230 



KNOWLEDGE 



[Jan. 13, 1882. 



Iinvp suffered from flyspopnia when living on mixed food, Imvo been 

 freed from tlie miilady after (und presumably on account of) a 

 chanjfe to a vep'tnblv diet. I wisli Uicro were epoco for his letter 

 in full, hut there is not. 



" A. (!. I'." has scon parts of Ursa Major in lat. 20° soutli, and a 

 shadow thrown by objects in lijjht of Venus. Ho asks what pheno- 

 mena beyond total absence of centrifugal force might reasonably be 

 eipcctod at the pole ? E.\treme cold, wo should say, the pole of 

 the heavens over head, sun visible for more than halt a year and 

 invisible for the rest of the time. Uiciiarm .\. I'm u tor. 



CHEAPENING OP APPARATtTS, AS AN AID TO TUE 

 DIFFUSION OF SCIENTIFIC KNOWLEDGK. 



[1S3] — At the beginning of a New Year, I venture to suggest that 

 the most worthy and laudable end to which Knowlkdgi: so especially 

 addresses itself — that of the diffusion of sound popular scientific 

 information — might be very considerablyadvancedby that invaluable 

 class of men, the optical and philosophical instrument maker.", if 

 they would only address themselves to the production of apparatus 

 at a cost to bring it within the means of those whose pecnniarv 

 means are (like the joint-stock companies) "limited." Conducting 

 some experiments in chromatics recently, I required a double-image 

 prism, for the purpose of superposing coloured discs ; but on a 

 friend inquiring on my behalf at two separate shops, he was asked 

 12s. Od. for one at one of them, and 15s. at the other! Now, we 

 know pretty well what Iceland spar costs in the rough, and I feel 

 tolerably confident that a prism mounted in a simple bit of brass 

 tnbing might be sold at a good profit for half-a-erown. Such, though, 

 is the trade consen-atism among those concerned in the manufacture 

 of such things, that they would much rather sell one prism at 15s. 

 than eight for a sovereign, ilutatin mutandis, these remarks wotild 

 equally ap)ily to the transit instrument. There can be no doubt 

 that a real want exists for a simple meridian instrument, which 

 shall be at once trustworthy to use and cheap to buy; but £15 

 represents the present price of the smallest transit made which is 

 really serviceable. Here, then, is an opening for some enterprising 

 scientific artificer, at once to benefit himself and the community at 

 large. An economical transit instrument once produced, it mav be 

 worth the while of clock and chronometer makers to try whether a 

 cheap compensated sidereal clock is quite an impossible thing to 

 construct ? The achromatic telescope has been cheapened, and a 

 really efficient photographic camera can now be bought for a small 

 snm. In other types and classes of instruments, however, the opti- 

 cians are content stare super vias aniiqvas. It is in hopes of 

 rousing a few of the more energetic among them to a course of 

 action which must ultimately benefit them as much as it un- 

 doubtedly must the great army of scientific students in this country, 

 that these lines are written. 



A Fkllow of the Royal Astronomical Society. 



INSTRUMENT FOR DESCRIBING THE CONIC SECTIONS. 



[186]— In answer to "F. R. H. S.," I may say that in Fig. 2 

 (p. 160) the central leg of the instrument is licld at an angle 

 corresponding with the amotmt of eccentricity of the intended 

 ellipse, while the moving leg slides up and down the same, thus 

 describing a section of a cylinder. 



As to Pig. 1, the idea was snggested by the shadow of the upper 

 part of the glolie on the gas bracket, cast by the glimmer of light 

 usually left in, on the walls of my bedroom, the style shown moving 

 ronnd a circle simply performing the part played by the rays of 



ligl't- TllllMAS 'I'llORP. 



A LUNAR ILLUSION. 



[187] — " S. H. W." will perceive, on reconsideration, that he has 

 made a mistake in his letter. No. 61, with the above title. The 

 illusion mentioned is only an illusion, and it is solely connected 

 with the projections of the moon and sun on the sphere of ■vision, 

 which may have any radius we choose to give it, and not with their 

 unei|ual distances from the eye. In the case supposed by him, viz., 

 when the moon is in the zenith, there will be no illusion ; the 

 observer would naturally stand facing the sun, and he would look 

 " square " at both moon and sun, and bo able to produce the sagitta 

 correctly, by the eye, to hit the sun. '' S. II. W." seems to bo 

 thinking of the plane containing his eye and both luminaries; ho is 

 thinking, too, of the sun itself, and not of its above projection. 

