Ill 



principal showers has advanced at a nearly uniform rate from October 

 12, old style, which was the date in A.D. 902, to November 12, new 

 style, which was the date in 1833; and finally that the meteoric 

 orbit, whatever it is, is but little inclined to the ecliptic, and that the 

 motion of the meteors where they enter the earth's atmosphere is 

 nearly perpendicular to the direction of the sun. 



Such being the facts, he proceeds to determine what inferences 

 may be drawn from them. From the dates of the showers he 

 ascertained that the node of the meteoric orbit the point of its 

 intersection with the earth's orbit has been since A.D. 902 advancing 

 in longitude nearly uniformly and at the average rate of 1 711' 

 annually. Allowing for the precession of the equinox, this is 

 equivalent to an advance of 29' in 33J years, measured from a fixed 

 point. This motion is accordingly direct, and Professor Newton 

 infers from this and from dynamical considerations that the motion 

 of the meteors in their orbit must be retrograde. He next considers 

 whether the meteoric orbit is wholly or only partly occupied by the 

 dense swarm of meteors. He first examines the hypothesis of an 

 elliptic orbit along which the meteors are distributed uniformly, and 

 which suffers such perturbations that it shifts about so as periodi- 

 cally to intersect the earth's orbit three times in a century. He 

 finds that this hypothesis must be rejected, because it involves an 

 apsidal motion so rapid as would require perturbing forces of an 

 intensity which we can satisfy ourselves do not exist. Accordingly 

 the meteors, leaving out of account the sporadic meteors which have 

 got separated from the main swarm, occupy only a portion of their 

 orbit. He next inquires what further can be learned about an orbit 

 of which the main swarm of meteors occupies only a portion ; and he 

 made the important discovery that only five orbits are compatible 

 with the observed return of the swarm to the earth at intervals of 

 33 years. One of these five, accordingly, must be the true orbit. 

 Professor Newton determined the periodic times in these orbits, and 

 thus ascertained the axis major of each. All that was then wanting 

 to fix the precise form and position of each of the five orbits was a 

 sufficiently accurate determination of the "radiant point," i.e., of that 

 direction from which the meteors are seen to enter our atmosphere. 

 On account of Professor Newton's representations, efforts were made 

 by astronomers to make this observation with the utmost care during 

 the great meteoric showers of 1866 and 1867. This direction, when 

 corrected for the deflection of the meteors by the earth's attraction, 

 furnishes the position in space of one tangent to the orbit. Knowing 

 then the focus, the axis major, and the position and point of contact 

 of one tangent of each of the five orbits, its exact form and situation 

 in space can be ascertained. Thus the five orbits become fully 

 known; and the next step was to determine which of them is the 



