Nov. 



1884.] 



♦ KNOWLEDGE ♦ 



391 



Again, at p. 338, col. 1, last line, yon say " it is evident that 

 K L = C H." If this were once granted, you would not need the 

 diagram in col. 2, you need only say " join H K. These triangles 

 have all their side? equal. Therefore angle C K H is equal to angle 

 K H L. Therefore the three angles of the triangle H C K are 

 equal to the three angles K C H, H C K, K H L, i.e., to two right- 

 angles. But any given right-angled triangle may be treated like 

 this, and any triangle may be divided into two right-angled trian- 

 gles. Hence the angles of any triangle are equal to two right- 

 angles." This proves Euc. I. 32, after which all is easy. 



Again, in col. 2, you claim to have 'proved Sirason's axiom that 

 two lines through a point cannot both be parallel to a third line. 

 But you have only proved this for "parallels" as you define them, 

 viz., "lines which have a common perpendicular," and so have not 

 proved Simson's axiom at all. This is, I fear, a logical flaw in your 

 argument. C. L. Dodg.son. 



Ch. Ch., Oxford. 



"WHAT LARGE EYES YOU HAVE, GEANDMAMMA!" 

 [1496] — Whoever walks through a good collection of fossils, 

 arranged according to strata, or turns over many engravings of 

 them, is likely to be struck by one distinction, I think, between the 

 majority of animals prior to a certain date and all those since. 

 Those that now have eyes, or that had them in the Tertiary, the 

 Cretaceous, or the Wealden times, have those organs very small 

 compared to the great eyes prevalent of old both in the vertebrate 

 and lower classes. If our crocodile be descended from the ichthyo- 

 sanr, for instance, or our shrimp or crayfish from a trilobite, might 

 not these moderns address to their ancestors Ridinghood's exclama- 

 tion to her sham grandam ? Is any reason yet suggested for this 

 seemingly sudden and unique change of fashion between the Oolite 

 and Wealden days ? If not, I can propose one; but would like to 

 hear first of any existent theory. E. L. Gaebett. 



MAY-FLIES. 



[1497]— Mr. F. W. Halfpenny's query (1487) will be found 

 answered in Knowledge for Oct. 10, p. 294. May-flies are the only 

 insects that cast the skin after acquiring wings. 



E. A. Butler. 



THE ECLIPSE OF THE MOON OF OCT. 4. 



Morar, Oct. 5, 1884. 



[1498] — I have witnessed many total lunar eclipses, but for im- 

 pressiveness, completeness, and length, none approach that observed 

 last night. 



In every sense of the word it was a total eclipse, the moon being 

 almost obliterated from the sky for an hour and a half. To the 

 naked eye, a mere nebulous blotch remained, recognised with difli- 

 ctilty, and a powerful binocular only just revealed the disc. 



The totality I have hitherto observed has always been confined to 

 a rusty or smoky veil, through which the lunar surface was dis- 

 tinctly traceable. In the present instance, an inky pall veiled the 

 orb entirely, and its nature was revealed at first contact, for it was 

 at once apparent that the moon was caught in the umbra, the 

 penumbra being barely visible (Fig. 1). During totality, you could 

 dimly discern through the inky pall a still blacker configuration. 



Fig. 2. 



as if of two triangular continents connected by a lengthy isthmus 

 (Pig. 2). This eclipse was further characterised by a combination, 

 I fancy, of rare occurrence, and that was an occultation. The 

 behaviour of the star was qnite new to me, for it certainly 

 descended (apparently) behind the moon, from its position to her 

 N.E., as regarding the top of the moon as N. I did not witness 

 emergence. 



Kindly let me know if you noticed at home the blackness of this 

 eclipse, and what I have advanced regarding the occultation. 



