20 i 



♦ KNOWLEDGE • 



[March 6, 1885. 



(recent. Had you not better send your two cases of "calling" 

 direct to Mr. Gurney ? Re (so-called) "Spiritual" literature, you 

 Beemto proceed upon the simple principle, " Credo quia impofsibile 

 eet," to have read the rubbish of those who have failed to find out 

 how the " mediums" do (often clumsily enough) their tricks, bat to 

 have studied little or nothing written by the very few scientific 

 men who have condescended to notice the subject. Heally, the 

 cleverest of the slate-writing fellows is a fool by the side of JI. 

 Verbeck, now performing at the Princes* Hall in London. 31. 

 Verbeck, though, is honest, and says that his marvels are 

 effected by sleight-of-hand. The " mediums " are scandalous 

 impostors, who trade upon the ignorance of their fatuous 

 dupes by pretending to be in communication -with the 

 other world. I could fill a quarter of this column -with the 

 names of these people, who have been successively detected in 

 their sorry cheating ; but it is really degrading the pages of Kxow- 

 LEDGK to admit any reference to " spiritualism " into them at all. — 

 Chas. Eose. I cannot suffer the question to drift into pure theo- 

 logy. — yv. H. Ba.\s.\ll, B.A. Miss Ballin, as you will see, has had 

 an opportunity of very promptly expressing her own opinion of the 

 letter of " Hallyards," who, moreover, as you will see from 

 his letter [1626] on p. 203, has himself promptly expressed 

 his regret at the tone in which he criticised her theory. 

 When our contributors themselves complain of the criticism 

 of their utterances by their readers, it will be quite time 

 enough for me to take action in the matter. — H. A. B. The 

 empirical law to which you refer can only possibly apply to a 

 partially-illuminated body seen through a great thickness of atmo- 

 sphere. Why, at the distance you name, the man's height would 

 subtend an angle of more than 6S" ; while that of the motes in the 

 air, rendered visible by Tyndall's Electric beam, is utterly insensible 

 and immeasurable. What angle do you suppose that a floating 

 gossamer-thread subtends at your eye, when the brilliant illumina- 

 tion of the sun renders it visible !■ The angular diameter of Mara 

 varies between 4" and 30"; that of the ball of Saturn, alone, 

 between 15" and 20" ; his astonishing ring-system much more 

 fhan doubling this. — Ax E.\rxest Thinker points out that there 

 are two misprints in letter lt'06 (p. 150) both in the second 

 line : " single " should be " simple," and " of it " appears for " fit." 

 — Dh. Lewins also complains that on line 25 of the second column 

 of his letter (1602), on p. 158, the comma should be after "such," 

 and not after " self," as printed. — L. H. Spkott. I am very sorry 

 that you have got a non-acliromatic telescope. With an achromatic 

 of the same apertui*e you would see enormously more. In reply to 

 your questions, you can see 10th mag. stars with your telescope ; 

 6th mag. only with the naked eye. You will perceive Mercury 

 most indifferently. The crescent of Venus you mijlit see towards 

 the end of this year, but that she will be so very badly placed. 

 The moon you will be able to see a little of when she is about 

 half -full, of Mars nothing. Jupiter as a most indifferent little disc, 

 bordered with orange light ; but you will pick up his four moons 

 easily enough ; Saturn's rings certainly not. The magnifying 

 power of your eyepiece is 30, but its component lenses ought only 

 io be H inch apart. The telescope is not worth any higher power. 

 — E. Gould (ISury St. Edmunds). Will you be good enough to 

 read the italicised paragraph which appeared on page -173, and 

 elsewhere in the last volume of Knowledge ? — S. S. Geeat- 

 HEED, M.A. " When the sun was a nebula, extending to the 

 orbit of Venus," I wonder what the condition of the earth 

 was. Suppose that it was itself a diffused nebulous mass, 

 equally illuminated throughout! — H.Camerox Gillies. Many thanks, 

 but such a subject would hardly fall within onr prescribed limits. 

 At the same time, I am greatly in sympathy with you, regarding, 

 £3 I do, that subject as on about a par with meteorology as a science, 

 — F. W. Reynolds proposes to fit a mercurial compensated pendu- 

 lum, making eighty-one beats per second, to an Austrian clock. The 

 steel rod, which is about '156 in. in diameter, weighs 2 oz., and the 

 mercury cistern itself is of such diameter that, when filled 4 in. 

 deep, it holds 1 lb. of mercury. The clock train is not strong enough 

 to drive a heavier pendulum than this. Can any practical horo- 

 logical correspondent tell him whether a depth of 4 in. of quick- 

 silver in such a pendulum as this (which obviously should be about 

 21i in. long from the centre of suspension to the centre of oscilla- 

 tion) will suffice to compensate it ? — W. R. K. Imprimis, I con- 

 gratulate you on your observation of the reappearance of Aldebaran 

 from occultation on Sunday week. It was more than any one saw 

 in this part of the world. In the next place, I rather fail to under- 

 stand your figures. Dublin is 25m. 22s. ivest of Greenwich. Con- 

 sequently, 7h. 37m. Dublin time is 8h. 2m. 22s. Greenwich time : 

 "because the earth is turning on her axis from west to east, and 

 Greenwich gets under any celestial body, or has that body (say the 

 sun), on its meridian 25m. before Dublin does. Hence, if you saw 

 Aldebaran reappear from behind the moon's limb at 5h.27m. Dublin 

 time, it must have been 5h. 55in. 223. at Greenwich at that instant. 



