9,4.7 



1879.] ***■* [Kirk wood. 



discovered." * The fireballs of August 11 and December 30, 1878, as well 

 as that of February 3, 1879, had very rapid motions, and we can perhaps best 

 account for the non-appearance of aerolites on the theory of their complete 

 disintegration.! The meteors, it is obvious, could not have escaped out of 

 the atmosphere. Events of this kind are doubtless of very rare occurrence. 

 We have, it is believed, no authenticated instance in which a fireball has 

 escaped after approaching within 39 miles of the earth's surface.}: Assum- 

 ing this as an inferior limit and taking 100 miles as the greatest height at 

 which such bodies become visible, it is easy to show that but one in thirty- 

 four can continue its orbital motion. 



Suggestion to Observers. 



In the theory of meteors it is a matter of first importance to determine 

 the form of their orbits. If any move in hyperbolas they must have had 

 a proper motion in space before entering the solar system. Now the 

 nature of a meteor's orbit is determined from its observed velocity. Un- 

 fortunately, however, the time of flight (on which the velocity depends) is 

 generallj 7 a very uncertain element ; the estimates of different observers 

 being very discordant. Persons therefore who report such phenomena 

 should train themselves to habits of exactness in measuring the time of 

 visibility. 



Stated Meeting, May 16, 1879. 



Present, 28 members. 



Vice-President, Mr. Fraley, in the Chair. 



A letter of envoy was received from Mr. A. Agassiz. 



A letter of invitation to the members to attend the last 

 session, May 9th, of the West Chester Philosophical Society 

 was received. 



Donations for the Library were received from the editor 

 of the Zoologischer Anzeiger, Leipsig ; M. Melsens, Brus- 

 sels; the Annales des Mines, Paris ; Meteorological and An- 

 tiquarian Societies, Cobden Club, and Nature, London ; 



•Met. Astr.,p. 65. 



fThe average height of shooting stars at extinction is about 55 miles; that of 

 aerolitic fireballs and detonating meteors at the time of explosion, about 25 

 miles. 



% This was the nearest approach of the great meteor of July 20, 1860. See Prof. 

 Coffin's memoir in the Smithsonian Contributions, vol. XVI. 



