286 M. A. de la Rive on the Diurnal Variations of 



cubic feet. But since the wave is propagated from the centre 

 with a velocity «, the distance of the maximum condensation 

 from the centre may at certain time tc^ become 100,000 feet, 

 in which case the whole quantity of matter is A + 400,000 

 cubic feet. Thus in the interval from /j to tc^ the analysis has 

 generated 360,000 cubic feet of matter ! After obtaining such 

 a result from the part of the argument to which Mr. Stokes 

 has expressed his assent, I am at a loss to conceive for what 

 reason he asserts that any onus probatidi rests with me. 



I have no doubt whatever that I have pointed out r^aZ con- 

 tradictions resulting from the suppositions of plane-waves and 

 spherical waves, of the utmost importance in hydrodynamics, 

 since they prove that the true theoretical value of the velocity 

 of sound cannot be deduced from those suppositions. By 

 another supposition which conducted to ray-vibrations, I ob- 

 tained in the Philosophical Magazine for February a value of 

 the velocity of sound very nearly agreeing with observation, 

 without meeting with any similar contradiction. To this sub- 

 ject, however, I hope to find time to recur on a future occasion. 



Cambridge Observatory, 

 March 22, 1849. 



XLI. On the Diurnal Variations of the Magnet Needle^ and 

 on Aurorce Boreales. By Auguste de la Rive, being an 

 Extract from a Letter to M. Arago*. 



ALLOW me to communicate to you, with the request that 

 you will make it known to the Academic des Sciences, 

 an extract of a memoir recently read before our Societe de 

 Physique et d'Histoire Naturelle, on the cause of the diurnal 

 variations of the magnet needle and of auroras boreales. In 

 assigning successfully these two classes of phaenomena to the 

 same origin, I have but followed the path you have pointed 

 out; for more than thirty years ago you established with in- 

 defatigable perseverance, by your numerous observations, the 

 remarkable agreement which prevails between the appear- 

 ances of the aurora borealis and the disturbance of the mag- 

 net needle. 



The following is my theory. You will observe that it rests 

 solely upon well-ascertained facts and on principles of physics 

 positively established. 



I had already, in 1836, in a notice upon hailf, attempted 

 to show that the atmospheric electricity owes its origin to the 



* From the Annates de Chimie et de Physique for March 1849. 

 f Bibliotheque Umverselle, vol. iii. p. 217, nouvelle serie. 



