NATURE 



[November i, 1900 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.^ 



The Leonids— a Forecast. 



In the Proceedings of the Royal Society for March 2, 1899 

 (vol. Ixiv. p. 403), will be found an account of the perturba- 

 tions suffered since 1866, November 13, by the Leonids which in 

 that month intersected or passed close to the earth's orbit. This 

 position in the meteor stream may be called station A (Fig. i). 



We have since investigated the principal perturbations affect- 

 ing two other points in the stream, viz., the station z, which 

 intersected the earth's orbit 360 days earlier, i.e. in November 



1865, and the station B, which intersected the earth's orbit 360 

 days later, i.e. in November 1867. 



We therefore now know the principal perturbations which 

 during the last revolution of the meteors have affected three 

 points, z, A and B, situated along an orbit (Adams's orbit) which, 

 at the commencement of the revolution, lay within the stream. 



The full results of the investigation will not be ready for 

 publication till after the time when the Leonid shower of this 

 year is due, and on this account it has been thought expedient 



To the^Suri/ 



The. poirvt b. 



,1,500,000 Miles 

 . . .1.000,000 Miles 

 500. 000 Miles 



^ccrtJTJs T*cLth/ 



Fig. 2. 



to publish beforehand such of the results as have special 

 reference to it. 



A point in the stream which in 1867 'ay along Adams's orbit 

 between A and B, but nearer B, and which we may call the 

 point b, will this year reach its descending node simultaneously 

 with the earth. This will happen approximately on 1900, 

 November, I5d. 3h. Greenwich mean astronomical time. 



Unfortunately, the orbit of a meteor situated near point b in 



NO. 1618, Vf'L. 63] 



the stream, instead of intersecting the earth's orbit as it did in 

 1867, will now pierce the plane of the ecliptic in a point which 

 lies about o'oi8 nearer the sun. Now, 0018 of the earth's 

 mean distance from the sun is 1,674,000 miles ; so that, of the 

 meteors which in 1867 intersected the earth's orbit, those which 

 will come nearest to the earth in the present year will not ap- 

 proach it nearer than a million and six hundred thousand miles. 



It is known from the duration of the great showers that the 

 width of the ortho-stream, if measured in the direction which is 

 parallel to the earth's path, is only about 300,000 miles ; but 

 there is reason to believe that the Leonids entered the solar 

 system under conditions which have made the section of the 

 stream much longer than it is broad, so that its trace upon the 

 plane of the ecliptic is something like what is represented in 

 Fig. 2. The longer axis of this cross section lay originally along 

 the radius vector from the sun, but perturbations have acted on 

 the Leonids for nearly 1800 years of such a kind as have probably 

 caused the section of the stream to incline in the direction 

 represented in the figure. 



If the section is long enough to reach the earth's orbit, we 

 shall have a great meteoric shower this year. It is, besides, just 

 possible that a sinuosity in the stream may so displace a part of 

 the section as to bring it sufficiently far out. But neither of 

 these seem likely to have happened ; so that the present in- 

 vestigation does not raise any hope of a great shower this year. 



If, contrary to our expectation, the axis major of the section 

 proves to be long enough to reach the earth's orbit, the conse- 

 quent shower of ortho- Leonids is likely to occur several hours — 

 possibly more than a whole day — earlier than 



1900, November, I5d. 3h. 



The number of hours by which it will precede that epoch depends 

 upon the angle which the axis major of the section makes with 

 the radius vector from the sun — an angle which is at present un- 

 known. If there is this year a shower of ortho-Leonids, the 

 time at which it occurs will enable us to determine this im- 

 portant datum. 



Station a in the stream (see Fig. i) intersected the earth's 

 orbit in 1866, but after completing a revolution it passed the 

 earth in November of last year at a distance of some 1,300,000 

 miles ; and z, the corresponding point for the preceding year, 

 which also intersected the earth's orbit in 1865, was on its re- 

 turn distant from the earth in November 1898 by about 960,000 

 miles. It thus appears that the displacements of the meteoric 

 orbits which have been brought about by the perturbations of 

 the last thirty-three years suffice to have prevented the meteoric 

 orbit from now intersecting the earth's orbit. This accounts 

 for our not having had any great shower in either of the last 

 two years, and unfortunately the conditions seem still more 

 unfavourable in the present year. 



Nevertheless, as there is always a possibility that one or 

 other of the contingencies mentioned above may carry a part 

 of the ortho-stream out as far as the earth, and as we have no 

 means of ascertaining whether those contingencies have arisen, 

 it is desirable that preparation shall be made for adequately 

 observing the shower, if it should unexpectedly come. 



The perturbations during the last revolution, which have for 

 the present carried the ortho-stream of Leonids so far from the 

 earth's orbit, belong to the class of perturbations which act at 

 different times with equal effect in opposite directions ; so that 

 there is reasonable ground for expecting that further perturba- 

 tions must at some future time bring this remarkable stream 

 back to the earth's orbit. It would be possible to ascertain 

 when this will happen, by an investigation carried over a sufh- _ 

 cient time forward upon the same lines as those which we have 

 pursued. G. Johnstone Stoney. 



October 24. A. M. W. Downing. 



Examinations in Experimental Science. 



You occasionally do us, who are humble teachers of Elementary 

 Science in schools, the very great kindness of giving us, through 

 your columns, the chance of reaching the ears of those eminent 

 men who are your frequent contributors, and who examine our 

 pupils. Will you, in the interest of that real science teaching, 

 so often advocated in your columns, allow me such a chance 

 now? I will be as brief as possible. In common with a few 

 individuals and many public bodies, I have spent a very large 

 amount of time, money and labour in introducing the teaching 

 of practical physics into my school, and trying to see that it shall 

 be of the best kind possible, and I am prepared to do more. 



