200 ROfll ESTER .\CAnE^rY OF SCIENCE. 



the meteorite turning- in the air, owing to change of form by erosion, 

 allows the vacuum behind it to fill suddenly, and detonation ensues. 

 In none of these instances can the phenomenon be properly called 

 an explosion. Scientists would seem to be agreed that in describint; 

 this breaking or bursting of a meteorite in its course, it shall not 

 be called an explosion in the sense of being due to a force acting 

 on the mass from within outward. But they have not come to an 

 agreement as to how the dismemberment and detonation are really 

 caused. We do know that the final bursting is a phenomenon in 

 that part of the meteorite's course where cosmic velocity is retained, 

 for the fragments then thrown ofif have still time to acquire surfaces 

 of secondary crust, which would almost surely not occur during any 

 distance of gravitational fall. This means, then, a height of at least 

 30 miles, a height indeed which is accorded to the horizontal path 

 of many bursting and detonating meteors. There has been a 

 curious speculative theory originated by Haidinger, and practically 

 accepted by Brezina and Doss, that there is a point "Hemmungs- 

 punckt" as he calls it , where the falling meteorite finally loses its 

 cosmic velocity, reaches for an instant a "Stillstand," and thence- 

 forth is left solely obedient to the power of gravity. Galle who also 

 accepts this theory adds to it a corollary, that the greater the origi- 

 nal cosmic velocity of a meteorite the less the gravitational velocity 

 with which it finally reaches our earth. It should be added that 

 this point of loss of cosmic impetus will be largely determined by 

 the angle between our earth's surface and the path of the bolide, and 

 the consequent amount of aerial resistance which is to be overcome. 

 We leave this subject with the single additional observation that in 

 any clearly seen fall of a meteorite the detonation comes usually 

 some seconds, often indeed, several minutes, after the fall, the sound 

 coming at only 1 100 feet per second, while the meteorite has traveled 

 much faster. We have also the curious fact that when there have, 

 been several successive detonations it is the last one of these which 

 is heard first, and so on back in the series. 



The force with which meteorites strike the earth is quite variable, 

 as we should expect from previous consideration of their relative 

 motions. Meteorites have been known to strike on thin ice and 

 rebound without breaking either the ice or themselves ; while a 500 

 pound siderolite that fell at Estherville, la., on May 10, 1879, pene- 

 trated a stifif, clayey soil to a depth of eight feet. Another piece of 



