ASTRONOMY 111 



existed. The pressure exerted by a beam of light, though 

 small, is still sufficient, as suggested by Arrhenius, to drive the 

 smallest known forms of life away from the earth, where they 

 might in time lodge on another planet such as Mars, and there 

 propagate themselves indefinitely. Whether this really occurs, 

 however, is very uncertain. 



SOME SPECIAL METHODS IN ASTRONOMY 



Before speaking about the present live questions in astron- 

 omy, I should like to point out one distinguishing feature of 

 astronomical methods. In many of its lines of enquiry, 

 astronomy is largely a statistical science. That is, it calls for 

 an immense number of observations of the same kind on dif- 

 ferent objects, and from these observations certain general 

 conclusions are drawn on the assumption that, while stars 

 differ largely as individuals, still the average star in one part 

 of the sky will resemble in its general properties the average 

 star in another part. This statement may seem rather vague, 

 but it amounts to the same thing as assuming that the aver- 

 age student in chemistry is as intelligent as the average student 

 in sociology, as is done in our grading system. A good exam- 

 ple of a problem attacked by statistical methods is the deter- 

 mination of the direction and velocity with which the sun and 

 its attendant planets are moving through space. The stars 

 outside our system, though they are often improperly called 

 fixed stars, are not really at rest in the heavens, but are mov- 

 ing, in some cases with velocities far greater than that of the 

 swiftest modern projectile. Owing to their great distance 

 from us, however, these movements appear so small that they 

 can be detected only by the most refined methods. The motions 

 have all conceivable directions, but we observe a general tend- 

 ency in one direction. Now, if you were floating on a lake, 

 and observed people on the bank moving in all directions, but 

 with a general tendency, for no apparent reason, in one direc- 



