Ava. 18, If 



KNOWLEDGE 



203 



oxygen and hydrogen, in snfficient quantities, to which I referred. 

 You seemed to forget that observers hare repeatedly been before, 

 daring and after thunderstonns, in the very region where this pecu- 

 liar constitution of the atmosphere should have existed, if you are 

 right. — Canadensis. Your suggestion is excellent. As to the P.D. 

 theory, it really seemed too bad to keep all J. M.'s effusions for my 

 own particular edification. — J. M. G. The subjects of your first three 

 queries will be very suitable for future articles, scarcely for replies 

 here. (4) There are not half-a-dozen stars whose distance can be 

 regarded as determined. I place no reliance myself on any 

 parallax but one — that of Alpha Centauri. (5) Let earth's mass 

 bo 81m, moon's mass m, distance of earth's centre from moon's d ; 

 let X be distance from moon where attractions neutralised. Then 

 by the known law of gravity m-hx' = Slm-i-(d — x)' ; whence 

 J! : d — x::l : 9 ; or x : d::l : lO ; thus the body's distance from 

 the moon must be one-tenth the earth's. — Ahthck Black. You 

 have rather misapprehended my reply to " Gradatim," and the 

 nature of the question he asked (naturally enough, as my answer 

 was meant for him only). "Gradatim's" original query referred 

 to a passage in Clerk Maxwell's little book on matter and 

 motion, on referring to which you will find that the point you 

 mention is fully considered. I wrote knowing " Gradatim " knew 

 this. All these " answers " are to be read as replies to individual 

 querists, not as independent statements. — Wm. J. Logeeman' (?). 

 (1) I cannot remember receiving the letter you refer to. The 

 subject, as you describe it, would not have suited these pages; but 

 you may be assured your kind offer would not have passed wholly 

 unnoticed. (2) Our electrical contributor added the remark that 

 accidents of that kind (from lightning) are of great importance— I 

 suppose in connection with the subject of risks from electric 

 lighting. (3) Thanks for note that Evora (town in Portugal) is 

 pronounced Ayvorah — accent on first syllable very strong, so that 

 the two next are very short. (4) We by no means peevishly limit 

 Knowledge to the abstract sciences. Cricket is a fit subject for our 

 pages in the same sense that it is a fit subject for literature. You 

 say, " If cricket is a fit subject for Knowledge, could you not find 

 place for an occasional paper on ' Philology, the science of 

 languages?'" Why, certainly; we could even do that if cricket 

 were an unfit subject (also for an occasional paper on 

 Logic).— BoTD Moss, F.R.C.S. It is only vivisection so in- 

 tended and so regulated that has ever found approval here. 

 — Charles L.\ssalle. No, I do not " suppose you require any 

 puflBngs of your works " ; because none of them have been sent to 

 the office of Knowledge for review, far less for " puffings." Articles 

 which, as I judge, have already been sent to other quarters, have 

 reached me, with certificates, laudatory comments, and so forth. 

 Should I have inserted the articles, or the certificates, or both ? — 

 W. S. Montague. I would use Todhunter, I think, for study ; but 

 read Williamson's, as treating the same subject from a different 

 standpoint. — Hon. Sec. Have inserted your query, not feeling 

 competent to express an opinion. — S. W. Desires to learn how to 

 fumigate mahogany or oak wood, so as to give a deep rich colour 

 and tone to new wood. — Jonx J. Nobrin, Jun. The energy is ex- 

 pended in both ways. First, on the brake as it checks and finally 

 stops the turning motion of the wheels ; then on the rails as 

 they check and finally stop the advancing motion of the 

 untuming wheels. — Zypero. Scarcely sufficient evidence given 

 to show that that is the sole and true cause of Fairy 

 Rings. — W. H. Stone. Alas, how can we find room ?— 

 T. W. W. Would much rather say " T. W. W. Yes." Would it 

 be long ? — C. 0. The subject of " mnemotechnic formula?," is, I 

 agree with you, well suited to Knowledge.— C. J. B. lu both 

 cases the denser body falls fastest j because in each case there is 

 a certain amount of resistance to overcome in falling : the heavier 

 body, having the greater momentum, does the work soonest. Only 

 in a vacuum does a light body of the same shape and size fall in 

 same time through a given distance from rest as a heavier body. — 

 Blind-wokm. (1) Astronomy knows no Vulcan. (2) I do not 

 insist on the sun being inhabited when cool enough. Without a 

 special warmer and light-giver he might not be a very comfortable 

 world. Perhaps there might bo inherent warmth enough for all 

 practical purposes, and a proper amount of light might shine from 

 his interior, through suitable aix-rtures on largo masses of floating 

 cloud. I do not know how this will be arranged. — J. C. Lloyd. 

 It is not certainly known at what time the Mammoth lived. 

 Analysis of water, if wc have space, later. Can scarcely find time 

 for discussing specimens of flowers sent for identification. 



