BY PEOFESSOR E. G. HOGG, M.A. 83 



Hobart is given by him in Vol. I. (1842) of the Tas- 

 man /an Jonrnal. 



A long gap is now met with in the magnetic history of 

 Tasmania. No absolute determination of the magnetic 

 elements was again made until the visit of Dr. Neumayer 

 to Hobart, in 1863. This observer determined the mean 

 declination of Hobart in 1863 as 10° 25' 9" E. If the 

 mean rate of increase inferred by Lieut. Kay from his 

 observations had held true over the period between 

 1843 and 1863 the declination should have been 

 10° 22' 53". The dilFerence between the observed 

 and computed values of the declination for 1863 

 may (in part) be accounted for by the fact that the 

 stations selected by Lieut. Kay and Dr. Neumayer were 

 not identical, though both were situate in the Domain. 

 I shall, later on, have occasion to refer to the part played 

 in magnetic work in Tasmania by the volcanic rock — - 

 known as Tasmanian greenstone — which occurs so widely 

 in the S.E. of the Colony, and, in particular, outcrops so 

 much in the Domain. Accepting Dr. Neumayer's result 

 as correct, the increase of declination of Hobart between 

 1843 and 1863 amounts to 31' 50", giving a mean annual 

 increase of 1' 36", as against 1' 27' computed from Lieut. 

 Kay's obsei'vations. The declination in 1881, when the 

 next determination was made, should have been 10° 53' 48"; 

 assuming the previous rate of increase to have been 

 maintained, but the observations made by His Excellency 

 Sir J. H. Lefroy at the station employed by Kay 

 discovered the declination to be only 8° 49' 2" E., a 

 qviantity somewhat more than 2° in defect of the com- 

 puted value. Although the instrument employed by 

 Sir J. Lefroy — a prismatic compass — is not the most 

 delicate instrument for determining the magnetic decli- 

 nation, the difference between the observed and computed 

 values of the declination in 1881 cannot be put down 

 entirely to errors of observation. The explanation is 

 not far to seek : between the dates mentioned the 

 declination must have attained a maximum, and was, in 

 1881, proceeding to a minimum. In the neighbouring 

 Colony of Victoria we know that in the 15 years pre- 

 ceding 1881 the declination decreased at the rate of 

 about 2' })er anmuu, and from what we know of the 

 secular variations elsewhere, it is permissible to assume 