 The sagitta never points to the sun itself, except at half-moon. If 

 we know the angle between the sagitta and the lino joining the 

 moon and the sun itself, and observed the angular distance of those 

 two luminaries, wc could get their proportional linear distances from 

 the earth. Hut we can never sec that angle, since wo are in the 



plane of it ; wo know, however, its value at half-mrjon, viz., zero; 

 but wc cannot tell, by the eye, any better than .\ri»tnrchuB, when 

 it in half-moon precisely, and therefore should fail, as he did, to j^et 

 the relative distances of moon and sun. M. 



OPTICAL ILLUSION. 



[188] — Take a strip of thin card about three inches long and one 

 wide, bend up the ends at right angles, perforate one end with a 

 pin-hole and the other directly and cenccntrically op)>oeito with a 

 square hole about ene-eighth of an inch square ; place a small pin in 



From Nature, vol. x.xiv., p. 53, 5t. 



this end, so that the'head may be in the middle of the square hole; 

 apply your eye to this end, and look through both holes towards the 

 sky, and the pin will be seen apparently beyond the pin-hole and 

 reversed in position. A. T. C. 



THE FIFTEEN PUZZLE. 



[189] — If we are allowed to turn the board we can pass from the 

 lost to the won position in thirty-nine moves. 



For instance, wo may move the pieces numbered 12, 8, 4, 3, 2. 1, 

 5, 9, 13, 15, 11, 12, 8, 4, 3, 2, 1, 5, 9, 13, 10, 14, 12, 8, 4, 3, 2, 1, 5, 9, 

 13, 10, 14, 11, 7, 6, 10, 14, 15. 



There is no solution in less than thirty-nine moves, for thirty-seven 

 moves are needed to move the blocks by the shortest (laths from their 

 fii'st to last places ; and the other two moves are needed because the 

 four blocks in the middle cannot move round until one of them 

 steps out into the outer row, from which it must again return. 



I gave the above solution and proof in the Brighton Herald in 

 May, 1880. — Yours, &c. Arthiti Black. 



J. Watson gives a similar solution, thus : — 4.8.12. R. — 12.3. U. — 

 5.9.13.L. — 12.14.15.D. — 3.4.8.R. — I.2.U. — 6.10.14.L. — 8.12.D.— 

 2.3.4.R.— 5.G.1.U.--9.13.L.— 11.14.D.— 6.7.R.— 9.5.U.— 13.14.15.L. 



PNEUMA,TIC BELL. 



[100] — The pneumatic bells referred to by me (letter 79, p. 100) 

 were supplied and fixed in my house for me by a firm at King's- 

 cross, where " E. li. C." can obtain every information. I could not 

 instruct in the manuf.icture of the article. Although the fixing of 

 the tliree requisites, \-iz. (1) pipe, (2) press-button, and (3) bell, is 

 very easy, yet space in Knowledge is, I apprehend, too ^-aluable 

 for such detail as gas-fitter's work. All I can say and assure 

 " E. R. C." of is this -a more simple, reliable, cheap, and instant 

 signal, produced only by compression of the ail- we breathe, cannot 

 possibly be obtained. — Y'ours truly, M. Testes. 



P.S. I am sorry to say I could not procure Knowlepge at the 

 bookstalls of Birmingham or Lichfield, where I have been staying 

 the last fortnight, so I have not seen the recent issues until my 

 return to London. 



INFLUENCE OP SEX ON MIND. 

 III. UisTOEicAt Evidence. 

 [101] — History is conclusive as to man's mental supremacy. 

 The inventing, creating, cogitating mind is masculine. Men carry 

 on the world's business in thought and action. The ideas on which 

 depend all the marvellous acts of human intelligence, scientific 

 discoveries, jurisprudence, political, civil, military institutions, 

 maintaining the social structure, are elaborated by men. In the 

 donmin of pure intellect it is doubtful if women have contributed 

 one profound original idea of any permanent value. Jlen legislate, 

 govern, invent, colonise, fight, build, and dig. So little demand is 

 there for woman's direct help in the mental departn>ents which are 

 men's special province, that, could all the male intellect in the 

 world be suddenly paralysed or annihilated, there is not in woman 





