TERRESTRIAL MAGNETISM. 695 



Observations were made for the variation with Gilbert's Azi- 

 muth compass, and also with Kater's compass. 



A great many observations were made in November 1824, and 

 a few in December and January. 



The mean variation was found to be 123° 21' 55" at Port 

 Bowen. 



At Neill's Harbour the mean variation was 118° 48' W. on 

 June 4, 1825. 



Since 1819 the variation had increased at Port Bowen from 

 114° to 123°. 



The maximum variation westerly was observed to occur between 

 the hours of 10 a.m. and 1 p.m. ; and the minimum between 8 p.m. 

 and 2 a.m. ; the quantity being seldom less than 1-|° or 2°, and 

 sometimes amounting to 5, 6, or even 7°. 



In connexion with these observations a regular series of hourly 

 experiments was made on the magnetic intensity with sl suspended 

 needle, which admitted of the intervals of vibration being observed 

 with minute accuracy. There was found to be a diurnal change of 

 intensity, generally showing a regular increase of intensity from 

 the morning to the afternoon, and as regular a decrease from the 

 afternoon to the morning. The changes seemed to depend on the 

 sun and the relative position of the sun and moon. The diurnal 

 maximum Variation occurred at llh. 49m., and the minimum 

 Variation at lOh. Im. p.m. 



[The importance of having needles delicately siispetided in- 

 stead of supported on a point for observing the magnetic dis- 

 turbances is noticed by Parry, and also by McClintock in his ob- 

 servations at Port Kennedy. Corresponding disturbances in the 

 dip were not observed, probably because the dipping needles were 

 too heavy and not delicately suspended. 



For measuring these disturbances the dipping needle may 

 be made very light, and be suspended by two threads attached 

 to two points in a horizontal axis, passing through the centre 

 of gravity of the needle. If the magnetic axis of the needle is 

 at right angles to the axis of suspension, the disturbances will 

 be indicated, even though the centre of gravity should not be 

 accurately on the axis of suspension. The importance of having 

 very light needles is shown by M. Weyprecht in his account of his 

 Arctic scientific work {see p. 709).] 



Observations ivere made hourly for Horizontal Intensity, 



The maximum intensity of the horizontal needle uniformly took 

 place at 7 p.m., but the minimum was not so well marked. 



The needle for estimating the diurnal changes in the horizontal 

 needle was frequently observed to vibrate in very small arcs in its 

 passage to and fro westward from 2 to 6 p.m., and also in its 

 passage eastward. During the same interval the intensity of the 

 horizontal needle was observed to be very changeable, and the 

 action of the suspended needles very irregular. 



Feb. 22, 2. a.m. — Aurora appeared in an arch from north to west 

 by compass, with bright streamers towards the zenith ; the needle 

 was not affected in any way whatever. 



