Sept. 7, 1883 



» KNOWLEDGE ♦ 



159 



SUNSET ON THE MOON. 



[921] — Watching the moon to-night, I was trying to imagine 

 what sort of event sunset would be if one could get over the little 

 difficulty of reaching our satellite, and be independent of such slight 

 details as respiration, extremes of heat and cold, &c. I was getting 

 along splendidh- with the intensely black sky, the myriads of stars 

 shining day and night, the enormous disc of the earth constantly in 

 almost one direction, part of it bright with reflected sunlight, part 

 just relieved from darkness by a faint reflection of moonshine, and 

 the shading from light to dark by the refraction of our atmosphere. 

 I supposed the sun's disc to have reached the horizon, and (roughly) 

 in about an hour of terrestrial time his last fierce ray suddenly to 

 fade, leaving not a suggestion of twilight to show he had been 

 there. But here I was stopped. Would he disappear in this way ? 

 Of course not. Every solar appendage that can be seen from the 

 earth during a total eclipse would be always visible from the moon. 

 Consequently, when the disc (such as we see it from the earth) had 

 passed below the horizon, there would still remain a gorgeous sight 

 for our adventurous traveller [please do not call him a lunatic]. 



Now, this is my point. Between the part of the moon which is 

 in darkness and the part which is fully illuminated there must be a 

 strip which is still receiving light from the parts of the sun that lie 

 beyond the disc visible to us. Is it possible to separate and examine 

 the light reflected to us by that strip of the moon's sm-face, and so 

 to add to our knowledge of its source .' Probably the idea is already 

 familiar to you, and its feasibility may have already been tested. 

 Be this as it may, if I have furnished you with a text for some 

 future article in Knowledge, I know I shall receive a hearty, 

 though silent, vote of thanks from its readers. [I likewise bows. — 

 B. P.] W. 



FIGURE-CONJURING. 



[922] — Referring to recent communications which have appeared 

 in Knowledge, under the head of "Figure-Conjuring," I should 

 now like to give an experiment, abridged from Professor Hoff- 

 mann's book on " Modern Magic," which I think will give com- 

 pleteness to the experiments above alluded to. 



The performer asks one of the company to take two dice and 

 shake them secretly in his hands ; he must then turn them down 

 on the table, shading them with his arm from all eyes but his own, 

 and must then write down privately on a piece of paper, the 

 numbers tm-ned up, placing the same apart from each other. The 

 performer then tells him to multiply the left-hand number by 2, 

 then add 5, then multiply by 5, and then add the right-hand figure. 

 He then tells the performer the total thus obtained, and the per- 

 former after mentally deducting 25 from the same, announces what 

 two numbers were turned np by the dice. Tims, supposing one 

 die showed 4 and the other 5, then the experiment would proceed 

 thus : — 



X 2 



8 

 + 5 



Add right-hand fi'ntro 



Deduction mentally by performer 25 



Leaving -1.5. the figures turned up. 



Now, if the figure-conjuror would show off this experiment in the 

 first place, then follow it up by the one given by myself (858), and, 

 in case of difficulty with some extra-sharp victim, cap the list by 

 going through llio one given by Mr. Proctor on page 2G, I think he 

 would then astonish his audience as much as if he had gone through 

 a clever porfornuinco with the cups and balls. I cannot close this 

 communication without thanking our indefatigable editor ior the 

 article above rel'orred to; I consider it quite a valuable addition to 

 the literature of the subject. To descend from the stars to the 

 constructing of an article on Figure-Conjuring may appear to sonic 

 persona a somewhat incongruous proceeding. I, however, am quite 

 of a contrary opinion to this. To the truly great and fully-developed 

 mind nothing is too great and nothing too small. The wonders 

 revealed by the microscope are quite as astonishing and mysterious 

 aa those revealed by the astronomer's tube. Ill truth, to (he 

 scientific eye, "great" and "small" are synonymous. (;. M. 



GREATEST CONTENT OF A PARCEL WHICH CAN BE 

 SENT BY POST. 



[923] — The problem on this subject, which you have solved on 

 p. 76 by the Differential Calculus, can be solved without it, as 

 follows : — 



First. — Suppose the parcel to be a parallelepiped. Since the sum 

 of the length and girth is Oft., the sum of half the length, the 

 breadth, and the thickness, or the three dimensions of half the 

 parcel is 3 ft. It is plain that the bulk of half the parcel will be 

 greatest when these dimensions are equal, each 1 ft. Therefore 

 the content of the whole parcel is greatest when it is 2 by 1 by 1 = 

 2 ft. 



