8 



PROFESSOR FORBES'S EXPERIMENTS ON' 



Table III. 



Additive Corrections applicable to fire Place Logarithms of the Time, for the 



effect of Temperature. 



16. I am disposed to think that the correction for temperature is always 

 open to a certain degree of doubt. Perhaps the condition of magnetism in the 

 needle is not necessarily that due to the temperature it possesses at the moment, 

 but rather to a temperature it had formerly. I think I have in some cases per- 

 ceived indications of this. The Needle No. 1, which is more slender than the 

 " Flat," seemed to be more steady in its indications than the other, and as I have 

 always placed more rehance upon its indications, so the effect of temperature was 

 determined with most care. 



17. IV. Variations in the Earth'' s Magnetic Intensity. These variations 

 must affect observations of the relative intensity at two places, if the observa- 

 tions be not simultaneous. These variations are either (1.) secular, shelving a 

 progressive change fi-om year to year ; (2.) periodical, that is subject to short pe- 

 riods of valuation and regular, as at different seasons of the year, and at different 

 hours of the day; or, (3.) accidental, arising from the aurora borealis, or from un- 

 known causes.* The numerical laws of these three, may be said to be almost 

 equally unknown ; the variations of the second class have indeed been studied by 

 Hansteen, Christie, Dove, and others, but the results are not sufficiently ac- 

 cordant to permit me to apply any of them to my observations. As, however, the 



* My friend Professor Necker of Geneva has pointed out to me one of the first recorded obser- 

 vations of the influence of the aurora upon the magnetic needle, the more interesting because the coin- 

 cidence was unnoticed (apparently) by the observer himself. In Saussure's Voyages dans les Alpes, 

 vol. iv. p. 300, that enterprising traveller notices an auroral appearance, observed from the Col du Geant 

 on the 12th July 1788, and in another part of the same volume (p. 308), records, amongst his magnetical 

 observations, the unsettled state of the needle during the whole of that evening. 



