M. Secchi on Shooting-Stars. 381 



Another remarkable circumstance is the small horizontal distance 

 of these meteors : in no case did it exceed 222 kilometres, or 2 

 geographical degrees. A somewhat curious consequence of this 

 is that no meteor seen from one of two stations, more than 444 

 kilometres asunder, can be identical with any of those seen from 

 the other. If the space that can be examined by an observer 

 from a given station were referred to a globe half a metre (in 

 diameter), we should find that a franc-piece would cover just as 

 much of its surface. 



This explains why, in certain showers of shooting-stars, the 

 latter have often been so concentrated in one place and wholly 

 invisible in another, and also why the shooting-stars of the 

 period of the 10th of August are not visible in the southern 

 hemisphere. We may also obtain from the above considerations 

 some conception of the prodigious number of these meteors ; for 

 if we were to take a circle whose radius is equal to the distance 

 from Rome to Paris, and to suppose the density of these meteors 

 to be 63 per hour (as actually found this year at Paris by M. 

 Coulvier-Gravier, and at Rome by ourselves), the number of 

 meteors falling thereon, daily, would be found to be 18,144. 

 This density, however, is very small, and the surface in question 

 is not even equal to the half that of the continent of Europe. 



In conclusion, we must, I think, admit that the height of our 

 atmosphere exceeds 200 kilometres (124 J English miles), and 

 that at this elevation the density of the air is sufficient to excite 

 light when violently compressed at the surface of these bodies. 

 I say to excite light, and not always to produce combustion ; for, 

 according to the observations where a veritable combustion was 

 seen to be determined in the middle of the star's course, it may 

 be questioned whether in reality every train is an actual com- 

 bustion, or whether it may not arise from the production of an 

 electric light developed during the violent friction of the meteor 

 against the air ; the heat accompanying which, however, may 

 sometimes elevate the temperature of the body to the point of 

 fusion. Subsequent observations must decide this question. 



In closing this letter I may observe that the point of depar- 

 ture of most of the shooting-stars always lay between Cephcus 

 and Cassiopeia, but that the parallax must necessarily cause this 

 point to vary for different stars as well as for the same star 

 as seen from different stations. Thus a star which to us ap- 

 peared to be altogether without tail, the eye being in its direction 

 from Cassiopeia, was seen from Civita Vecchia with a pretty long 

 tail and in another part of the heavens. Two, indeed, were ob- 

 served to have opposite directions ; but these manifestly moved 

 verij slowly. 



