116 



* KNOWLEDGE 



[Feb. 6, 18&5 



said I thought a mutton-chop would stop the nausea. He greeted 

 tliis with a pitying smile ; but, being at home, I immediately 

 ordered one. I ate it, and was all right in five minutes. 

 " Experientia docet melius quam mille magistri." 



Hallyards. 



LETTERS RECEIVED AND SUORT ANSWERS. 



Bemkose & Sons. Received. — Hexkv J. Madge aska Mr. V,'. F. 

 Allen if he will show how much of the equation of time is referable 

 to the obliquity of the ecliptic, and how much to the eccentricity of 

 the earth's orbit ? — F. F.iKMER. I have not the least idea of the 

 name and address of the gentleman who had back numbers of 

 K.N'OWLEuge to sell. I noticed his communication and tore up his 

 letter straightway. Were you to advertise that you wished to buy 

 such, he might probably communicate with you. — J. F. Hobaet {':). 

 Be good enough to read the reply to " F. T. B.," with which the 

 "Letters Received and Short Answers" commence on p. 38. — 

 R. W. Pope. Xot yet published. — Dr. Groth. Tou have no one 

 whatever but yourself to blame. Your letters were addressed to 

 The Editor of K.vowlkdoe, and in your reproduction of them and 

 the replies to them (omitting, by-the-bye, that on p. 452 of our 

 last volume) you are guilty of the gross breach of literary etiquette 

 of mentioning the gentleman you merely suppose to have written 

 such replies by name. What would be thought of the taste and 

 gentlemanlike feeling of a person who, having invited Editorial 

 comment on letters sent to the Times or the Standard, were to 

 reproduce his communications, and such comments, under the 

 heading of " CoiTespondence between Mr. Smith and Mr. G. E. 

 Buckle — or Mr. W. H. Mudford," as the case might be :•' Aa 

 a matter of fact, Mr. Proctor was some 4,000 miles off when 

 your communications reached this office, has never set eyea 

 upon a tingle one of them, and is absolutely innocent of 

 having answered them. 1 must insist upon your immediate with- 

 drawal of his name from the heading of your pamphlet. It is 

 utterly unjustifiable. — J. B. N. informs "Sigma" that lime water 

 will effectually neutralise the free acid in rancid butter, and thus 

 make it sweet. — D. H. I cannot make head or tail of your quotation 

 from Norie's " Navigation." Astronomers have always commenced 

 their day at the instant when the sun's centre is on the meridian. 

 For example, Oh. 30m. a.m. civil time on January 1, has invariably 

 been known as December 31, 21h. 3Um,, and it was not until the 

 fii'st of the current month that the Astronomer Royal at Greenwich 

 began to reckon the astronomical day coincidcntly with the civil one, 

 by putting its commencement twelve hoars back. See p. 486 of 

 the Nautical Almanac for this year. — Protea. The great red spot 

 on Jupiter, has definitely disappeared for more than a year in all 

 ordinary telescopes, but it was still perceptible in April last in the 

 great IS'5. inch Achromatic of the Dearborn Observatory at Chicago. 

 The reason why different spots give differents rates of rotation of the 

 planet is simply on account of their actual drifting inter ^e. — Tnos. 

 J. Savage. I was careful to recognise the merit and sound common 

 sense of yonr suggestion anent printing the value of all postage- 

 stamps boldly and conspicuously on them. As far as I understood 

 the rest of your letter, it appeared tome to consist of the contention 

 that because, after a lapse of years, the Queen's features would 

 alter, that hence, it was useless to attempt to give a portrait of her 

 at any given date ; and that the present head of a youthful maiden 

 should be retained. As it is impossible that yon can have expressed 

 yourself obscurely, it must have been my stupidity which caused 

 me to misinterpret your meaning. Ilave you ever seen the 3 cent. 

 Newfoundland stamp r — M. V. The snn is certainly not due east 

 at 6 a.m., or west at 6 p.m. all through the year. He will be so 

 very approximately at Greenwich this year on March 1^0, and Sep- 

 tember 22 ; but on June 21 he will be 15' 7' north of east at 

 6h. a.m., and as mnchN. of W. at Gh. p.m. : a condition of things 

 precisely reversed, in the sense of his being as much south of east, 

 on Dec. 21. — A Chairman of School Board asks Mr. Mattien 

 Williams to recommend a simple and cheap catechism of cookery for 

 school girls. — Fred. W. Fo.ster. I am most sincerely obliged to you ; 

 and have forwarded your communication to the proper quarter, in 

 order that the valuable hints contained in it may receive the atten- 

 tion they merit. — Dr. Kegan, A. Jikes Allen. Received with 

 thanks. Undoubtedly the day is to commerce all over the world 

 at Greenwich mean midnight, and this is the time that clocks 

 everywhere must show for scientific purposes. Whether any local 

 concurrent, or, so to speak, vulgar time will also be kept, though, 

 for the ordinary purposes of life may be a matter for discussion. 

 I should think that there must be; at all events at first. — W. A. 

 Thanks ; but it is a subject with which at least two members of 

 our staff are competent to deal. — A. SaL'L has made a pair of Bell's 

 telephones from the directions given in a book — but cannot make 

 them speak ; and asks for hints in woiking them. As I have Tiever 

 seen the directions referred to, this would be somewhat difficult. 



