42 



KNOWLEDGE 



[Februaky 1, 1896. 



course, made tbe comet brighter, since it brought it 

 closer to na. lUit there is another point about tlie orbit of 

 not less interest : the motion is of such a character that 

 every ninety-five years it must come under tlie inlhicncc of 

 .lupiter; but the action of tliat planet will not alwnys 

 be favourable to its continuance with us, and in lli:!7, 

 had the comet pursued the even tenour of its way, it 

 might have been dtflectcd into a hyperbolic path. 

 It would have been highly gratifying had the comet 

 remained visible till 

 wo could have ac- 

 counted for its disap- 

 pearance as satisfac- 

 torily as for its intro- 

 duction. And this >nay 

 happen yet, for the 

 comet bus shown Ihictu- 

 atingvalues inbrilliancy 

 and may recovir. If 

 we consider a conit't to 

 shine by reflected light 

 it is evident that its 

 brilliancy should vary 

 inversely as tlie product 

 of the squares of its dis- 

 tance from us and from 

 the sun. On this sup- 

 position we can compute 

 its theoretical brilliancy 

 for the first and last 

 time at which it has 

 been seen at each ap- 

 parition. Of course the 

 quantities are not strict- 

 ly comparable. Proxi- 

 mity to the sun, or the 

 presence of moonlight, 

 may rob us of the comet 

 where under better con- 

 ditions it would have 

 been visible in our 

 telescopes. The values 

 of this brilliancy are 

 set out in the following 

 table, and the apparent- 

 ly liopeless expectation 

 of seeing it again is em- 

 phasized by the fact 

 that in 1890 the bril- 

 liancy exceeded 8-0, and 

 though sought by many 

 experienced observers 

 the comet remained 

 invisiljle. 



Fig. 1. — Spunisli Chestnut Tree, 



In taking into consideration the causes that may 

 prevent the due return of a comet, we must not leave out 

 the chance of a collision with a small planet, when the 

 comet might be scattered, or more probably detlected into 

 a new path and therefore become unrecognizable to us. 

 The chances of such a catastrophe would depend very 



much upon the angle that the comet's path made with the 

 ecliptic. The smaller tliis angle, evidently the greater 

 the chance of encountering an asteroid ; and usually this 

 angle, known as the inclination of the orbit, is small in 

 the case of the comets we are considering. The inclina- 

 tion of the orbit of Brorsen is, however, very con- 

 siderable — greater than that of any of the short 

 period comets. Nevertheless, the orbit is so situated tliat 

 it can approach certainly three of these small objects, 



namely : Hebe, within 

 0'07;i ; llesperia, within 

 0-0 II! ; and Artemis, from 

 which it may be distant 

 one-tenth. Such dis- 

 tances, when turned into 

 miles by multiplying 

 them by the earth's 

 distance from the sun 

 (more than i»2,000,000 

 miles), seem to oiler 

 ample space for a comet 

 to pass unmolested ; but 

 taking into account the 

 probable large number 

 of small planets yet 

 undiscovered, it is im- 

 possible to deny the 

 chances of unexplained 

 perturbation. 



The second comet on 

 the list is the one re- 

 ferred to in connection 

 with that of Tempel 

 1878. This may now 

 be regarded as lost, 

 seeing that it has been 

 unsuccessfully looked for 

 at several returns, and 

 the mean motion must 

 now be very uncertain. 

 Jupiter, whose influence 

 has on several occasions, 

 as we have seen, been 

 exercised to increase the 

 number of our comets, 

 has, in this instance, 

 operated to remove one. 

 Remembering that .Jupi- 

 ter's period is 11 -80 

 years, and that this 

 comet had in 1879 a 

 period of 5-98 years, it 

 is evident that at each 

 alternate revolution the 

 comet and Jupiter must occupy, approximately, the 

 same relative positions. In 1.S70 the comet was only 

 32 distant from the planet, consequently in 1881 

 there was another close approach. The perturbations on 

 these two occasions were excessive, and the elements of 

 the orbit of 1885 differ very materially from those in 1867. 

 If we consider only the period, the most important for our 

 purpose, we have — 



Approaches Jupiter n'itliiu 032. 

 No approach. 



Approaelies .Tupiter within 0'55. 

 No approach. 



This increase in the period, or in the semi-axis major, is 

 accompanied by a corresponding increase in the perihelion 

 distance, so that the comet can never be closer to the sun 

 than twice the earth's mean distance— that is to say, con- 



