478 Intelligence and Miscellaneous Aj^ticles, 



that the duration of collision increases with the 7nass of the striking 

 body and almost proportionally to that mass. 



2. Influence of the height of fall of the striking hody. — The experi- 

 ments were made with balls of steel, and showed that, when the height 

 of fall is increased, the duration of the collision is diminished, 



^. Influence of the radius of curvature of the striking body. — ^Four 

 '^cylinders, let fall from the same height upon the plane, were of the 

 same dimensions and the same mass ; but their lower extremities 

 were spherical caps of different radii: and it was demonstrated that 

 the duration of collision diminishes when the radius of curvature of the 

 striking body increases. 



4. Influence of the length of the striking body. — "With cylinders of 

 the same weigjit, terminated by identical spherical caps, but of dif- 

 ferent lengths, the author found that the duration of collision augme^its 

 with the length of the striking body. 



5. Collision between two identical balls. — Here the author distin- 

 guishes two cases : — that in which the ball struck is suspended 

 freely ; and that in which it is fixed to the vertical face, on this 

 occasion, of the steel cylinder. 



Calling a the duration of the impact of the first ball against the 

 steel cylinder, b the duration of the impact of rhe second ball against 

 the other when it rests against the cylinder, and c the duration of 

 the impact of the second ball against the first freely suspended, we 

 ^ave 5 = 3^^ 4^^ 



6. Collision between bodies of different dimensions suspended freely. 

 — Of two balls of different dimensions, the author caused now the 

 small ball to strike against the large one originally motionless, then 

 the large one against the small — or, again, a ball against the large 

 cylinder freely suspended, then the cylinder against the ball ; and 

 he ascertained that, in the collision of two elastic bodies, it is immate- 

 rial for the duration of the contact whether it is the larger or the 

 smaller of the two which strikes the other. 



7. Numerical valuation of the duration of collision. — The duration 

 of the collision of two elastic bodies is always very short. To give 

 an idea of it, M. Schneebeli cites a valuation he made of it in a parti- 

 cular case : a steel cylinder of 695 grammes weight, falling from a 

 height of 33 milliras. upon the face of the large steel cylinder, remains 

 in contact with it during a time 



t = 0-00019 second. 

 — Bibl. Univ., Jrch. des Sciences, vol. xliv. pp. 332-335. 



SPECTKUM OF THE AURORA. BY ED^VARD I. HOLDEN, 

 SECOND LIEUTEXAXT OE ENGINEERS. 



I have this evening succeeded in observing the spectrum of a very 

 fine aurora, which appeared about 7 p.m., and lasted perhaps 20 

 minutes. It first appeared as a rosy cloud about 15° wide and 

 perhaps 30° high, hearing N. 30° W. by comjmss. Afterwards it 

 spread to the zenith, and was principally in the shape of a band 