Second. — Suppose the parcel to be a cylinder. As the bulk of a 

 cylinder has a constant ratio (that of 4 to tt) to the bulk of a 

 parallelopiped of the same length and girth, the maximum 

 cylindrical parcel must have the same length and girth as the 

 maximum parallelopiped, that is, its length must be 2 ft., its girth 



4 ft., its bulk ^ t. Algeenox Bray. 



[The problem was given as an illustration of the application of 

 the differential calculus to simple questions. — B. P.] 



LETTERS RECEIVED, AND SHORT ANSWERS. 



Jas. M. Rodger. A "Bat upon a circle" may, for aught I 

 know, be j'gth of the circle itself. I do not happen to know what 

 a flat on a circle is. — Alg. Bray. Latest letter on flight of missile 

 correct, but no space for so long an investigation. Thanks for 

 other letters. — Thos. Saver. Will shortly put the list of Nos. in 

 which articles on " How to Get Strong" have appeared at head of 

 one of the forthcoming articles. — H.iKVEST Moox. There are 

 " other causes than cold condensing moisture in the atmosphere to 

 produce what is known to us as the harvest moon." — E. C. H. 

 Thanks. Highest tide about three tides after full moon. — B. J. 

 Jenki.ns. Thanks for correction. Reasoning in other letter scarcely 

 convincing.— Jas. Cram. Squares causing so much anxiety that I 

 shudder at the thought of magic cubes. — W. S. Why should that 

 meteor not belong to our system ? Meteors often behave so. — 

 Francis H. Have as yet had no opportunities to examine the 

 system. — G. G. H. Fear no space for numismatics, though the 

 subject is, no doubt, interesting. — A. M. Cobeold. The subject of 

 spelling reform — and especially of ways in which practical 

 attempts might be made to introduce it — is too wide for these 

 columns. — H. H. Map of the Moon accompanied first of 

 "F.R.A.S.'s" interesting papers. I have not vol. iii. by me for 

 reference ; know the map is in that vol. — J. S. The angle cannot 

 be trisected by simple geometry ; for the equations corresponding to 

 the geometrical requirements is a cube. I am obliged to you for 

 your kind expression of sympathy, though your " pleasure on 

 hearing of my gradual recovery" might be misinterpreted.— W. 

 Wilson. Ton are right in thinking that the Southern Cross affords 

 a perfect proof of the earth's rotundity. Yet not quite as you 

 present it ; for yon write as if the Southern Cross were a Polar 

 constellation ; and you overlook the difference in rotation-time at 

 which, in the opposite directions indicated by you, the Cross is 

 visible. An earth-flatteuer of the foolish sort would find a ready 

 answer to the argument derived from mere difference of direction 

 in the journeys taken towards the icebai-rier. But you mistake in 

 supposing the Pretty Proofs part of a discussion with the eai-th- 

 flatteners. They are so few that such a discussion would be waste 

 time. Proofs of the earth's rotundity ai-e interesting to those who 

 know perfectly well that the earth is rotund. The existence of a 

 Southern Celestial Pule round which as uniformly as round 

 the Northern pole the stars revolve in parallel circles, is 

 an absolute jiroof of the absurdity of the flat earth theory 

 for all who have brains; but it is not sufficient for the 

 flat earth men. — Q. Jordan. The Lunar Theory about which 

 the Astronomical Society was talking is not what you mean 

 by the Lunar Theory. The subject of the lunar acceleration is in 

 no way connected with the elementary phenomena of the moon's 

 movements. You are in deeper water than you think. 1 do not asso- 

 ciate you with Hampden & Co. As to them 1 mention their ignorance 

 and folly, only because of theirviolenco and rudeness.— A Would-be 

 Observer. Cannot do that.— M. U. B. Knowing nothing can say no 

 more. — F. LEriRiNGTON. Question outsideour scope. — Nigel Doble. 

 Such a telescope would be almost worthless. — R. Jones. The 

 answer relating to Text-book matter did not relate to the trisection 

 of an angle. Cannot remember answering any question recently on 

 this subject. Possibly some answer to another correspondent has 

 misled you. 1 remember only that the question related to text- 

 book matters, which would have occupied many pages of explanation 



