JtJLY 1, 1898.] 



KNOWLEDGE 



159 



%tttttn. 



[The Editors do not hold themselves responsible for the opinions oi 

 statements of correspondents.] 



TITE SUXS STELLAR irAGXTTCDE. 

 To the Editors of Knowledge. 



Sirs, — Mr. Holmes had evidently not seen Mr. Gore's 

 article in Knowledge for June, 1895. The method adopted 

 in that article seems to me an improvement on the earlier 

 ones. 



But now that the spectra of stars are being carefully 

 studied, I think another method has become available. 

 Select the binary stars with pretty certain orbits whose 

 spectra approach most closely to that of the sun. If the 

 surface brilliancy of these stars appears to be pretty nearly 

 the same in all cases, we shall have a fair measure of the 

 surface brilliancy of the sun. If, then, we can get one of 

 these stars whose parallax as well as its orbit (in angular 

 measure) is ascertainable, and whose stellar magnitude is 

 known, we have the requisite data for comparison. The star 

 best suited for this purpose is, I think, Procyon. At present 

 the orbit is not certain, owing to the very recent discovery 

 of the satellite whose existence had previously been only 

 a subject of computation. But the photometric measures 

 and determinations of parallax in the case of Procyon are 

 remarkably concurrent, and the type of the spectrum 

 appears to be decidedly solar. If the spectrum approaches 

 that of the sun as closely as I believe it does (1 have no 

 spectroscope of my own), a good orbit for Procyon is pro- 

 bably all that we require in order to make a better estimate 

 of the sun's stellar magnitude than has hitherto been 

 accomplished. We have got so much into the habit of 

 calling stars hri'/lit that give a large quantity of light that it 

 seems useless to attempt to use the word in any other sense. 

 The word hi-illiant seems less appropriated to this use, 

 and we might therefore, perhaps, employ the word lirif/htne^s 

 with reference to the magnitude of the star (or quantity 

 of its Ught). and the word luilliiiucy with reference to its 

 intrinsic luminosity. Though there may be little difference 

 in the popular use of the two terms, it is desirable to have 

 two different words to express these qualities when we are 

 dealing with them scientifically. But the brilliancy of a 

 star, as thus explained, only represents the luminosity of 

 the surface unit on the assumption that all stars have the 

 same density. A diminished density wiU have the same 

 effect as an increased luminosity of the surface unit, and 

 as long as we are unable to measure the disc of the star 

 we can hardly form a decisive opinion as to which of these 

 causes a high or low degree of brilliancy is to be ascribed. 

 The phenomena of Algol and some other stars of the same 

 type, however, seem to indicate a low density in the Sirian 

 stars, and it is, therefore, not unlikely that their high 

 brilliancy is due rather to their great extent of surface 

 than to the great luminosity of the sui-face unit. Mr. 

 Gore's figures were based on this supposition of greater 

 surface, or, in other words, greater diameter. In a 

 paper which I contributed to the British Astronomical 

 Association nearly at the same time, I proceeded on 

 the assumption of greater brilliancy of the surface 

 unit. Most probably both causes combine. As a star 

 cools and condenses the surface shrinks and becomes 

 less luminous at the same time. But figures worked out 

 on either theory can be easily translated into the other. 

 They represent facts which may be interpreted in different 

 ways ; but different modes of interpretation cannot change 

 facts to fictions. 



W. H. S. MONCK. 



To the Editors of Knowledge. 



Sirs, — In reply to Mr. Holmes, I beg to say that if he 

 will again refer to my paper in Knowledce for March, 

 1898 (bottom of first column), he will find that I refer to 

 my paper in Knowledge for June, 1895, in which I have 

 computed the sun's stellar magnitude as - 27, and this is, 

 I think, a more probable value than -25'5, which was 

 formerly adopted. 



With reference to the term " brightness," Mr. Holmes 

 is right in thinking that I mean " quantity of light." If 

 he will read my paper again he will find that when I refer 

 to bri'ihtness of surface, I use the term " intrinsic bright- 

 ness," or " brightness of surface." In the case of Sirius, 

 I have shown that, taking its mass as 2-3(3 times the sun's 

 mass (as computed by Dr. See), and assuming its density 

 and liriijhtness of xurfuce to be the same as that of the sun, 

 the i/uantitij of lii/ht which it would emit would be only 

 1-773 times what the sun would emit at the same distance 

 as Sirius ; but its apparent '■ magnitude " shows that it is 

 17-38 times brighter than the sun would be at equal 

 distances. Hence, the " quantity of light " which Sirius 

 emits is about ten times greater than it should be, con- 

 sidering its mass. I think the sentence quoted by Mr. 

 Holmes expresses this with sufficient clearness. The 

 term " brightness " used alone, always, I think, means 

 " quantity of light." Thus a first magnitude star is said 

 to be 2-512 times brighter than a second magnitude 

 star, etc. Mr. Holmes speaks of y Leonis, but this star is 

 not mentioned in my last paper, as its orbit has not been 

 accurately determined. J. E. Gore. 



Dublin, June 10th, 1898. 



Experiments are in progress, under the Essex Technical 

 Instruction Committee, having for their object the de- 

 struction of charlock in barley and other crops : and it 

 appears that a two per cent, solution of copper sulphate, 

 applied at the rate of twenty-five to fifty gallons an acre, 

 by means of a " knapsack strawsometer," during dry 

 weather, and at an early period of growth, has been found 

 completely successful in suppressing the charlock without 

 injuring the barley. 



During the past month the South-Eastern Union of 

 Scientific Societies has held its annual congress (the third) 

 in Croydon, the president elect being Prof. G. S. Boulger, 

 and the retiring president the Rev. T. R. R. Stebbing, f.r.s. 

 The place of meeting for next year is Rochester. The aim 

 of the Union is "to win for science such benefits as are 

 found to accrue in manufactures from division of labour ; 

 and in trade, commerce, and finance from co-operation." 

 A perusal of the local reports of the papers read by the 

 members in the Town Hall, where the congress was 

 graciously received by the Mayor and Mayoress, indicates 

 that the scope of study and research extends to every 

 branch of natural history ; and the mutual enthusiasm and 

 goodwill which prevailed among both hosts and visitors 

 augurs weU for the future of the Union. 



Experiments in wireless telegraphy are, we understand, 

 in progress under newspaper auspices by Mr. C. Dolbear, 

 son of Prof. Dolbear, and messages, it appears, have thus 

 far been successfully transmitted over a distance of some 

 fifteen hundred feet. Prof. Slaby has also devoted some 

 attention to the subject, and is said to have sent intelligible 

 Morse signals thirteen and a quarter miles, using two 

 balloons filled with hydrogen to elevate the ends of the 

 conductors to the height of one thousand feet in the air. 



