April 24, 1885.] 



♦ KNO\VLEDGE ♦ 



339 



,u\ ILLUSTRATED 



MAGAZINE OF SCIENCE 



PLAlNLYWORDED-EXACTLYDESCRlBia 



LONDON: FRIDAY, APRIL 24, 1885. 



OONTKNTS OP No. 1S2. 



FAGB 



Tlif Raddv Eclipsed Moon. Bt 



B. A. Proctor ". 339 



Our Household Inserts. {lUut.) By 



E. A. Batlfr 3J2 



BTolntion of the Sense of Beauty. 



By ConsIMc-e C. W. N'aden 313 



The Younc Electrician. {Ittui.) 



By W. Slineo 3« 



Critical Methods of Detecting 



Errors in Plane Sarfaces. {lUun.) 



By John. A. Braahear 345 



Exposition at Xew Orleans. By 



R A. Proctor 3kS 



TbeKalevala. TI. By Edward Clodd 348 



First Star Lessons. {With Hap.) 

 By R. A. Proctor 350 



TricTclcslin 1885. (lltu.-.) 361 



Chapters on Modem Domestio Bco- 

 nomv 352 



Editorial Gossip 353 



Faoo of the Sky. ByF.R.A.S 361 



Correspondi^nce : The Death-Watch 

 — Evils of the A^e : Jind 

 Language, &c 



Miscellanea 



Our Inventors* Column 



Our Whist Column 



Our Chess Colonm 



354 

 367 

 368 

 359 

 360 



THE RUDDY ECLIPSED MOON. 



By Richard A. Proctor. 



MR. W. MATTIEr WILLIAMS, whose views 

 respecting the physics of astronomy have been 

 suggestive and interesting, even where not theoretically 

 sound, has maintained with his customary ability and 

 liveliness the theory that the moon has a heated tufaceons 

 surface, showing a red glow due to a temperature of about 

 600" Fahrenheit [Gentleman's Magazine for February, 

 1882); and he attributes to this high temperature the 

 ruddiness of the moon at the time of total eclipse. I am 

 not proposing to attack this theory, which seems fully 

 entitled to die a natural death : otherwise I might point 

 out that Prof. Langley's refined experiments with his 

 delicate heat-measurers, indicate for the moon's surface 

 even when she is full, or exposed to the full blaze of sun- 

 light, a temperature many degrees below the freezing- 

 point. I might also dwell on the circumstance that nearly 

 three hundred years ago a black eclipse of the moon was 

 observed by Kepler, when not a trace of the moon could be 

 seen during part of the totality though the stars were 

 shining in a clear sky. On three occasions since then the 

 moon has been lost to view, even to telescopic view, at the 

 time of mid-totality. This at any rate would dispose of 

 that part of Mr. Williams's theory which involves the 

 cooling down of the moon measurably within comparatively 

 short intervals of time, — as between the eclipses of 2.'5rd 

 August, 1877, and December 6th, 1881. But in truth the 

 red-hot moon theory needs no a posteriori attack, being 

 altogether devitalized by h jyriori disease. 



What is really worth correcting is Mr. Williams's mis- 

 taken argument respecting " the usually accepted theory of 

 the red moon within the shadow." (Even correct theories, 

 I may remark in passing, are usually more valuable for the 

 facts they co-ordinate than for their own intrinsic merit, — 

 and it may be noticed frequently that an erroneous theory 

 does more harm through the false ideas associated with it- 

 false ideas respecting facts — than because of its own in- 

 herent falsity.) 



This " usually accepted theory " Mr. Williams evidently 



misunderstands. And when a man of his ability and clear- 

 sinhtedness misunderstands a theory, wo iiiuy take it for 

 granted that many others must misunderstand it too. 1 

 rtinember that the pri'sent Astronomer Royal, in dealing 

 with Mr. Ilrett's amusing theory about Venus, as a shiny 

 nietallie planet enclosed in some sort of vitreous shell 

 showed manifest misapprehension of the way in which the 

 refractive action of a planet's atmosphere bends t.ho sun's 

 rays round the planet's body, geomctrieally intervening 

 between the sun and the observer on earth. Wo may be 

 well assured multitudes of " general readers " err, when 

 mathematicians and physicists — oven though it be but 

 through carelessness — go astray as Mr. Christie and Mr 

 Williams have done in this matter, though not both in the 

 same way. 



The usually accepted theory, says Mr. Williams, is that 

 the illumination of the moon in shadow is due to the re- 

 fraction of sunlight by the earth's atmosphere and the 

 transmission of the lw!li;/ht of the lover almos/ilieric strata. 

 The first jiart of this is right, the italicised portion is alto- 

 gether wrong. It is not the transmission of the twilight 

 of the lower atmospheric strata, but of the sun's very own 

 rays, which causes the red light of the eclipsed moon, — a 

 very different matter. Indeed the mistake is so utter that 

 when I first read the above sentence, I thought it was only 

 a carelessly worded expression of the real fact. But one 

 has only to read on to see that Mr. Williams imagines the 

 usually accepted theory to explain the red light as due to 

 the ruddiness pervading our atmosphere at the time of 

 evening and morning twilight. He quotes a sentence of 

 Sir John Herschel's in which reference is made to the tints 

 of our glowing sunsets, a sentence which might readily 

 mislead any one who had got hold of the erroneous idea 

 Mr. Williams has adopted. The atmospheric strata, says 

 Herschel, " will impart toall theraysthey transmit theruddy 

 hue of sunset, only of double the depth of tint which we ad- 

 mire in our glowing sunsets, by reason of the rays having to 

 traverse twice as gi-eat a thickness of atmosphere." This 

 of course is quite correct, but Herschel is talking of the 

 rays of the sun on their course from the sun to the moon, 

 not of the transmission to the moon of the rays from our 

 illuminated air. " According to this," proceeds Mr. Williams, 

 speaking of Herschel's remark as misunderstood by him, 

 " the eclipse of October 4 should have shown the moon as 

 an excessively ruddy ball, for we have had excessively 

 ' ruddy hues of sunset ' all over the world of late, and on 

 the date of this eclipse the sunset was especially magnifi- 

 cent, in this part at least." Of course, if the rays of the 

 sunset skies transmitted through our air were the cause of 

 the ruddiness of the moon, this argument would be perfectly 

 just : but the ruddiness of our skies indicates just the 

 reason (probably) why the moon looked so dark on October 

 4. As Mr. Williams himself says, in trying to explain 

 why his red-hot moon could not show (poor thing !) on 

 October 4, the recent sunset glows indicate the extinction 

 of a large amount of red — " quite as much as the eclipsed 

 moon," he proceeds, " heated to 600°, is supposed by my 

 theory to radiate." If the sun's rays stop on their way to 

 light up our atmosphere with an unusual amount of ruddy 

 lustre it stands to reason they cannot get through in usual 

 amount to light up the moon's surface during totality. 



Farther on, Mr. Williams speaks of a stratum of air two 

 miles high being all that can be illuminated by the sun's 

 rays, and then compares the earth shine of the new moon, 

 which is due to the light reflected by the whole surface of 

 the earth with the sun shining fully upon it, on the one 

 hand, with " the little burnished edge of the circular out- 

 line of the dark earth which shines upon the moon during 

 the eclipse" on the other. It is manifest that it is siinlil 



