April 14, 1882.] 



• KNO\VLEDGE • 



521 



I will name his coutradictioDS 1, 2, 3, 4, 5, to save space : — 



1. Bird. Cents. 1879. 634. 



2. ., ,. 1880. 472-471. 



3 & 4 1879. 106-108. W. Paulson. 



5. ., ,. 1880. 868-870. W. H. 



P. C. 



TRICYCLES. 



[382] — As a bicycle and tricycle rider of many years' standing, 

 and one who has tried many kinds of tricycles, I would strongly 

 warn your rcadera against taking Mr. Browning's advice, and 

 getting a "creeping thing" in the way of a tricycle, with small 

 wheels, under any circumstances. It is obvious that the smaller 

 the wheels, the more must inequalities in the ground and stones 

 inconTcnience the rider, and this is an important point. Large 

 wheels pass unjoltingly over an obstacle that would clash the teeth 

 on a small 36-iu. wheel. 



In case of an emergency, I should not like to have my toes in a 

 stiiTup, as with the Monarch. It might be very awkward. 



My settled opinion is, that a 50 or 52 in. Cheylesmore is the per- 

 fection of tricycles. The machine is beautifully finished, very light, 

 and yet strong, and is also one of the fastest runnci-s and easiest 

 hill-climbers. Then comes the Kucker, the Premier, and perhaps 

 the National. The Ilumber is fast, but I concur in Mr. Browning's 

 remarks upon it in other respects. 



It is a great mistake to have too heavy a machine, no matter the 

 rider's weight, and strength can be secured without ponderosity. 



I have just had a three days' ride to and along the salt coast and 

 back on a Cheylesmore (about 190 miles), and rode every hill both 

 up and down (although I crossed the South Downs), and experienced 

 no fatigue whatever. 



I fear the rotarj- Coventiy is too narrow to bo safe from over- 

 turning at a high speed. 



The Cheylesmore is, I find, much less fatiguing on a long ride, 

 infinitely more comfortable, and averages as fast a pace as a 

 bicycle. Of this I am perfectly convinced from my experience. 



I should like to dilate on the delights of tricycling and incidental 

 matter, but I fear your space is too valuable to be afforded for the 

 purpose. I should, however, be happy to give any practical informa- 

 tion, should readers desire it. — Yours faithfully, Ex-Bicyclist. 



HALO ROUND THE MOON. 



[383] — On January 29, between 6.20 pnd 6.40 p.m., I saw a very 

 peculiar halo round the moon. I was in Tokiyo (Yedo) at the time, 

 and the same a^■pearance was seen by many of my friends at Yoko- 

 kama, 18 miles distant. It was like the figure herewith. At first 

 I was nnder the impression that the figure was slightly elongated 

 on the outer circle, that is to say, the distance from the centre of 



the moon to the edge of the first halo was less than that from the 

 edge of the first halo to the farthest point in the second ; but on 

 drawing it I am convinced that this was an optical illusion. The 

 lower part of the second halo intersected the lower horn of the 

 moon in a peculiar way, as given in the drawing. The moon was 

 nearly half full.— Yours, &c., 

 Yokohama, Japan, Feb. 13, 1882. H. Pbyeb. 



PERSONAL ILLUSION. 



[384] — I observe in your issue of March 18 that a correspondent, 

 under the nom-de-piiim« of "Jumbo," mentions as an instance of 

 the above the fact that to him " horizontal lines appear plainer 

 than perpendicular'' ones. I beg to suggest that this phenomenon 

 more probably depends on a physical than a psychical cause. It is 

 produced, in the great majority of cases, by a difference of refrac- 

 tion in the vertical and horizontal meridians of either the lens or 



cornea of the eye. The effect of this is to cause the one set of 

 lines to be brought to a focus on the retina, while the ones at right 

 angles to them aie not. This condition car be corrected in most 

 cases, to a great extent, if not entirely, by the use of cylindrical 

 glasses. G. A. Hkk.schell, M.D. Lond. 



[Answered by W. R. D., F. T. W., and several others.— Ed.] 



REPLIES TO QUERIES. 



