November 1, 1894.] 



KNOWLEDGE 



257 



to ob%-iate this evil by means of wash-towers, a dry filter 

 made of brushwood, gorse, etc., was suggested, and this 

 has been found to arrest almost the whole of the arsenic 

 fumes in the escaping gases. In one case, where there 

 was no such filter, a test of the chimney gases showed the 

 presence of as much as 7'4 grains of arsenic m a cubic 

 foot of the gases passing into the air, whereas, with dry 

 filters of the kind, the average is now less than one-tenth 

 of a grain per cubic foot. 



The above remarks refer to but a few of the points, taken 

 almost at random, m. this most interesting report. Its 

 author for rather authors, for Mr. Fletcher's contribution 

 is supplemented by other reports from the inspectors for 

 the different districts) would be the last to claim that 

 the work had by any means approached perfection. But 

 the short sketch just given will enable the readers of 

 Knowledge to appreciate the fact that an immense deal 

 has already been done in the way of preventing the escape 

 of noxious gases and vapours into the atmosphere, with a 

 corresponding benefit to the health of the community and 

 a large saving of valuable material. The climate of our 

 country is too damp to allow of the air here being as clear 

 as that of more southerly and drier regions ; but we may 

 at least look forward to the time when increased knowledge, 

 and its judicious application, will bring about such an 

 improvement in the air breathed by the people of our 

 towns and manufacturing districts as to make life Ln 

 these both pleasanter and healthier. 



A CLASSIC DOUBLE STAR. 



By Miss A. !M. Clerke, Authoress of " T/ie System of the 



Stars" and " A Popular History of Astronomy during the 



Ninetfenth Century," <(•(•., i(-c. 



NOT far fi-om seventy years ago, Friedi-ich Struve 

 declared that the mathematical probability of a 

 physical connection between the stars of 

 61 Cygni was actually gi-eater than that of the 

 sun rising the morrow morn. Yet these " odds 

 beyond arithmetic " possibly misled him. It cannot, 

 indeed, for a moment be supposed that the luminaries are 

 of independent origin, or have had independent histories. 

 Their common proper 

 motion — the largest be- 211" 

 longing to any known 

 compound object — -of itself 

 estabUshes a close tie 

 between them ; it has, 

 moreover, been learnt, 

 since Struve wrote about 

 them in 1827, that their 

 apparent vicinity is real, 

 their parallaxes being 

 sensibly the same ; finally, 

 they are both yeUow stars 

 showing a spectrum of the 

 second type. Their photo- 

 metric magnitudes are 5-5 

 and 6-3. 



Our records of them go 

 back to 1753, when Brad- 

 ley noted their separate 

 transits ; and an ample 



21-0" 



20-9" 



20-8' 



20-7'' 



amoimt of relative motion, otherwise nothing could prevent 

 their falling together ; it shows, however, no sign of the 

 anticipated orbital character. The stars apparently travel 

 along right lines, inclined to each other at an angle of about 

 two degrees, the smaller with a velocity slightly inferior to 

 that of the larger. The inference that they were separating 

 never to be reunited was arrived at by Captain Jacob as 

 early as 1858 ; and a fi-esh discussion, after sixteen further 

 years of observation, led M. Flammarion to the same 

 conclusion. Nevertheless, orbits for the pair, based no 

 doubt in part upon illusory observations, were calculated 

 Ln 1883 by Mr. Mann, of Rochester, New York, and Ln 

 1885 by the late Dr. C. F. W. Peters ; but neither of them 

 claims any longer a shadow of authority. At the same 

 time, Prof. Hall's opinion, founded upon determinations 

 made by him at ^Yashington dm'ing the years 1879 to 

 1881, is slightly Ln favour of the star's binary natiu'e ; and 

 an attempt to fix the centre of gravity of the system gave 

 3'1 to 1 as the proportion of the masses of its components. 

 But no indications whatever of a definite orbit could be 

 traced. Mr. Burnham, on the other hand, has no doubt 

 that the march of both stars is absolutely rectilinear ; that 

 they are separating, and will continue to separate ; and 

 that the evidence of their present conjunction must, Ln the 

 distant futm-e, become wholly obhterated. If this be so, 

 they were formerly much closer together than they now 

 are, and each must, in all probability, have executed a 

 hyperbohc sweep round their common centre of gravity. 

 This little adventm-e might not be inconsistent with 

 the state of things sketched out by Prof. Newcomb, since 

 we are acquainted with comets which — without finally 

 renouncing then- allegiance to the sun — have described 

 hyperbolas round Jupiter. Not that the two cases bear 

 detailed comparison ; but tlieu- Ukeness, though elementary, 

 is instructive. 



" The only conclusion open to us," remarked the last- 

 named authority in connection with Mr. Burnham's verdict 

 regarding the status of these stars, " is that each of them 

 describes an immense orbit, which may be several degrees 

 in apparent diameter, and in which the time of revolution 

 is counted by thousands of years." An enormous mass 

 would, however, be needed in order to control the velocity 

 of 61 Cygni. For here an important distinction comes Ln. 

 Only the small relative motion of its components has to do 



21-1" 



21-0" 



20-9" 



20-8" 



Oct Dec. Feb. Apr. June Aug. Oct. Dec. Feb. Apr. June Aug. Oct. Dec. Feb. Apr. June Aug. Out. 

 1890 1891 1892 1893 



Wilsing : Changes in Distance of the Components of 61 Cygni. 



20-7" 



store of accurate determinations of their distances and 

 position-angles has been collected since lS2o. Yet no 

 certaLnty has yet been obtaLned that they form a true binary 

 system. They possess, as a matter of coui'se, a certain 



with determining the nature of their association ; but the 

 swift movement which they share must be accounted for in 

 any attempt to divine the organization of a grand system 

 including them as co-equal members. Their high rate of 



