170 



normal to these lines. They were performed at intervals from 

 1833 to 1836. The method employed was that of counting the 

 vibrations of an equipoised needle set to vibrate in the plane 

 of the magnetic meridian and dip, in a given time. From 

 these observations corrected for temperature, and calculated, 

 results were obtained which shewed that the magnetic force 

 increased to the northward and westward. 



In 1838, Col. Sabine sent me some needles which, in his 

 researches, had been suspended by untwisted silk fibres, and 

 submitted to horizonal vibration. The rate of this vibration is 

 determined by the magnetic force (I) but not by the whole of 

 it ; it is, in fact, a part of that force which grows less and less 

 as the dip ( S ) augments, and may be represented by I X Cos 

 of the dip. 



By comparative observations with these needles at Tortington 

 (Sussex) and London, by Col. Sabine, and at York and West- 

 bourne Green, near London, by myself, we obtain, after suitable 

 corrections for temperature, &c. 



Total Intensity at York ... 1-0149 

 „ „ London... I'OOOO* 



From a series of vibrations observed by Capt. Sir James Ross, 

 in 1838, the ratio of intensity at York and London was found 

 1-0094. 



The relative intensity of magnetism at York and London 

 was examined by two other courses of experiment. In the 

 statical method of Dr. Lloyd, an originally equipoised needle 

 is magnetized, and afterwards thrown out of equipoise by the 

 addition of a known weight toward one end. By this arrange- 

 ment the needle is made to take up positions under the double 

 influence of gravity and magnetism : and by these positions, 

 compared with that assumed under the influence of magnetism 

 alone, the relative influence of the two becomes known. By 

 removing the small weight the dip is determinable separately. 

 Professor Christie made a different arrangement : by placing a 

 small weight under the axle, the equipoise of the needle was 



* Keports of the British Association for 1838. 



