Baron de la Tour on Metallic Strings^ ^c. 201 



almost exactly double, viz. 3° 18^ 41". If this difference, 

 which is far too great to be accidental, shall be confirmed by 

 future observation, it will prove that in the arctic latitudes, and 

 in those periods which abound with Aurora?, the excursions of 

 the magnetic needle are diminished ; while in our own latitudes 

 the causes which produce Aurorse increase the excursions of 

 the magnetic needle. 



In the present state of our knowledge, it would be idle to 

 speculate respecting these apparently opposite effects. Nume- 

 rous accurate observations, made in different points of the mag- 

 netic meridian, can alone furnish us with the data which such 

 speculations require. On a subject involved in so much difficul- 

 ty, and so far withdrawn from experimental research, it would 

 be presumptuous to pronounce any rash opinion ; and those 

 who would fix us down to any dogmatical explanation, or 

 shackle the freedom of discussion where every hypothesis must 

 present so many vulnerable points, can neither be acquainted 

 with the history nor imbued with the spirit of inductive 

 science. 



Art. II. — On the Elasticity and Change of Volume of Me- 

 tallic Strings while in a state of Vibration* By Baron 

 Cagniard de la Tour. * 



It is well known that if we draw out a plate of caoutchouc, it 

 becomes thinner and narrower as its elongation is increased, so 

 that in changing its form it does not seem to change its vo- 

 lume, since it loses in one dimension what it gains in another. 

 The same effect might be expected in metaUic strings ; but M. 

 Cagniard de la Tour has proved, that if we submit a metallic 

 string to a moderate elongating force, the diminution of its dia- 

 meter is less than it ought to be if its decrease of thickness 

 were exactly compensated by its increase of length. Hence it 

 follows, that in drawing a wire, a sensible vacuum is left be- 

 tween its molecules. 



• This abstract of Baron Cagniard de la Tour's paper is translated from 

 Le Globe, Jan. 5th 1828. jj-j . 



