October 2, 1899.] 



KNOWLEDGE 



227 



The weather in the second and third weeks of February, 

 aided by the moon, made observations and estimates 

 difficult and unsatisfactory. 



Memphis, Tenn., U.S.A., David Flanery. 



29th June, 1899. 



1^*. — 



THE DISTRIBUTION OF STARS IX SPACE. 

 To the Editors of Knowledge. 



Sirs, — With reference to Mr. Gore's letter in your issue 

 for August, I quite agree with his remarks about the MUky 

 Way. It appears to me very improbable that the galactic 

 stars are of the same order of brightness as our sun. There 

 can be no reasonable doubt that many stars of the (ith 

 magnitude, and even brighter, belons? to the system of stars 

 composing the Milky Way. I believe very few astronomers 

 now consider it likely that there is any great difference in 

 the distance of the stars in this system ; but if stars of the 

 Gth and 10th magnitude are at the same distance, the lith 

 magnitude stars must be ten thousand times as bright as 

 the Ifjth magnitude stars. With equal intrinsic superficial 

 luminosity, this implies a difference of volume of a million 

 to one ; if, therefore, we assume the llith magnitude stars 

 in the Milky Way to equal our sun m size, it follows that 

 the (ith magnitude stars in the Milky Way must be a 

 million times as great. If it is considered, that, of the 

 stars concerning whose mass we knew something, none is 

 more than forty times the mass of our sun, it must be 

 admitted that the existence of stars of a million times the 

 size is very improbable. It is far more likely that the 

 brighter stars in the Milky Way are of the same order of 

 brightness as our sun, and that the fainter stars are much 

 smaller. 



The following table gives the results of some of my own 

 gaugings in the region of the Milky Way compared with 

 similar gaugings in other parts of the heavens : — 



Nnmber ' 

 of stars I 

 visible. 



Number of Additional stars 



visible with eacli increase 



of aperture. 



Aperture. 



IJ-in. 2i-in. 3.in. I 3|-in. 



Siun of 22 gaugings in various') 

 parts of the Milky Way .. j 



Sum of 100 gaugings in various^ 

 parta of the heavens, ex- j- 

 cluding the Milky Way . . j 



61 

 118 



174 

 348 



Relative density of the stars') 

 in the Milky Way / 1 



3-8 



36 



198 

 342 



4-2 



In taking gaugings in the Milky Way, I found it im- 

 practicable to use a field of the same area as in other 

 cases, owing to the large number of stars to be counted. 

 The area of the field was about '025 of the area used in 

 the one hundred gaugings of the stars generally, and this 

 difference of area has to be taken into consideration in 

 making a comparison of the results. The last line of the 

 above table shows that the Milky Way, as far as these 

 observations go, is about four times as rich in stars as 

 other parts of the heavens. It will be noted that the in- 

 crease in density is distributed among stars of various 

 magnitudes, with some approach to uniformity. This fact 

 certainly suggests the conclusion that galactic stars are at 

 about the same distance as other stars. 



One result of my investigations in this subject is a 



conviction that the assumption frequently made of the 

 magnitude of a star depending chiefly on its distance is 

 not justified by facts. My belief is that real difference in 

 actual brightness, and not difference in distance, is the 

 more important element in determining the apparent 

 magnitude of a star. G.^vix J. Burns, b.sc. 



[In other words, the range in actual brightness is greater 

 than the square of the rawje in distance. The conclusion 

 is one of great importance, but I think Mr. Burns is justified 

 by the facts which he has adduced, and by the general 

 trend of discovery, in adopting it. — E. Walter Maunder.] 



SOME SUSPECTED VARI.IBLE STARS. 

 To the Editors of Knowledge. 



Sras, — In his paper on this subject, Mr. Gore does not 

 appear to have considered the effect of " personal equation " 

 in colour between different observers. In the case of 

 o Leonis (p. ITU), a bluish star, being compared with y, 

 which is yellow, the difference of colour will affect their 

 \ apparent relative brightness. The Harvard observers 

 j made yellow stars distinctly brighter, relatively, than I do, 

 and it is probable that the difference is even greater 

 between them and Mr. Gore. 



There is also the question, which has recently been so 

 much discussed, of " position angle " to be considered and 

 : allowed for ; the correction being probably variable for 

 I different observers. Therefore it seems to me hardly 

 : correct to say that if y Leonis be seen distinctly brighter 

 I than p, that is certain proof of the variability of one of 

 them. 



I do not, however, on the whole, dispute Mr. Gore's 

 conclusion ; to me the relative brightness of the two stars 

 undoubtedly varies, sometimes one and sometimes the 

 other being the brighter, though it is difficult to decide 

 which is the variable. 



Sunderland, T. W. Backhouse. 



September IGth, 1899. 



TREE STRUCK BY LIGHTNIXG. 

 To the Editors of Knowledge. 



SiBs, — One of your readers, in a letter subscribed ''A. C." 

 (Knowledge, -January, 1899), desires to have an answer to 

 the question : " What actually takes place when a tree is 

 struck by lightning ? " 



This certainly is a question of general interest, and I 

 have had the occasion to observe the effects of lightning 

 several times in different countries. Before all, I must 

 say that the result of a lightning-stroke on a tree may be 

 of very different character, and depends firstly on the 

 species to which the tree belongs, and secondly, on the 

 condition in which the tree is at the time it is struck. 



If lightning strikes a tree after a long period of 

 heavy rain, when the whole surface of the tree is damp, 

 it generally does very little harm to the tree, and often 

 none at all. On the other hand, if a tree is struck when 

 its surface is dry, it is more severely damaged, because 

 then the electric spark will descend by a line of lower 

 resistance along the damp wood under the bark of the 

 tree. In this case the heat of the spark instantly produces 

 steam of very high pressure under the bark, and the 

 latter is generally blown up in a long band. 



In such cases, pine trees, and other trees of that kind, 

 are much less damaged than other trees, because the wood 

 of the pine is much more dry, and contains a greater per- 

 centage of isolating resin. In a leaf-tree there is no isolating 

 substance, and if the whole interior of the tree is damp, 

 it often happens that a large quantity of steam is produced. 



