August 1, 1890.] 



KNOWLEDGE. 



183 



^T^ AN ILLUSTRATED "^.^ 



MAGAZINE OF SCIENCE | 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: AUGUST 1, 1890. 



CONTENTS. 



Binary Stars of Short Periods. By J. E. Gore, I'.R.A.S 

 M.R.I.A 



House-flies and Bluebottles.— VI. By E. A. Butler 

 The Bread-Fruit Tree and the Nettle Family. By R. 



Camper Day, B.A.Oxon 



Letters :^ — Evans; Lord Grimtfiori'E 



New Variable Star 



Teeth and their Variations. By R. Ltdekker, B.A.Cantab 

 Weismann's Theory of Heredity. By E. Clodd 



The Face of the Sky for August 



Sadler, F.R.A.S 



Whist Column. By W. Montagc Gatiie 

 Chess Column. By I. Gdnsberg 



Bv Herbert 



183 



1,S7 



i;m 



191 



m.i 



193 

 19,5 



199 

 200 



202 



BINARY STARS OF SHORT PERIOD. 



By J. E. (ioRE, F.R.A.S., M.R.I.A. 



Til]'] Biuary, or revolving double stars, are among 

 the most interesting objects in the heavens. 

 The number now known probably amounts 

 to nearly 1,000. In most of these, however, 

 the motion is very slow, and in only about 

 60 cases has the relative change of position since 

 their discovery been sufficient to enable an orbit to 

 be computed. In most cases the plane of the real 

 orbit, or ellipse, described by the companion round 

 the principal star, is inclined to the line of sight. 

 and is therefore foreshortened into a more elongated 

 ellipse. Indeed, there is only one case in which we mj . i 

 see the )val orbit of a binary star, viz. in the star fi 

 l^raconis, the orbit plane of which, according to 

 Berberich, is at right angles to the line of sight. But 

 the computed period — 6-18 years — is very long ; and 

 the orbit, therefore, of somewhat doubtful accuracy. 

 The relation of the (ijijiaivnt ellipse, which we see, 

 to the mil ellipse, will be understood by the follow- 

 ing illustration. Suppose a cyhnder, or rod, of an 

 elliptical, not circular section, to be cut across 

 obliquely to its axis. Then this oblique section will 

 represent the nnl orbit of a binary star, and the 

 section at right angles to the axis the (ip/nirfnt orbit. 

 The angle between these two sections will represent 

 the inclination of the real orbit to the plane of pro- 

 jection or backgroimd of the sky. In tlie apparent 

 orbit the primary star, which is supposed to bo situ- 

 ated in one of the foci of the mil ellipse, does not lie 



in the focus of the apparent ellipse ; and from its position 

 in this latter ellipse we can deduce mathematically the 

 particular angle at which the oblique section must be 

 made, to agree with the observed place of the primary 

 star ; and other details respecting the real ellipse. 



The periods of revolution of the computed orbits vary in 

 length from llj to 162.5 years. Of the 60 orbits which 

 have been computed, there are about twenty-one with 

 periods under 100 years, and these form the subject of the 

 present paper. The orbits of many of these have been 

 well determined, and some account of the most remark- 

 able, with diagrams of the orbits, may prove of interest 

 to the general reader. 



1. 8 Efptleii. — This has the shortest period of any 

 known binary star.* Over three complete revolutions have 

 been described by the companion star round the primary 

 since its discovery in 18.52. The period foimd by the 

 Russian computer Wrublewsky — 11-478 years — is pro- 

 bably not far from the truth, but, as his orbit does not 

 represent the measures very satisfactorily, I have not 

 drawTi the apparent orbit. It is, however, a very elon- 

 gated ellipse, owing to the high inclination of the i*eal 

 orbit. Burnham found only "a slight elongation"' in the 

 star with the great 36-iuch refractor of the Lick Obser- 

 vatory in .July 1880. The distance between the compo- 

 nents does not at any time exceed half a second of arc, 

 so that it is beyond the reach of aU but the lai'gest 

 telescopes. 



2. I Sat/ittarii. — This bright southern star is also a close 



* That is, of any binary which has been seen with a telescope as 

 double. Mr. Gore necessarily leaves out of account in this paper 

 such binary systems as those of ^ Ursie Majoris and Algol, the 

 existence of which has only been proved by the indirect evidence of 

 the spectroscope and photometer. But our knowledge with respect 

 to the orbits of some of these binary systems of very short period is 

 rapidly increasing, and continued observations will probably give us 

 data from which all the elements of their orbits may be determined, 

 even though we may never be able to see them as double with the 

 telescope. In view of the evidence we already possess, one may 

 assert with confidence that masses of matter, large enough to be 

 seen as stars, revolve about one another in periods ranging from less 

 than eight hours upwards. — A. C. Rastard. 



^l--is ../ nnili I/ <= A>'c<-o»i</ 



Fig. 1. — Apparent Orbit of I Saoittarii. 



