June 15, 1882] 



NATURh 



working they may be coupled up with the engine working 

 the stern rudder, and all three rudders worked together. 

 The dead wood has been cut away aft to a large extent in 

 order to reduce the resistance to turning. 



DOUBLE STARS 



SOME stars when looked at in a telescope are seen 

 really to consist of two stars so near together that 

 the naked eye is not able to distinguish them, but sees 

 them as a single star. 1 The knowledge of some of these 

 objects may be presumed to be almost as old as the 

 telescope. In fact Hevel remarked some of them in the 

 middle of the seventeenth century, but no attention was 

 paid to them, as it was thought that they were really far 

 asunder in space, and merely appeared close together in 

 the heavens, because they were nearly in the same visual 

 ray. It thus escaped notice that one star frequently 

 moves round the other, and Lambert, as late as 1761, 

 founded his opinion that those fixed stars that appear 

 near others, were in no physical connection with them, 

 upon this absence of relative motion, because, as he says 

 in his " Cosmologische Briefe," if they do not move round 

 each other, and still gravitate towards each other, they 

 must long ago have collapsed. But a few years after the 

 Rev. John Michell applied the rules of the calculation of 

 probabilities to the stars in the Pleiades, and showed that 

 it was exceedingly improbable that these stars could 

 appear so near together, if their proximity was the result 

 of a random scattering of the stars over the heavens, and 

 he showed that among 40,000 stars, one could expect to 

 find only one pair within twelve seconds of arc of each 

 other, and none nearer. These speculations were, how- 

 ever wholly conjectural, as longas no proper observations 

 were available, and it was therefore to the purpose when 

 the highly merited Jesuit, Christian Mayer, of the obser- 

 vatory at Mannheim, founded by the Elector of Pfalz, 

 commenced to search for, and systematically to observe, 

 double stars. But he met with no support trom his con- 

 temporaries, and had to defend his opinions in several 

 polemical pamphlets. His instrument, a mural quadrant 

 by Bird, was scarcely sufficient for the purpose, and his 

 opinion, that, "satellites " of the brighter fixed stars were 

 found at a distance of as much as three degrees, was cer- 

 tainly wrong in the instances he adduced, though Madler 

 has shown that stars as far asunder may possibly be phy- 

 sically connected. 2 We must, therefore, consider William 

 Herschel to be the first who proved the existence of 

 double stars. This he did by aid of micrometric inci- 

 sures, 3 which he originally had made with the view of 



1 Mizar and Alcor, the " lest-star," in the gre:U bear, present to the naked 

 eye very much the -ame appea'ance as a double star doe?, when seen in a 

 telescope. Their distance is about a lunar radius. It is sometime* said that 

 distances less than five minutes are not visible to the eye, but when wearing 

 glasses I see e Lyra:, the distance of whose components is less than four 

 minutes, double. 



- It was evidently Chr. Mayer's opinion that the smaller star shone in 

 reflected light. The term " double star" had been applied by previous 

 observers, who little dreamt that these stars would become so interesting. 



3 It may be as well to explain in a feiv words the instrument with which 

 such minute quantities are measured. It is called a micrometer, and forms 

 the eyepiece of a large telescope. It is well known that in the focus of the 

 object-glass of a telescope, the image of the object contemplated appears 

 inverted. Now in this focus are stretched two parallel spider lines, at 

 least one of which is moveable by an extremely fine screw. The magni- 

 tude of the object or the distance between the two comprnents of 

 the double star are thus measured in revolutions or fractions of a revo- 

 lution of this screw; for which purpose the head cf the screw is 

 divided into hundredths, the tenths of which are estimated by the eye fsee 

 Fig. r ). The value in seconds of arc of each revolutic n of the screw is eithi r 

