MAGNETICAL OBSERVATIONS IN SCOTLAND. 107 



identical with the difference indicated by the results at Helens- 

 burgh before and immediately after the accident. 



The ratio of the intensity of terrestrial magnetism between 

 Dublin and Helensburgh as inferred from the results of July 

 22nd in Dublin and July 27th in Helensburgh, (being both pre^ 

 vious to the accident,) is 1-0067 Helensburgh to 1 in Dublin; 

 and as inferred from the results of August 2 and September 

 13 and 14 in Helensburgh, and October 4th in Dublin, (all sub- 

 sequent to the accident,) the ratio is 1*0066 and 1*0059 Helens- 

 burgh, to 1 in Dublin. I have therefore taken the result in 

 Dublin of the 4th October as comparable with all the observa- 

 tions made with this needle in Scotland, excepting the first 

 results (July) at Helensburgh, and those are comparable with 

 the first results (July) in Dublin. The same counterpoise was 

 used throughout. 



Before the values of the terrestrial magnetic force can be 

 derived with accuracy from the angles of deflection, it is 

 necessary to apply a correction for the variations of tempe- 

 rature of the needle itself in the observations at the different 

 stations. The temperatures were observed by a thermometer 

 placed in the circle with the dipping needle, and remaining 

 dui'ing the course of the observations. For the purpose of 

 ascertaining the value of this correction for needle S (2), the 

 needle was suspended horizontally by fibres of unspun silk in an 

 earthen vessel glazed at the top, standing in a larger earthen 

 vessel, into which warm water might be poured to raise the 

 temperature of the air and needle in the inner one. Several 

 folds of flannel enveloped the whole apparatus, being drawn 

 close round the upper part of the inner vessel, to keep the tem- 

 perature steady for periods of sufficient duration. The tempe- 

 rature of the needle was shown by a thermometer suspended 

 horizontally across it, not being in contact with any part of the 

 apparatus. The needle was then vibrated alternately in the 

 natural temperature of the room, and in the artificially raised 

 temperature. An arc for measuring the extent of the vibration 

 was placed beneath the needle, which was drawn out of the 

 meridian, and released at pleasure by a suitable contrivance. 

 The following observations were made at Limerick on the 30th 

 of October. 



