

40 



stone here crops out, forming an island surrounded by basalt, 

 whicli, however, does not approach within 100yds., and is then 

 of moderate thiclrness. 



Observations of Magnetic Variation at the Hobarton Observatort 

 OF 1841-8, each the Mean of 5 Sights. 



■ Tliese were observed by Mr. G. A. Lefroy. 



I think there must have been an error in the time on the 

 4tli ikpril, and it was not observed on quite the same spot as 

 the rest, but I do not feel at liberty to omit the observation. 

 The rest are very faii'ly accordant. As we know the diurnal 

 movement here, I have applied the proper correction. The 

 mean of the whole is 8deg. 49*3min. east. 



Being desirous of ascertaining whether any local influence 

 could be supposed to cause the variation at the Observatory 

 to be less than it should be, I repeated the observations at 

 Lady Franklin's Museum, and at, or near, the Cascades 

 Brewery.* Those two stations were selected as likely to show 

 the influence of the massive basaltic formation capping Mount 

 Wellington, if it is capable of exerting any at two miles 

 distant. The bold cliff's called the Organ Pipes bear about 

 W. 60deg. S. from the Museum, and SY. 15deg. S. from the 

 Brewery. Those stations are about two miles apart, with hills 

 between them. Mean of two sets on the 22nd January at the 

 Museum, 8deg. 59 rain. S.E. ; mean of four sets on the 28th 

 January at the Brewery, 8deg. 35-Omin. E. We have here 

 concurrent evidence that the variation is considerably less than 

 it must have been in 1863. To which I might add an 

 observation at Port Arthur, taken on the 27tli January, but, 

 as it was a hurried one, I will only say that it also points to a 

 decrease. t 



* In the field beyond, near the gate, 

 t The result was lOdeg. 43"9min. 



