On Magnetic Variations. 25 



Art. II . — On Magnetic Variations in New South 

 Wales, by H. C. Russell^ Esq., B.A. 



Eead July 12, 1871. 



Eeom the foundation of the colony up to 186J;, observations 

 on the variations of the magnet at Sydney had generally shown 

 a gradual increase of easterly variation ; it then appeared that 

 the maximum had been reached, and westerly motion had begun. 

 Subsequent investigation has shown that such is the case, and 

 that the westerly motion is at about the same rate as the easterly 

 motion had been. 



I purpose stating briefly our present knowledge on this sub- 

 ject, and putting the information already obtained in a form 

 easily accessible, so that all who are interested in the magnetic 

 variations at any particular time may be able at once to see if 

 there are any records of the period, and what is the probable 

 value to be given to them ; and if none are recorded, then to 

 find from the general curve the probable variation. It will of 

 course be understood that the curve only gives the probable 

 direction, and was formed by drawing lines from the several 

 observed points after they had been marked on the diagram. 



Erom what is to be shown presently, it will appear that the 

 common practice of taking the average annual variation between 

 two periods, and thence calculating the variation at another 

 period, is by no means a safe one ; and to assume that because 

 the variation at Sydney has changed from an easterly to a westerly 

 motion, therefore the same change has taken place all over Aus- 

 tralia, would be contrary to experience in other parts of the 

 world. 



Erom the observations which have been made at London and 

 Paris, since 1580, it appears that although the variation at the 

 two places was decreasing, and the magnetic coincided with the 

 true meridian at London in 1657, the same coincidence did not 

 take place at Paris until 1669, or 12 years later. A similar 

 difference is noticeable here, for while the easterly increase 

 ceased in Sydney in 1858-9, it did not cease at Melbourne until 

 1867 (8 years.) 



It has also been shown that in Europe the variation may alter 

 as much as 18' in one year, and at other times alter only 46' in 

 35 years, or li' per annum ; at London the annual change of 



