain Flinders ascertained, that the medium error or ano- 

 maly for 8 points deviati(m of the Investigator's head, on either 

 side of the magnetic meridian, was very nearly -J-^ of the dip, -05 

 the decimal expression of which he considered to be the com- 

 mon multiplier to the dip, for obtaining the radius of error at any 

 situation in the southern hemisphere ; and "053 to be the com- 

 mon multiplier, from England to the magnetic equator. This, how- 

 ever, can only be correct within certain limits, as on the magnetic 

 pole, where the anomaly would probably be equal to the dip, or 

 90% the decimal multiplier would require to be increased to 1-0. 

 Hence it has been suggested, by an officer on board oiie of the 

 vessels now in search of a north- west passage, that in those parts 

 of the globe where the dip is 90', the compass needle would pro- 

 bably always stand N. and S., by the attraction of the ship. Tliis 

 position clearly follows from the inference al)ove, provided the 

 compass be placed near the ship's stern in midships; but if placed 

 as described in inference No. S, the ship's head by the compass 

 on the starboard side of the main deck would always appear to 

 be east, and on the larboard side west. 



G. A 



