July 17, 1885.] 



SCIENCE, 



47 



oldest. Some of the volcanoes are younger, 

 and a considerable number of smaller cones 

 may have been built within a few thousands 

 of years. C. E. Button. 



7 A 



Fig. 1. 



COMETS II AND III OF 1884 ^ 



It is quite remarkable, that, of the five comets 

 visible during the year 1884, four should have 

 been periodic, and two of these of short 

 period, and observed apparently for the first 

 time at this return. By short period is gener- 

 allj' understood a period of somewhere in the 

 neighborhood of five 3'ears, of which we have 

 well - known ex- 

 amples in the 

 comets of En eke 

 (3.3 years), Bror- 

 sen (5.5 3'ears), 

 Win ne eke (5.7 

 years), Fa^'e (7.4 

 years), etc., — 

 twelve in all. 



The new com- 

 ets referred to are 



comets II and III of 1884, — the first discov- 

 ered by E. E. Barnard of Nashville, Tenn. ; 

 and the second, by Max Wolf, a student at 

 Heidelberg. Neither of these comets has been 

 a conspicuous object, — not even visible to the 

 naked eye, I believe, — but they are fair rep- 

 resentatives of the class known as ' telescopic ' 

 comets. 



As I have intimated, the orbit of comet 1884 

 II (Barnard), is elliptical with a period of about 

 five and a half years. Making allowance for 

 necessar}^ uncertaint}^ the elements show a 

 certain resemblance to those of DeVico's 

 'lost comet,' 1844 I, which, though certainly 

 elliptical, has not been seen since, if we except 

 a single rather doubtful observation made at 

 Paris in 1855. The period agrees very well 

 with that determined for DeVico's comet by 

 Briinnow (5.469 years) ; but Berberich has 

 pointed out that their identity cannot be as- 

 sumed, for the time elapsed since 1884, forty 

 years, does not correspond to an}^ whole number 

 of revolutions. He notes, also, that the physi- 

 cal appearance would seem to be against this 

 identity ; DeVico's comet, in a similar position 

 with respect to the earth, having been visible 

 to the naked e3'e. Leverrier thought it ver}' 

 probable that this comet of DeVico's was 

 identical with one observed in 1678 by La 

 Hire ; and Laugier and Mauvais concluded that 

 it was identical with the comets 1585, 1766 II, 

 and 1819 III or IV. 



Below are the elements of the two comets, 

 brought together for comparison. DeVico's 

 comet was computed b}^ Briinnow ; Barnard's, 

 by Frisby. 



Comet 1884 1 1 (Barnard). 



T = 1884, Aug. 16.2895, G-reeiiwich M. T. 



77 = 306° 10' 9'^4 1 



log a 

 Period 



= 5°23'5r^2 ( 

 = 300°46a8'^2 ( 

 = 5' 24' 48''.7 j 

 = 34°5r49".3 

 = 0.474164. 

 = 689''.858. 

 = 1878.65 days. 



Mean equinox, 1884.0. 



Comet 18U I (DeVico). 



1844, Sept. 2.511238, Berlin M. T. 



342° 30' 49' 

 63° 49' 0".] 

 2° 54' 50". c 

 38° 8'42".( 

 : 0.0742308. 

 649".1503. 

 Period = 1996.46 days 

 Motion direct. 



Mean equinox, 1844, Sept. 0. 



logg 



Let me try to show how these elements rep- 

 resent the orbit of a comet, and to give an 

 idea of the shape of this orbit, and its position 

 in space with respect to the sun and earth. 

 By far the most satisfactor}^ wa}^ of doing this 

 would be to construct from the elements a 

 cardboard model, which I think can be done 

 with little difficulty from the following direc- 

 tions. 



We know, that, in obedience to the law of 

 gravitation, comets must move about the sun 

 in some form of conic section, — the ellipse, 

 parabola, or hyperbola. As a matter of fact, 

 for the majority of comets, the orbit is given 

 as a parabola ; a few are known to be elliptic ; 

 but it cannot be said with certainty that any 

 are hyperbolic.^ 



We are first to fix the shape and dimensions 

 of the curve, and then its situation with refer- 

 ence to the plane 

 of the ecliptic, in 

 which the earth 

 moves. 



Suppose, for a 

 moment, that the 

 orbit is an ellipse, 

 the sun being at 



one focus (F, fig.l). Two of the ' elements ' 

 determine the form of the ellipse : — 



1. The semi-major axis CP, which is den6ted 

 by the letter a. 



2. The eccentricity e, the ratio of the dis- 

 tance from the centre to the focus, to the semi- 

 major axis ; that is, 



CF 

 ^-CP' 



1 Newcomb's • Popular astronomy.' 



Fig. 2. 



