TRANSACTIONS 



OF THE 



NEW ZEALAND INSTITUTE, 



1873. 



I.— MISCELLANEOUS. 



Art. I. — On the Variation of the Declination of the Magnetic Needle in the 

 Southern Portion of the Middle Island, and Remarks on the Desirahility of 

 Establishing Magnetic Observatories in New Zealand. By A. H. Ross. 

 {Read hefore the Otago Institute, 2Uh April, 1873.] 

 When a magnetized steel bar is placed on a vertical axis throngli its centre 

 of gravity, on wliicli it is free to revolve, the axis being between its poles, it 

 "will oscillate on each side of a certain determinate position, in which, at 

 length, it will come to rest. When in this position a vertical plane passing 

 through the axis and the poles is called the magnetic meridian. This plane 

 generally forms an angle with the plane of the true meridian of the place in 

 which the magnet is situate. This angle is called the declination of the 

 magnet. It is, perhaps, better known to British sailors and others as the 

 variation of the compass. It is to this property of the magnetic needle that 

 I propose to direct attention in this short paper, more particularly, however, 

 to some remarkable variations in declination which occur in different localities 

 in this Province which have come under my notice. 



The declination of the magnetic needle is subject to variations of several 

 kinds — secular variation, annual and diurnal variation, accidental variations 

 or perturbations, and local variations. 



Observations carefully taken in London and Paris, and extending over a 

 period of nearly three hundred years, show that from about the year 1580 the 

 declination was E. of N. in those places, but deci^easing, which it continued to 

 do, until 1657, when the magnetic and terrestrial meridians were coincident, 

 and remained so until 1663. A westerly declination then commenced, and 

 continued increasing, though not regularly, until 1818, when, at London, a 



