HUBERT ANSON NEWTON. 271 



shower, although exceptional in size. For this he calculated the 

 elements of the orbit which would give the observed path and velocity. 

 But the determination of the velocity in such cases, which depends 

 upon the estimation by the observers of the time of flight, is neces- 

 sarily very uncertain, and at best affords only a lower limit for the 

 value of the original velocity of the body before it encountered the 

 resistance of the earth's atmosphere. This would seem to constitute an 

 insuperable difficulty in the determination of the orbits of meteoroids, 

 to use the term which Professor Newton applied to these bodies, 

 before they enter the earth's atmosphere to appear for a moment as 

 luminous meteors. Yet it has been completely overcome in the case 

 of the November meteors, or Leonids as they are called from the 

 constellation from which they appear to radiate. This achievement 

 constitutes one of the most interesting chapters in the history of 

 meteoric science, and gives the subject an honorable place among the 

 exact sciences. 



In the first place, by a careful study of the records, Professor 

 Newton showed that the connection of early showers with those of 

 1799 and 1833 had been masked by a progressive change in the time 

 of the year in which the shower occurs. This change had amounted 

 to a full month between A.D. 902, when the shower occurred on 

 October 13, and 1833, when it occurred on November 13. It is in 

 part due to the precession of the equinoxes, and in part to the motion 

 of the node where the earth's orbit meets that of the meteoroids. 

 This motion must be attributed to the perturbations of the orbits of 

 the meteoroids which are produced by the attractions of the planets, 

 and being in the direction opposite to that of the equinoxes, Professor 

 Newton inferred that the motion of the meteoroids must be retrograde. 



The showers do not, however, occur whenever the earth passes the 

 node, but only when the passage occurs within a year or two before 

 or after the termination of a cycle of 32*25 years. This number is 

 obtained by dividing the interval between the showers of 902 and 

 1833 by 28, the number of cycles between these dates, and must 

 therefore be a very close approximation. For if these showers did 

 not mark the precise end of cycles the resultant error would be 

 divided by 28. Professor Newton showed that this value of the cycle 

 requires that the number of revolutions performed by the meteoroids 

 in one year should be either 2+^ or 1+^ or ^JL. In other words, 

 the periodic time of the meteoroids must be either 180*0 or 185*4 or 

 354*6 or 376*6 days, or 33*25 years. Now the velocity of any body 

 in the solar system has a simple relation to its periodic time and its 

 distance from the sun. Assuming, therefore, any one of these five 

 values of the periodic time, we have the velocities of the Leonids 

 at the node very sharply determined. From this velocity, with the 