 ascertained by measuring some known distance or by measuring the length 

 of a revolution in parts of an inch (a very small quantity) and dividing this 

 by the focal length of the object-glass expressed in inches. 'I he value of a 

 revolution is generally found to vary a little with the temperature, as the 

 steel of which the screw is made expands or contracts with the heat, but 

 this is Counteracted by changes in the focal length of the object-glass. It is 

 of more importance to exam.ne the irregularities of the screw, the different 

 turns of which might not exactly be of the same size. Even parts of each 

 turn might be slightly different. However modern engineers have carried 

 the making of screws on the lathe to so high a perfection, that there are 

 screws made in which no errors can with certainty be ascertained. It also 

 deserves to be remarked that it is more easy to make a screw accurate the 

 finer it is. — But the screw gives us enly the distance of one star from the 



finding the parallax of fixed stars, similar observations 

 having previously been attempted by the Rev. Roger Long, 

 of Pembroke College, Cambridge, who, however, had not 

 been very successful. 



Herschel commenced micrometric measures in 1776, 

 when he observed 6 Orionis. In 1779 he began syste- 

 matically to search for and measure double stars, and as 

 early as 1782 he laid his first " Catalogue of Double 

 Stars " before the Royal Society. It contained 269 ob^ 

 jects, but few of which had been observed by Chr. Mayer. 

 Subsequently he published other catalogues, which, how- 

 ever, contain many stars more than 32" asunder, which 

 are not now considered as double stars proper ; but of the 

 latter Herschel discovered between four and five hundred. 

 He measured double stars micrometrically up to 1785, and 

 again for some years after 1790. The measures showed 

 some discrepancies, but it was impossible "< decide 

 whether the relative motion of the components— for 

 changed their position some of them evidently had — was 

 rectilinear or otherwise, and whether it in some cases per- 

 haps arose from the proper motion of one star. However, 

 already in 1794 Herschel explained how they must move 

 in curved paths on account of their mutual gravitation, and 

 in 1803 appeared that famous "account of the changes 

 that have happened during the last twenty-five years in 

 the relative situation of double stars," in which he, from 

 actual measures, proved this to be a fact. But Conti- 

 nental astronomers were nevertheless slow to give in to so 



novel and startling a revelation. In France even Lalande 

 openly expressed his want of faith in these disclosures, 

 notwithstanding his high regard for Herschel's merits in 

 other respects. — At the end of his active career Herschel 

 had the pleasure to see his son John Herschel take up 

 this subject with fervour. South, the friend of this 

 illustrious astronomer, joined him in this work, and ob- 

 tained thus a place in the scientific world, to which his 

 own merits, only for this connection would scarcely have 

 entitled him. J. Herschel went in 1834 to the Cape of 

 Good Hope, where he discovered and made some mea- 

 sures of above two thousand double stars on the southern 

 sky with his 20-feet reflecting telescope. He continued 

 to take an active interest in these stars till his death in 

 187 1, when he left behind unfinished manuscripts that 

 showed that he had been engaged on a general catalogue 

 of double stars and the observations made of them. It 

 contained about 10,000 entries. 



Meantime Struve in Russia had commenced a series of 

 double-star measures, which is even now unsurpassed, as 

 well with regard to extent as to consistency. In 1824 he 

 received a 10-inch refractor, mounted equatorially, from 

 Fraunhofer of Munich, and with this magnificent instiu- 



other. We require to know al.o in what dir ot .. n it is situated. For this 

 purpose the plate on which the micrometric screw and the uires are fixed tan 

 be revolved, and the wires placed parallel t > the line j ining the centres cf 

 the two stars. I he angle is read off . n a circle in firm t onnection with the 

 tube. This, the so called an^le of p siticn is counted from the line c. n- 

 necting the principal star with the role. Fiom n rth o' round thr. ugh east 

 90°, south iSo°. and west 270' (see Fig. 2). Mhis circle is adjusted ly a'low- 

 ing the stars in their daily motion to run frcm east to west alcng the wires. 

 The index should then point to oo°or 170°. Eut it must be kept in view that 

 the images are inverted, so that eg. when look.ng southwards, ncrth 

 appears down. 