BOTANICAL. 

 H. H. H. (Liverpool). I do not know how to make skeleton 

 Irnvos, but 1 bolievo you will find a recipe in the book called 

 • Enquire Within." Corcagiensis (Cork). Soldinclla alpina is a 

 jilant of the primrose family, closely allied to Dodecalheon, the 

 .-ijcalled American primrose. It is not related to tho Edelweiss. 



The botanical name of the Papaw tree is Carica papaya. I hsTC 

 often heard that its leaves have the power of making tough beef- 

 steak tender, but I never experimented upon it, though it grew in 

 my garden in Jamaica. This is the first time, however, that I havo 

 ever heard of its use in preventing meat from decomposing, and I 

 suspect that your informant was mistaken. The leaves, pressed 

 against the head, make an excellent cold compress for tho headache. 

 — High School (Nottingham). I don't quite understand the nature 

 of your objection to the phrase " double calyx." I see it is used by 

 Mr. Bentham, and, I think, by all other botanists. The calyx of 

 potentilla is double, because it consist of five sepals, alternating with 

 five external bracts. Of course, if you choose, you can say the 

 bracts are not part of the calyx ; and if so minded, you may call 

 them an involucel. But I don't see that it matters as far as the 

 facts are concerned; everybody would understand at once what one 

 means. On the whole, I prefer the old statement that language is 

 meant to express otir thoughts, to Talleyrand's amended version. — 

 G. 0. Dephaga. To preserve plants, first, if possible, got a perfect 

 specimen, i.e., one with root, stem, leaves, flowers, and fruit 

 together ; but as this is sometimes practically inconvenient (for 

 example, in an oak or even a cherry tree), get as near it as you 

 can. Lay it flat between several sheets of coarse, unsized 

 paper (not blotting-paper), taking care to place the jiarts as 

 separate as possible. Press it with a big, heavy book. Change 

 the paper once or twice, till the plant is dry. Thick- 

 stemmed and thick-leaved specimens may bo split with a knife. 

 Verv succulent plants may be dipped in boiling water ; but 

 the "flowers must not be immersed. You had better try Bentham's 

 ' British Flora," which will not only give you full instructions for 

 preserving specimens, but will also enable you to identify all 

 flowering plants, ferns, horsetails, or club-mosses, indigenous or 

 naturalised, in the British Isles. lint you must be prepared to 

 take a little trouble. — A Reader (London-street, Edinburgh). 

 Sporting in cultivated turnips may be due to any of three causes — 

 first, to reversion; second, and most frequently, to changed condi- 

 tions of soil, climate, manuring, Ac. (for all plants sport most 

 often under cultivation) ; and third, to hybridisation by insects 

 with the other ctiltivated varieties of Brassica campestris, namely, 

 swedes and rape-seed, or with closely-related wild species, such as 

 charlock ; but this last cause would, I think, operate less often. 

 It is impossible to be sure in any case, except by isolating your 

 causes. You may experiment for yourself, but where the conditions 

 have occurred spontaneously you can only surmise. 

 CHEMICAL. 

 X. Y. Z. The papers, according to your description, are pre- 

 pared correctly; but the indicati(ms afforded are uncertain, as other 

 causes affect the paper. Thus, the rapidity of the action is modified 

 by the humidity and temperature of the air ; further, paper once 

 coloured becomes again decolourised by exposure. Other gases 

 also liberate the iodine, and thus colour the starch. Honzenn 

 prefers litmus paper, slightly reddened, and impregnated with 

 potassium iodide; this paper is rendered blue by ozone, the same 

 colour not being produced by any gas except ammonia.— F. T. 

 Drake. It is not wise to inhale or administer laughing gas, except 

 by and with tho advice of a medical man or other person duly 

 expcrieBced in the use of the gas (such as a qualified dentist). 

 The gas, when required for breathing purposes, should also bo 

 prepared and purified by a comjietent chemist. 



^ir iHattjtmatical Column. 



EASY LESSONS IN THE DIFFERENTIAL CALCDLUS. 



No. VIII. 



Bv Richard A. Proctor. 



THE following examples may servo conveniently to illustrate tho 

 ajiplication of even tho first principles of tho differential cal- 

 culus to problems which otherwise would present considerable 

 difficulty : — 



Pboblkm I. — I7i« captain o/ a racing boat has a vacancy in his 

 crew, and is desirous of so Jillini) it as to counterpoise ths boat as 

 much as possible on the side of the new comer. The breadth of tht 

 thwart tn be filed is 2/f. ; assuming a man's u-eight to be proportional 

 to the cube of the linear distance h» occupies on tht Ihvart, and that 

 a man who would occupy but haJf the thw<trt weighs six stone, what 

 would be the beet leeight/or the new oarsman .' 



A G B M C 



Fig. 3. 

 Lot AC, Fig. 3, represent the width of the thwart, \ M that of 



