Fisher on the Variation of the Compass. 99 



disturbing force would act nearly perpendicularly to the plane 

 of vibration of the needle. 



On the Diurnal Motion of the Needle at Spitzhergen, 

 August, 1818. 



The observations here given were made with an excellent 

 variation transit, made for the purpose by Dollond. The transit 

 had a motion in azimuth, by which the telescope of the instru- 

 ment having a microscopic cap attached to the object glass, 

 might be brought parallel to the needle which was fixed on a 

 centre under the axis of the instrument in a glass cover, by this 

 means the slightest motion in the needle could be observed, and 

 the variation read off with considerable accuracy on the azimuth 

 circle. The observations were made on a small island on the 

 north-west coast of Spitzbergen, in lat. 79°.40'.20",6 N., and 

 long. 11°.7' E. The transit was firmly fixed on a foundation of 

 solid rock, on a spot where a tent had been previously erected 

 for the purpose. On the 12th of August, having by observation 

 put the instrument into the meridian, the variation was observed, 

 and observations continued at equal intervals of about four hours, 

 both day and night, during our stay on the island. The needle, 

 although of the most delicate construction, was yet so extremely 

 sluggish as often to differ four or five minutes in the readings 

 when taken immediately after one another. Each of the fol- 

 lowing observations is, therefore, a mean between five distinct 

 observations, taken immediately after one another, the needle 

 being put gently in motion between each observation, and its 

 motion accelerated by slightly tapping the glass cover. 



The observations were taken alternately by myself and Mr. 

 Charles Palmer, midshipman of the Dorothea, to whom for his 

 skill and accuracy in observations of this kind I was particularly 

 indebted. 



H 2 



