106 SIXTH REPORT — 1836. 



should preserve its magnetic state unchanged or nearly so during 

 the whole series of the observations. I had had this needle in my 

 possession rather more than a twelvemonth, and had ascertained 

 by frequent trials at the same station that its magnetic power was 

 diminishing, but at a very slow and uniform rate, well admitting 

 of interpolation. Being desirous of shortening as much as pos- 

 sible the period within which interpolation might be necessary, 

 (which would naturally have been the interval between my 

 leaving Dublin and my return to it,) I made more than the 

 usual number of observations at Helensburgh, the first station 

 I observed at in Scotland, five days only after I had observed 

 in Dublin, designing to return to Helensburgh for the purpose 

 of verification once, or oftener if occasion required, iu the pro- 

 gress of the observations. 



The first place to which I went from Helensburgh was the 

 island of Great Cumbray. In disembarking the instruments, 

 there being a good deal of sea, the case containing the needle 

 fell from the table to the deck of the cabin. The needle was 

 securely and immoveably fixed in the case, but the soft iron 

 keeper which connected its poles was allowed a slight spring, 

 arising from its own elasticity, to prevent its pressing too hard 

 on the points of the needle. This occasioned a slight jar to 

 take place ; very slight, but still sufficient to be audible. 



I conjectured immediately that the magnetism of the needle 

 might be affected thereby ; and the observations at Cumbray 

 strengthened this conjecture, by showing a greater difterence 

 from the Helensburgh results than was likely to be due to the 

 geographical distance between the stations. I returned to 

 Helensburgh the following day, and on repeating the observa- 

 tions there, found that the counterpoise which had deflected the 

 needle 89° 33'-9, now deflected it 91° 15'-6, showing that the 

 magnetism of the needle had been lessened. Needles have been 

 frequently reiuarked gradually to lose magnetism for some time 

 after it has been first communicated to them, luitil they arrive 

 at what appears to be a permanent condition for each particular 

 needle ; after which their magnetism remains steady. As far 

 as can be conjectured, the jar above described seems to have 

 brought this needle at once to its permanent state ; for on re- 

 turning to Helensburgh a third time, after an interval of 42 

 days, the observations being repeated, the same counterpoise 

 again deflected the needle 91° 18'* 7, a result almost identical 

 with that obtained on returning from Cumbray. Further, on 

 my return to Dublin early in October, I found, on carefully 

 repeating the observations at the same place I had observed at 

 in July, a difference in the magnetism of the needle almost 



