of Terrestrial Magnetism. 445 



order to affect the needle ? That such an action does exist, how- 

 ever, appears to be proved by the fact, that in our climates the 

 needle performs its oscillation with the greatest regularity 

 during calm and serene days, and that on change of weather 

 this regularity is invariably disturbed. We have, in proof of 

 this, a year's observations at Home : and it would be well to 

 discuss magnetic observations more from the meteorological 

 point of view than has hitherto been done. From the few ob- 

 servations which we have made, it appears that light and pass- 

 ing overcloudings have more effect on the needle than tempests 

 themselves. We have already remarked elsewhere, that at 

 Rome the perturbations exhibit themselves in that particular 

 state of the atmosphere in which there are slightly phosphorescent 

 clouds having at night the appearance of the rudiments of the 

 aurora borealis. This fact was observed by us a second time on 

 the evening of the 27th of July. We were making some ob- 

 servations on stars in the meridian, when towards half-past nine 

 we were interrupted by a slight overclouding coming from the 

 north ; a very rare occurrence, since generally with us the sky 

 begins to overcast in the south-west. While we were waiting 

 for it to clear, the cloud appeared slightly luminous at the 

 edges, so that there seemed to be a diffusion of the milky way 

 in unwonted parts of the heavens. Soon after this it cleared, 

 and the observations were continued ; but almost immediately 

 the same overclouding recommenced with the same luminous 

 appearance. I then remembered the fact observed before, that 

 a similar state of the atmosphere had been accompanied by mag- 

 netic perturbations, and on going to look at the magnetometer 

 I found it more than 20 divisions (about 7-|') out of its usual 

 position, and the regular observation made at 9'35 had been 

 marked by the observer as being an extraordinary one for that 

 hour. This was the more striking from the needle having per- 

 formed its diurnal oscillation with the greatest regularity during 

 the whole of the preceding season. This was, without doubt, a 

 phsenomenon of the kind which accompanies the aurora borealis. 

 But it may be asked, was the condensation of vapours the cause, 

 or the effect, of the perturbation ? It is generally considered 

 to be most probable that the perturbation is the effect ; but is 

 this certain ? M. De la Itive has proposed, in his Memoir on 

 the Aurora Borealis *, a theory Avhich accounts with some feli- 

 city for the effects of atmospheric electricity on the needle ; but 

 it may be doubted whether this cause is sufficient to explain all 

 the facts to which the author would apply it. An accurate 

 study of the laws to which the extraordinary perturbations of 

 the needle arc subject, combined with the study of the aurora 

 borealis, can alone throw light on this question. 



* Bibl. Unki. Arvhiv. des Sciences Nufurelles, vol. xxiv. p. 337. 