Sprague's "Electricity" embodies as gcod practical details con- 

 cerning the construction of the telephone as you are likely 

 to get anywhere. — M. E. Mavrogoedato. How can I possil.'ly 

 explain a most abstruse mathematical question within such liniils 

 as are here prescribed to me ? See Glazobrook's " Physical 

 fiptics," pp. S25 to 331.— Poulto.k-le-Fvlde and M. J. H. The 

 A'l'ic roe?; rinics article is a not particularly brilliant specimen of 

 joking upon a small snbsti'atum of fact. Everybody knows that 

 the images of objects persist on the retina in tivie, e.ij., a burning 

 stick whirled round looks like a circle of light. A similar per- 

 sistance in sjiace on the retina is known as irradiation, and, in 

 virtue of this, all bright objects appear larger than they really are. 

 You can demonstrate this veiy easily by taking a sheet of white and 

 a sheet of black paper, and cutting out a disc from each of pre- 

 cisely the same diameter, say one inch. Then the black disc is to be 

 pasted upon a sheet of white paper, and the white disc upon a 

 black sheet. If now we regard these two discs, the white one will 

 seem notably the larger of the two, since its image encroaches on 

 the dark background, while in the case of the black disc the white 

 background encroaches on that. For the same reason, the white 

 disc continues visible at a distance whence the black one cannot be 

 seen. Hence it is true that any one dressed in white does appear 

 slightly larger than when clothed in black. The paragraphs con- 

 cerning the fat woman, the New York cars, the steamer, and the 

 battering-iam, belong strictly to the category of the anecdote of 

 the gentleman who ran so fast round a tree that he saw his own 

 back. — Tri'th will Prevail. A quasi-communistic book is sent to 

 this journal for review, and is handed for notice to a reviewer 1 

 supposed, from his knowledge of Sociology, to be competent to 

 judge of the questious discussed in it. Is he merely to prophesy 

 smooth things, or to give a candid and unbiassed opinion of 

 the woik? You are under an entire hallucination as to my 

 being " in the Tory camp." I have the most supreme con- 

 tempt for party politics. Pray obtain and read forthwith " The 

 Man reruns the State," by our greatest English thinker, 

 Herbert Spencer. Yon can buy it for a shilling. — The Duke of 

 NoRTHC.MBERLAND. Received. — P.4TERsox & Cooper. Received. — 

 Student. I am ignorant where the present series of Gilchrist 

 Lectures are being delivered. Can any reader inform our corre- 

 spondent ? It seems to me that the Gilchrist trustees take sin- 

 gularly little trouble to make the localities of their lectures public. 

 — E. R. Edis. "Spirits of salts" is really an aqueous solution of 

 liydric chloride ; which, in the old chemical nomenclature, was 

 known as hydrochloric, or muriatic acid. It does not contain any 

 zincochloride. The latter is added to it (as workmen say) to " kill 

 it;" and such" killed" hydricchloride forms, I fancy, " Baker's 

 fluid," sold for soldering purposes. — A Beginner. The only 

 method of rigidly measuring the area of a sun spot is by the aid 

 of a micrometer; but this is out of the question with a 3-inch 

 telescope not mounted equatorially. A very fair approximation, 

 however, may be made in the following manner. If you will refer 

 to page ITS of Yol. IV. of Knowledge, you will find a description 

 of the method of viewing the sun by projecting his image on to a 

 sheet of cardboard. On such a sheet, then, describe a circle of 4 in. 

 i"adius, or eight inches in diameter, and divide such diameter into 

 32 equal parts, each, of course, j in. long. Through them rule 

 parallel lines. Now cross them with 32 other parallel lines, also 

 3 in. apart, at right angles to the first ones; then shall we have our 

 Sin. circle cut up into little sqnares, \ in. each way, or the -j^th of 

 an inch in area. The image of the sun is to be thrown upon this, 

 and the card and the focussing-tube shifted about until the sun's 

 image precisely coincides with, and exactly covers, the circle. Now, 

 at the earth's mean distance, the sun's angular diameter=32'"04. 

 So that each of our divisions will represent 1' very nearly indeed. 

 But on the sun's surface (at his mean distance) 1' = 27,018 miles aid 

 a "square minute," of course, 729,072,324 square miles. Hence, 

 by noting how many of our squares and parts of squares, a spot, or 

 gronp of spots fills, we can determine its dimensions. The angular 

 distance between two stars in the sky may be determined with a 

 sextant ; on a celestial globe with the flexible quadrant ; and by 

 trigonometrical calculation from their pilaces in a catalogue. If you 

 know anything of trigonometry I will willingly explain the last- 

 named method. — Uncle John points out that the quotation from 

 Shakespeare in Knowledge for Jan. 16 (p. 51) is from " Hamlet," 

 act ii., scene 2, and should be : " Doubt thou the stars are fire. 

 Doubt that the sun doth move." Pray don't be timid. — D. Fran- 

 ciscA DE Barros e Cunha. Subject to the due acknowledgment 

 of which you speak, I shall be very pleased indeed to grant you 

 permission.— Rev. J. M. Brackexburt. I have communicated with 

 " E.R.A.S." on the subject, and he says, in reply : " The width of 

 the slots in the black disc of my anorthoscope is exactly 2 inch," 

 whence it is pretty obvious that you have hit upon the right cause 

 of your preliminary failure. — Miss C. A. Martineau. Thanks, 

 but our columns are just now somewhat overcrowded as it 



