39 



ON THE MAGNETIC VARIATION OF HOBART. 

 By His Excellency Sir J. H. Lefroy, K.C.M.O., C.B., F.R.S. 



Bead 12th Sept., 1881. 



It is, I believe, about 18 years since there has been deter- 

 mination of the magnetic variation here, at all events 

 published. It may have been, and very probably was, 

 observed by the American Transit of Venus Expedition in 

 1874 ; but, if so, it does not appear to have been published. 

 The most recent authentic observation I can find is that of 

 Dr. Geo. Neumeyer in A^rnl, 1863, as included in his " Results 

 of the Magnetic Survev of the Colony of Victoria."* He made 

 it lOdeg. 25-15min. E.' 



Dr. Neumeyer's station appears to have been in the field to 

 the south of the approach to Government House. He 

 describes the spot as 9*4 chains, or 206'yds., south of the 

 former Observatory, and S. 4deg. 27'76min. E. of it. 



Captain J. H. Kay, R.N., the former director of the 

 Observatory, made it 9deg. 57'2niin. E., reduced to mean 

 scale reading, for the mean epoch January 1, 1843, and inferred 

 the annual rate of increase to be l-46min., at which rate it 

 should have been lOdeg. 2G-8min. E. in April, 1863, and 

 lOdeg. 52"7min. E. in January last. 



I make it a considerably less quantity, and the inference is 

 that it has passed its maximum amount, and is now somewhat 

 rapidly decreasing. The annual rate of change from 1843 to 

 1863 was, in fact, rather more rapid than Captain Kay 

 supposed, being l"9min. 



My observations have been made with a 4in. prismatic 

 compass, the property of Kew Observatory, The limb is 

 divided to degrees on silver. It has one directing magnet, and 

 is a good instrument of the kind, although inferior to the 

 declinometers novv^ carried by scientific travellers. Moreover, 

 I have laboured under the disadvantage of not knowing my 

 time with absolute accuracy. I have been in the habit of 

 setting my watch, a good lever one with compensation balance, 

 by Mr. Abbott's regulator, which is kept to time I am told by 

 transits, and I do not think it has in these observations been 

 many seconds from the truth. I hope, therefore, that while 

 individual results differ more than I could wish, the mean of 

 the whole may be relied upon. 



The following are my results : — They were observed on the 

 site of the Observatory of 1840-1848, which appears to have 

 been judiciously selected as free from local effects. The sand- 



* Mannheim, J. Schneider, 1869, 4to. 



