On Magnetic Variations. 27 



approximate result. Rowe, in 1822, made it 9° 6' east, but the 

 three determinations made by this observer during the same visit 

 to Sydney differed nearly 2°, or from 8° 40' to upwards of 10°. 



Brevs^ster, in 1822, during the month of October, made the 

 variation 8° 47' 48". Up to this date the time of day when the 

 observations were taken is not stated by any of the observers, 

 there is, therefore, an uncertainty amounting to several minutes 

 of arc in all the results. 



In 1823, Mr. Eumker, at Parramatta, made the variation at 

 noon in April, 8° 47' 41" east, which corrected to Sydney is 

 8° 51' 41", this result was obtained with a small magnetic transit 

 by Dollond. The transit could be converted at will to a telescope 

 for observing the sun on the meridian, or a microscope to observe 

 the end of the magnet. The supports of the transit turned on an 

 azimuth circle, and the true meridian was obtained by making 

 the wire bisect the sun at the instant of meridian passage, noting 

 the reading of the azimuth circle. Changing the instrument to a 

 microscope and observing the magnet, a second reading, giving 

 the magnetic meridian, and the difference by taking one from the 

 other, the variation. In 1823, probably with the same instru- 

 ment, Sir Thomas Brisbane made it 8° 45' in Sydney ; the date is 

 not given. 



Brewster, 1824, during January and February, made it 8° 56' 

 ■east. Dunlop, astronomer at Parramatta, at noon in April, made 

 it, when corrected, to Sydney, 9° 8' 4" east for 1825, and 9° 21' 7" 

 ■east for September, 1832. In 1841, Sir J. C. Eoss made the 

 variation on Grarden Island (Sydney Harbour), 9° 57' 19" east 

 (vol. 2 page 41), being a mean of results obtained from 21st to 

 the 28th July that year ; there is strong probability that this is a 

 misprint for 9° 27' 19", for two reasons, first, because the first 

 result does not agree with any other results ; and, second, 

 because a few pages further on he gives the variation as 9° 42' east 

 at 30° 52' south latitude, and 154° 8' east longitude — that is 3 

 degrees east of Sydney less than at Sydney— while the variation 

 increases to the eastward of Sydney ; assuming 9° 27' 19" to be 

 the correct result, it agrees very well with those of Dunlop and 

 Admiral King. 



Blackwood, in February, 1844, made it 9° 25' east. Admiral 

 King, at Parramatta, using the same instrument that Dunlop 

 and Erumker had used, made it, when corrected to Sydney, 

 9° 42' 52" east at noon in April, 1851, and 9* 47' 56" noon in 

 April, 1851. Captain Denham, in 1857, made it 10°, in 1858 the 

 same; and the ship Novara,hi 1859, 9° 59' 54", in December. 

 Government Astronomer, 1864, 9° 49' 4" East 



1866, 9° 42' 54" „ 



1867, 9° 40' 40" „ 



1870, 9° 36' 36" „ 



1871, 9° 35' 0" „ February. 