[318] — Vegetarian books and pamphlets are numerous, but 

 perhaps G. A. S. would find what he wants in Dr. Nichol's " How 

 to Live on Sixpence a-Day," price 6d. [Nichols & Co., 23, Oxford- 

 street, London]. Of cheap cookery books there are, " How to 

 Spend Sixpence," price Id., " The Penny Vegetarian Cookery," and 

 the " Food Reform Cookery Book," price 2d., to be had from the 

 Secretary of the Vegetai-iau Society, 56, Peter-street, Manchester. 

 I should be glad to answer any letters addressed 46, Meadow-street, 

 Moss Side, Manchester, as a full answer to G. A. S.'s query would 

 occupy more space than the editor could spare. — A Fellow of the 

 Chemical Society. — [Many other replies received ; but the above 

 will suffice : and more also. — Ed.] 



[337] — Self- Acting Blowpipe. — Such is for sale in the better 

 class of tool-shops. A vessel with a screw top holding spirit, has a 

 small pipe leading from near the top of its interior through the 

 bottom, where it is turned at a right angle. A spirit-lamp under- 

 neath heats the spirit in the vessel ; the vapom- given off is expelled 

 through the pipe, and passing over the flame of the lamp is ignited. 

 A long roaring flame is the result, but it is not so hot as a good 

 blowpipe flame, as the combustion is only that of the spirit without 

 admixture of air. The flame is a large one, and is quite unsuited 

 for soldering, but I have used it for manipulating glass tubing, as it 

 allows the use of both hands. — Clake. 



ginstoers to Corif^pontifntisf* 



*^*AU eommunuafions for the Editor requiring earltf attention should reach the 

 Office on or before the Saturday preceding the current issue of Knowledgb, the 

 increasing circulation of which compels us to go to press early in the Keek. 



Hints to Coeeespoxdbnts. — 1. No questions asking for scientific information 

 can be ansicered through the post. 2. Letters sent to the Editor for correspondent* 

 cannot be forwarded ; nor can the names or addresses of correspondents be given in 

 answer to private inquiries, 3. Correspondents should vrite on one side only of 

 the paper, and put drawings on a separate leaf. 4. Each letter should have a title, 

 and in replying to a letter, r^erence should be aade to its number, the page on 

 which it appears, and its title. 



ASTRONOMY AND MATHEMATICS. 



J. MtTRRAY. How severe the P. D. system must be, which will 

 not allow Venus to be more than 5,000 miles in diameter ! It is 

 some comfort to know that, as " gravitation or retardation is a 

 compound body, it is hard to say to a 1,0U0 miles off-handed." 

 Yet you say, " Without a dought " (the worst spell of doubt I have 

 yet experienced) Mars is only about 1,600 miles in diameter. — 

 A. N. S. (1) Thanks for calling my attention to the undoubted 

 mistakes occurring in the times noted p. 468. I took down the 

 Nautical Almanack for 1881, instead of 1882. Fortunately the rea- 

 soning is in no sense affected by the mistake. (2) Astronomers find 

 it convenient to take the sun's centre ; that is the only reason I can 

 think of. — D. Maxwell. Your reasoning is incorrect, though plausible. 

 You cannot so simply deal with the attraction of the segment of a 

 sphere, as to assume that it acts as though the whole mass were at 

 the centre of gravity. The sphere (either of uniform density, or 

 composed of concentric shells, each of uniform density) is the only 

 solid whose resultant atti-action on a particle is the same as though 

 its whole mass were collected at the centre of gravity. As a matter 

 of fact, the force of gravity within a globe of uniform density 

 diminishes as the distance from the centre. Note also, that if 

 tlirough a particle in such a globe, a spherical surface con- 

 centric with the surface of the sphere be sui^posed to pass, 

 the portion of the globe outside that inner spherical surface 

 produces no effect whatever on the particle. — M. Boyce. 

 Your friend was right. The sun is far north of east at 6 a.m., 

 either mean or solar time. By solar time, he crosses at 6 a.m. 

 and 6 p.m. the great circle passing through the east and west points 

 and the pole of the heavens (a declination circle), not the great 

 circle passing through the east and west points and the zenith. — 

 A. S. T., M. H. C, Erin-go-Bkagh. Thanks. — Noblehill. I beg 

 to inform you that pressure would not vanish at the earth's centre. 

 Pressure there is not the result of gravity there, but of gravity else- 

 where, and therefore your reasoning has no weight. The matter 

 above the earth's centre has, on the contrary, a great deal of 



