Perturbations of the Leonids. 405 



long that it takes between two and three years to pass each point in 

 its orbit, so that the configurations in which the several parts are pre- 

 sented to the disturbing planets are markedly different. Accordingly, 

 perturbations must have produced in this long stream both sinuosities 

 and an unequal distribution of density ;* and the first step towards 

 increasing our acquaintance with these and other kindred phenomena, 

 as well as towards gaining a better insight into the past history of the 

 swarm, is to aim first at securing a more intimate knowledge of the 

 perturbations. 



With this end in view it was decided, as a first step, to compute the 

 actual perturbations of a definite part of the stream over the whole 

 of one revolution, taking that part of the ortho-stream of which Adams 

 had determined the orbit, and extending the computation over the 

 revolution from the date of the great shower of November, 1866, until 

 that day in January, 1900, when the same part of the stream will return 

 to the earth's orbit. 



Adams's calculation was based on determinations of the radiant point 

 which were made in 1866, before photography had lent the aid to 

 astronomy which it now yields. Moreover, the circumstance that the 

 earth deflected the meteors which were then observed by an amount 

 which varied as the shower progressed, was not at that time attended 

 to by observers. Owing to these imperfections, there is a considerable 

 probable error in the mean of the determinations which were made in 

 1866, and a corresponding uncertainty in the values of the elements 

 computed from that mean. We are accordingly only justified in em- 

 ploying Adams's orbit as approximate. But, fortunately, an error in 

 the orbit, of such an amount as is at all likely to exist, will not ma- 

 terially affect the perturbations of the orbit, which are what we have 

 at present in view. 



The main stream of Leonids the ortho-stream is narrow and very 

 long, and it is convenient to divide it into segments, each of which 



between a great body of them the ortho-Leonids which are travelling round 

 the sun in nearly identical orbits, and another class of Leonids which we may call 

 clino-Leonids, that are pursuing courses which differ in a more considerable 

 degree from the ortho-orbit. By the ortho-orbit is to be understood the mean of 

 the orbits of the ortho -Leonids. 



The ortho-Leonids at present form a compact stream of such a length that it 

 takes nearly three years to pass each point of its orbit, and so narrow that when 

 the earth passes obliquely through it the transit occupies only some five or six 

 hours ; whereas the clino-Leonids form a less dense and wider stream, which has 

 spread itself the whole way round the ring, and which produces in every 

 November, when the earth passes through it, a feeble meteoric shower that lasts for 

 several days. 



* One consequence of the existence of irregularities in the stream of ortho- 

 Leonids is that the ortho-orbit at one cross-section of the stream (i.e., the mean 

 of the orbits of the meteors occupying that situation in the stream) is in general 

 not absolutely identical with the ortho-orbits at other cross-sections. 



