334 Professor Dewar on the Story of a Meteorite. [1885-6. 



You have, no doubt, frequently observed what are called shoot- 

 ing stars, as they apj^ear to emerge from the dark sky of night, 

 pursue a short and rapid course, burst, and are dissipated in shining 

 fragments. 



From the velocity with which these bodies travel, there can be 

 little doubt that they are small planets which, in the course of their 

 revolution round the sun, are attracted and drawn to the earth. 



Eeflect for a moment on the consequences which would ensue, 

 if a hard meteoric stone were to strike the room in which we are 

 assembled with a velocity sixty times as great as that of a cannon- 

 ball. 



The dire effects of such a collision are effectually prevented by 

 the atmosj)here surrounding our globe, by which the velocity of the 

 meteoric stone is checked, and its living force converted into heat, 

 which at last becomes so intense as to melt the body and dissipate it 

 into fragments too small probably to be noticed in their fall to the 

 ground. 



Hence it is that, although multitudes of shooting stars appear 

 every night, few meteoric stones have been found, those few corrobo- 

 rating the truth of our hypothesis by the marks of intense heat which 

 they bear on their surface. 



The average result of the experiments of Joule and Thomson 

 showed that the wire was warmed 1° Cent, by moving at the velocity 

 of 175 feet per second. 



The highest velocity obtained was 872 feet per second, which 

 gave a rise of 5° -3 Cent. ; and there was no reason to doubt that the 

 thermal effect would go on continually increasing with the square of 

 the velocity. Thus at a mile per second the rise of temperature 

 would be, in round numbers, 900° Cent. ; and at twenty miles per 

 second, which may be taken as the average velocity with which 

 meteorites strike the atmosphere of the earth, 360,000°. 



The temperature due to the stoppage of air at the velocity of 

 143 feet per second is 1° Cent. Hence we may infer that the rise 

 observed in the experiments was that due to the stoppage of air, less 

 a certain quantity, of which probably the greater part is owing to 

 loss by radiation. It beiug also clear that the effect is independent 

 of the density of the air, there remains no doubt as to the real nature 

 of ' shooting stars.' 



These are small bodies which come into the earth's atmosphere 

 at velocities of twenty miles per second and upwards. 



The instant they touch the atmosphere their surfaces become 

 heated far beyond the point of fusion — even of volatilisation — and 

 the consequence is that they are speedily and, for the most part, com- 

 pletely burnt down and reduced to impalpable oxides. It is thus that, 

 by the seemingly feeble resistance of the atmosphere, Providence 

 secures us effectively from a bombardment which would destroy all 

 animated nature exposed to its influence." 



[J. D.J 



