432 



SCIENCE PROGRESS 



therefore, expect to have more transits in November than in 

 May ; as a matter of fact, there are more than twice as many. 



The intervals between successive transits of Mercury are 

 much shorter than those between transits of Venus. At present 

 a transit of Venus occurs, on the average, once in sixty years. 

 This can be deduced from the table. Those of Mercury, how- 

 ever, are occurring, on the average, at the rate of one in eight or 

 nine years, seven times more frequently. 



There are three fundamental periods for transits of Mercury 

 derived in precisely the same way as the two for Venus. The 

 first contains seven years, and it is quite possible to have 

 November transits at the ends of this period ; May transits, 

 however, occur less frequently, and the seven-year period does 

 not hold for them. The second period is one of thirteen years 

 and is much more exact, so that transits may be regarded as 

 probable when separated by this interval. It holds both for 

 November and for May. But the third period is so much more 

 accurate that it can be used with great confidence to predict 

 the transits of the future. It contains forty-six years, and 

 rarely fails. 



Twelve transits of Mercury occur during the present century, 

 nine of them in November and three in May. If we tabulate 

 each set we shall see how the interval-periods work out. 



The first transit of the 

 took place on 



The next occurred on 



Another will occur on 



Another will occur on 



Another will occur on 



Another will occur on 



Another will occur on 



Another will occur on 



Another will occur on 



century 

 Add 



Add 



Add 



Add 



Add 



Add 



Add 



Add 



1907 



7 



1914 



13 

 1927 



13 

 1940 



13 



1953 

 7 



i960 

 13 



1973 

 13 



1986 



13 



1999 



November 14 

 years 



November 6 

 years 



November 8 

 years 



November 12 

 years 



November 13 

 years 



November 6 

 years 



November 9 

 years 



November 12 

 years 



November 14 



