150 A XX UAL REPORT SMITHSONIAN INSTITUTION, 1921. 



densed head of a comet with the earth is not impossible; it may 

 some time occur; but comprehensive studies of this question, based 

 upon observational data concerning many of these bodies, lead indu- 

 bitably to the conclusion that we must not expect these collisions 

 to occur, on the average, more than once in 15 or 20 million years. 

 The so-called shooting stars, which we have all observed in the night 

 sky, are in many cases, perhaps in all, though we do not know, the 

 burning of minute pieces of comets which have disintegrated and 

 disappeared as comets forever from our sight. Colliding with the 

 earth, rushing through the upper strata of our atmosphere with 

 speeds up to 40 or more miles per second, the frictional resistance 

 of the air heats them to the burning point, and they are turned into 

 ashes and the vapors of combustion. A very few get through to 

 the earth's surface and are found and placed in our museums. It is 

 not certain that any of those in the museums are parts of disinte- 

 grated comets, but some of them probably are. The number of small 

 foreign bodies which collide with our planet every day is very great ; 

 a conservative estimate is 20,000,000. Except for our beneficent at- 

 mosphere man would suffer many tragedies from the bombardment. 

 There is reason to believe that the earth is growing larger very 

 slowly, from these accretions, and this may have been the process 

 by which the earth grew from a small nuclear beginning up to its 

 present size. 



Astronomers have determined that our solar system is very com- 

 pletely isolated in space. We are widely separated from our neigh- 

 bors. I shall not try your patience by quoting the tremendous dis- 

 tances in miles, for they are incomprehensible to all of us. Eays 

 of light sent out by the sun require a little more than eight minutes 

 to reach the earth. The outermost known planet in our system, 

 Neptune, would be reached in four hours and a half. Rays of light 

 leaving the sun at the same time and traveling at the same rate, 

 186,000 miles per second, must travel continuously during four years 

 and a half to reach our nearest known neighbor in space, the bright 

 double star, Alpha Centauri. If the distance from the sun to the 

 earth is 1, the distance to our outer planet is 30, and the distance 

 to Alpha Centauri is 275,000. There appears to be an abundance 

 of room in the great stellar system to meet the requirements of all. 

 The spectrograph attached to the Lick telescope has determined 

 that our sun and its family of planets is traveling through the great 

 stellar system with a speed of 12£ miles per second, equivalent to 

 400,000,000 miles per year. The earth is certainly hundreds of mil- 

 lions of years in age, the sun is no doubt at least as old, and the 

 early youth of the earth was lived, not where we now are, but far 