K. F. Hutchinson-, M.D. 



[Dr. Hutchinson's observation, made at a distance of upwards of 

 4,500 miles from Greenwich, as measured on a great circle, possesses 

 high interest ; as showing that the extraordinary obscuration of 

 the moon during the late eclipse, so far from being local, was seen 



over a very large area of the earth's surface ; and lending addi- 

 tional weight to the idea that it must have had its origin in the 

 existence of some semi-opaque material in our own atmosphere 

 which hindered the ordinary refraction of the solar light into the 

 earth's shadow. The markings shown in Dr. H.'s sketch of the 

 totally-eclipsed moon I am unable to explain. Numerous minute 

 stars were occulted in England during the eclipse, but, of course, 

 parallax would carry the moon wide of these as viewed from 

 Gwalior. — Ed.] 



TWO-SPEED GEARINGS. 

 [1499] — C. Webb can have Hirst's two-speed gearing applied to 

 his front-steering tricycle. The increase in the weight of the 

 machine would be about 5 lb. to 6 lb. The change can be made best 

 from speed to power while travelling. The same crank will answer 

 if there is length suflicient to take the lower clutching apparatus. 

 As regarding price, I must refer Mr. Webb to the maker. 



Jonx Bkowning. 



A NEW PHOTOMETER. 



[1500]-=-The foUoiving is, I believe, an entirely new discovery, 

 and, at all events, sufficiently important to bring before your 

 readers. It is due to Dr. Gorham, of this town. It is a well- 

 known fact that the iris performs the duty of regulating the amotint 

 of light which passes into the interior chamber of the eye by en- 

 larging or contracting the diameter of the pupil. But has it ever 

 occurred to any one before that the eye can be used as a photo- 

 meter ? By means of a little instrument of his own invention. Dr. 

 Gorham is able to measure the diameter of his pupil, and has 

 proved, by a series of measurements, the number, of which he 

 reckons by thousands, that, for the same kinds of light, the mea- 

 surement of his pupil is invariable. The gradual diminution of the 

 size of the pupil as dawn gives way to daylight, and the increase 

 as daylight gives way to evening twilight — the minimum being 

 reached about an hour or an hour and a half after midnight — on 

 each successive day, combined with numerous experiments on 

 candle-light, gaslight, and other lights seems to place it beyond 

 doubt that Dr. Gorham's pupil photometer is an accurate way of 

 comparing the intensities of various sorts of light. Is it not a 

 curious thing that this fact was not discovered years ago ? To my 

 mind, this photometer far excels those of Rumford or Bnnsen. 



Tonbridge. E. A. TindALL. 



[That Dr. Gorham's "Pupil Photometer" will ever supersede 

 those of Rumford or Bunsen as a quantitative measurer of light, is, 

 I think, very doubtful. Will Dr. Gorham— or Mr. Tindall— favour 

 us with a more detailed description of it, though ? — Ed.]. 



TRADUCING A TALISMAN. 



[1501]— The "potent talisman" in letter 1490 is only our old 

 friend the " magic square " thinly disguised. 



Add 1479 ( = 29 X 17 X 3) to each of the numbers 9 to 16, and you 

 may imagine yourself " to have full command over demons, fairies, 

 and enchanters." But whether this happy restUt is produced by 

 the magic square arrangement or by the addition of 1479, probably 

 only the Asiatics (whoever they may be) can say. 



If " A Lover of Things Occult " wishes to probe the dread secret 

 of his talisman, he may at his leisure verify the statement that 

 there are exactly 72 ways in which, by adding together four of the 

 numbers on it (more or less sj-mmetrically arranged), he may 

 obtain 2,992. I do not understand "things occult," but I should 

 imagine this ought to be done at midnight amidst suitable sur- 

 roundings — skull, cross-bones, &c. W. 



LETTERS RECEIVED AND SHORT ANSWERS. 



J. McKR.w. — Yon need not be under the slightest apprehension 

 of ever becoming too wise ; or that, during your brief span of life, 

 you will, in respect to knowledge, ever be like Alexander, who wept 

 because he had no more worlds to conquer. — SiBius states that last 

 year's phenomenon of a green moon is now recurring, and was seen 

 at Anerley on the 26th ult. If you read through the articles on 



