JcsE 3, 1SS5.] 



• KNOWLEDGE 



471 



MAGAZINE OF SClENCr ^ 



.£/ 



PLAINEfWORDED-EXACTLYDESCRIBm 



LOXDOX : FRIDAY, JUNE 5, 1885. 



Contents op No. 188. 



The KnddT Eclipsed Uoon. Bj 

 Kich»rd i. Proctor Kl 



Oar Honsi-hold Insects. (/Km.) By 

 B. A. Butler 473 



Thooght and Language. X. By 

 AdaS. Ballin V* 



Arago on Shooting Stars 475 



Animab of the Present and the 

 Past. Bv Richard A. Proctor ... 177 



Origin and'Home of the Diamond. 

 Bt W. J. Harrison 47S 



Chats OD Geometrical Measure- 

 ment. (Illut.) Bv Richard A. 

 Proctor 479 



PIOS 



Watchmakiog at the Inventions 



Exhibition. (/Hu..) ISO 



Editorial Gossip -Wl 



Reviews 483 



Face of the Sky. Bt F.R.A.S 186 



Lunar Eclipse of Ma'roh 30 ISO 



A Mathematical Theory of Evolu- 

 tion 186 



Correspondence : Conventional Dar- 

 winism — Thought Reading — Con. 

 ception and Sensation— Generic 



Images, .Vc -kS? 



Our Inventors* Column 491 



Out Cheeg Column 192 



THE RUDDY ECLIPSED MOOX. 



By Richard A. Proctor. 



I AM very sorry to iind that Mr. Williams tLinks I 

 have handled his theory about the moon's being red- 

 hot " rather roughly " ; for the elucidation of truth, which 

 as he truly remarks is the object we both have in view, is 

 never aided by roughness of manner. Neither is it well, 

 if one can avoid it, to adopt a tone of confidence — as of 

 one who knoics and feels competent to explain to those who 

 do not know, or who have not as yet fully under.stood : yet 

 unfortunately, I can adopt no other tone in this matter. 

 The account given in my article on the " Ruddy Eclipsed 

 Moon " was not meant as a defence of any particular 

 "views," but as an explanation of certain facts ; and 

 beyond admitting that my explanation has been manifestly 

 insufficient, since it has not made matters clear to .Mr. 

 Williams, one of my readers, I have no fault to find with 

 it. I see, though, that the last sentence has been misun- 

 derstood by a correspondent, Mr. Harold Taylor (Answers, 

 p. 401) ; and I think he i.s quite right in objecting to the 

 wording of that sentence ; for in it I fay that under very 

 favourable conditions, we get -500 times full moonlight 

 from a totally eclipsed moon, where I meant to say that 

 we get as much light from the moon as though she were 

 illuminated by .500 times full moonlight. The totally 

 eclipsed moon may, under favourable conditions, be 500 

 times as bright as a surface of weathered sandstone illumi- 

 nated by full moonlight. 



Mr. Williams's remarks may be divided into those wliich 

 relate to physical considerations, where he is at home, and 

 those relating to optical and mathematical considerations, 

 where he is otherwise. 



In regard to the first, I note that he limits his remarks 

 to Prof. Langley's experiments with the bolometer. 1 

 myself attach so little weight to those experiments that in 

 lecturing on the moon I constantly quote Prof. Langley's 

 results in company with those obtained by Lord Ros.se, and 

 with the remark that I regard the latter as altogether the 

 more trustworthy. I believe that the moon, however 

 rapidly she may radiate the heat phe reciives, bsconies 

 exceedingly hot at the time of lunar midday. I am some- 

 what surprised, by the way, to find that Mr. V.'illisms's 



red-hot moon theoiy depends on this sun-supplied heat ; 

 for, in the closing pai'agraph of his letter, ho speaks of the 

 "ruddy glow fading (hiriiKj totalih/." I had supposed the 

 moon was ruddy, according to Mr. Williams's theory, v. ith 

 primeval heat. If slie warms up to redness at lun.ir mid- 

 day, and darkens measurably during total eclipse, then I 

 cannot understand why, according to Mr. William.s's theory, 

 there should be any ditterence between the amount of rud- 

 diness in different lunar eclipses. The full moon ought 

 always to be red hot. As a matter of fact, the moon has 

 disai)peared entirely in several lunar eclipses, — ancient as 

 well as recent. She has also been often ob.served to glow 

 with more light in the latter half of totality than in the 

 former. 



However, I am not concerned to .show that I have 

 under.stood more of Mr. Williamss theory than that it 

 undertakes to explain the ruddiness of the eclipsed moon 

 by inherent heat, primeval or sun supplied, or both. The 

 point which I (to understand is the way in which that 

 ruddiness is really caused by red light from the sun 

 himself. 



But here I find myself in a difficulty, because Mr. 

 Williams's remarks on this point arc such as to show that 

 my explanation should have begun somewhat farther back. 

 For example, when Mr. Williams says that if the sun were 

 a bright point, behind the centre of the earth a.s looked at 

 from the moon, he would not be visible at all (and perhaps 

 still more when he regards thig as deducible from Sir John 

 Herschel's demonstration) he — how shall I put it without 

 seeming to "handle" the matter " roughly 1" — well, he 

 shows that optical considerations have not received the 

 same amount of attention from him as some others which 

 he has dealt with in such a way as to instruct and interest 

 great numViers of readers. 



To begin with, Mr. Williams supposes that I have mis- 

 under.stood Sir John Herschel's account of the matter. But 

 as I had never read it, that is manifestly impossible, 

 jjossess Herschel's " Outlines," and have read large 

 ]:ortions of it ; but, of course, I have not thought it neces- 

 sary to read his explanation of a matter which I had already 

 inquired into myself. I remarked on certain sentences 

 quoted by Mr. Williams, as manife,stly meaning something 

 flitierent from what he had supposed (manifestly that is to 

 iiiP; who had already considered the optical relations in 

 question) ; but I had not read Herschel's explanation. I 

 have, however, within the last ten minutes read Arts. 421, 

 et seq. of the " Outlines of Astronomy." I need hardly 

 say that I have found every part of Sir J. Herschel's 

 explanation correct and excellent. The only exception I 

 can take is that Hersohel assumes on the part of his 

 readers a fuller understanding of optical considerations 

 than probably most of them possessed. 



To those who understand optical principles it is hardly 

 necessary to .say that Herschel's scoount is not directed 

 to the behaviour of jmicils of rays emanating from dif- 

 ferent points of the fun, but only to the limiting directions 

 of single rays which have passed through our earth's a<mo- 

 si)here. There is not any remark about foci, nor indeed 

 can any one discuss the/oca^ convergence of rays through 

 such an atmos|)here as our earth's, or any shell of that 

 atmosphere. All that could be done in that way would be 

 to consider the chai'acter of any small ;• -ncil of rays from 

 any point on the sun, after that particular pencil had 

 passed along its own track through our atmosphere ; and 

 I need hardly tell the optical student that any such 

 pencil of rays, divergent necessarily at entering our 

 atmosphere, would be divergent after jiassage through it. 

 The axes of all the pencils, infinite in number, proceeding 

 from different points on the sun towa'ds the earth's 