Tour difficulty about the discrepancy between these corrected times 

 of your observations, and those predicted by " F.R.A.S." in his 

 " Face of the Sky," admits of a very easy solution. The times he 

 gives are those at which the phenomena will happen at Greenwich. 

 But Dublin is 2' north of Greenwich, which, in itself, must ap- 

 parently depress a body so close to the earth as the moon is, in 

 the sky ; moreover, it is 6' 20' to the west of the prime meridian, 

 which will seemingly shift the moon towards the Ea^t. From these 

 considerations you will see that the occultation of stars will take 

 place at different times, and at different parts of the moon's limb 

 in Dublin to what they will at Greenwich ; nay, it may happen that 

 a star will be occulted at Greenwich, of which the moon will pass 

 quite clear in Dublin — and vice i-ersi. — F. C. Go into any assembly 

 of men famous in literature, science, or art, and you will all but 

 invariably find that the most distinguished among them are little 

 men. Your strapping handsome fellow six feet high takes his 

 share in muscle to the loss of brain power. With reference to the 

 other part of your letter, it almost seems to me as though Medical 

 advice was reauisite. — Thos. Agxew i Sons. I was unfortunately 

 unable to avail m.yself of it. — H. B. L. Read the masterly critique on 

 " The Unseen Universe" in Vol. I. of the " Lectures and Essays," 

 by the late Prof. Clifford, published by Macmillan & Co. — Hard- 

 ware. They can be placed in 120 different ways. — John Hampden. 

 You vri I have your little joke. — T. W. Backhouse. I gravely 

 question your view of the action of the light of a star (just at the 

 instant of its disappearance as a point) on the retina. I further 

 fail to share in your wonder that streaks and wisps of milky light 

 attached, as it were, to the galaxy are not included in lists of 

 isolated nebula?. I cannot say, off-hand, how the author of the 

 paper arrived at the figures you quote. — W. M. Get " Experi- 

 mental Physiology : its Benefits to Mankind," by Sir Richard 

 Owen, published by Longmans & Co., two or three years ago, 

 and look through back volumes of the Medical Press and 

 Circular. — Tanxaker Buhicbosan. See Knowledge for Feb. 6, 

 p. 110. — H. R. D. M. So far from having been "bored," I have 

 read your long letter through with a considerable amount of in- 

 terest, and thank you sincerely for the courteous and friendly 

 spirit which pervades it. Let me deal with the chief points upon 

 which you touch seriatim. To begin with the velocity of light. 

 The way in which the 192,000 miles per second was arrived at was 

 by the observation of the apparent retardation of the phenomena 

 of Jupiter's satellites, as observed from opposite extremities of a 

 diameter of the earth's orbit. The amount of its retardation was, as 

 I have said, simply a matter of observation, and knowing accurately 

 the diameter of our orbit in miles, a very simple sum would suffice 

 to show the velocity of light in miles per second. By combining 

 the results of the transits of Venus of 1761 and 1769, Encke, just 

 fifty years ago, calculated the solar parallax at 8 571", which would 

 place the earth at a mean distance of 95,370,000 miles from the 

 sun. But in 1854 Hansen threw grave doubts upon the accuracy 

 of Encke's determination, by proving that the motions of the moon 

 demanded a reduction of something like a'oth of the whole amount 

 of this parallax ; and the recent more accurate observations of the 

 transits of Venus, the parallax of Mars, &c., all combine to show 

 that the solar parallax does not differ much from 8 82", which 

 would, in turn, reduce the diameter of the earth's orbit and the velo- 

 cityof light. Meanwhile, Fizeau,Foucault,Cornu, Young, Forbes, and 

 Michelsen had, by the aid of most ingenious apparatus, been measur- 

 ing the actual rate at which light travels on the earth's surface, and, 

 as the final and most trustworthy result, Glazpbrook, in his" Physical 

 Optics," gives 186,771 miles per second as that rate ; a quantity in 

 the most perfect agreement with the increased parallax of the sun. 

 Of course, the linear (not proportional) dimensions and masses of 

 the whole solar system (save those of the earth) have had to be 

 reduced, as you will see if you will compare a table of them given 

 in every book on astronomy thirty years old, with the corresponding 

 one in a similar work published recently. Your account of the 

 destruction of the pre-historic mounds in your neighbourhood is 

 at once interesting and painful. The utter disregard of many 

 of our most precious archa!ological relics is simply a scandal to us 

 as a nation. I fear that the mere iuspeetion of your primitive 

 " Macadam," would scarcely enable anyone to form a definite 

 opinion as to whether it had been employed for purposes of 

 smelting or of cremation. On the subject of glacial epochs in 

 these islands and their existing traces, read Ramsay's excellent 

 book, " The Physical Geology and Geography of Great Britain," 

 published by Stanford. Coal-fields, by the bye, afford evidence of 

 " a moist and equable climate," by no means of a tropical, or even 

 sub-tropical one. I quite join with you in the denunciation of 

 the American vulgarism "at" for "in." The distinction that used 

 to be made may be illustrated by saying that formerly it would 

 have been written that a man was stopping at Capel Curig, but 

 that he lived i» London,— Jonx Harhs. Received; bnt it is not 

 a matter suitable for discussion in these eolamne. 



