loO 



KNOWLEDGE 



[F£B. 20, 1S85. 



bearings in a tricycle the following all-important qualifications : — 

 " The interposition of a lubricant modilies this law in a very 

 remarSablc maoner, so that it is necessary to calculate upon a [larti- 

 cular co-etBcient of friction dependent upon the nature and state of 

 the lubricant, and upon its mode of supply whenever any of this 

 class of material is used." Without this qualification the statement 

 of the law is simply misleading. Asa general rule, "the tliiimer 

 the lubricant the better ; " but this rule again requires the qualifica- 

 tion, " if a constant supply of the lubricant be insured." 



I was not writing with the intention of making a strictly accurate 

 theoretical statement, but only giving rough practical rules for 

 keeping the bearings of tricycles in good order with the least 

 possible trouble. For this reason I directed attention particularly 

 to the lubricant to be used. 



When I was writing I had in my mind some practical experiments 

 on friction recently made by a Committee of the Mechanical Engi- 

 neers, of which I can inKxonLKDGE only quote the result ; — "Before 

 considering thoroughly lubricated bearings, it will be well to state 

 the laws of fluid friction— that is, the friction between surfaces of 

 liquids and .solids. We have seen that friction between solids 

 depends on the intensity of pressure between the surfaces, also on 

 the area in contact, on the nature of the surfaces in contact; but 

 is, at all events, for metals, independent of tJ'c t-ehxily. On the 

 other baud, the friction between a solid and a liquid is independent 

 of the intensity of pressure and of the nature of the solid : though 

 it depends on the licjuid, it also depends on the area in contact and 

 on the velocity. These differences will repay consideration. The 

 exjioriments have shown that with good fitting and thoroughly 

 lubricated journals, the friction is indcpoidcnt of the load as long 

 as the luhricanf is not squeezed, out, and that it depends on the 

 relocitij. This fact points out that the friction is really fluid friction, 

 and the importance of this cannot be over-estimated. It means 

 that the bearing must fit well to insure a uniform continuous film 

 of lubricant, and that the intensity of pressure must not be so 

 great as to squeeze out the lubricant. Of course, the heavier the 

 pressures the thicker the lubricant must be." Jou.n Bkownixg. 



LETTERS RECEIVED AND SHORT ANSWERS. 



A. Howard. Tour quotation (of the source of which I am 

 ignorant) is a mere quibble. Of course, the stars (suns) which 

 were in existence prior to our sun emitted light, as did, in fact, 

 our entire solar system in its inchoate or nebulous stage. What 

 " men of science . . . once ridiculed as absurd " — and, I may add, 

 ejntinue to ridicule — is that datjlijht, or day and night, existed, or 

 could possibly exist, on the earth before the sun assumed its 

 present ci ndition. — BECc.^isr.NGA sends an extract from a con- 

 temporary on testing butter, in which it is stated that, if a knife- 

 blade be drawn through oleomargarine, the place where it was pressed 

 will look white, and its track be marked with little beads of water. 

 Genuine butter isalleged to retain its colour under these cumlitions. 

 Again, oleomargarine is said to sputter in a fryinf.-,jan ; butter 

 merely to melt and bubble. He asks if there is any truth in this ? 

 — B. Valers (?). The twentieth century obviously cannot begin 

 until the termination of the nineteenth — i.e., of the year I'.WO. 

 When a baby is born, it enters its first year; but is, of cause, 

 not one year old until it has Ih.'CU 3G5 days in the world. In 

 the same way (if we assume the correctness of our orthodox 

 reckoning), a.d. 1 began with the birth of Jesus, and was not com- 

 plete until the expiration of twelve months, whence it must be 

 immediately apparent that the First Century was not completed 

 until the end of the year a.d. 100. The first day of the Twentieth 

 Century, then, will be January 1, 1901. — TRfxn Will Prevail. 

 Had you spent the single shilling necessary to purchase Herbert 

 Spencer's " The Man eersus the State," and read it dispassionately 

 through, you would have found our Reviewer was not " asj^ersing cer- 

 tain jjolitical princiiples," but stating a mere matter of hard, dry fact. 

 These columns are not the pdace for any mere political discussion ; 

 but, if publishers send books for review, they must bo content 

 to have them honestly noticed — or consigned to the waste-piaper 

 basket. For m^ self, I would absolutely exclude alike the dogmatism 

 of Lord Salisbury aiul the rant of Mr. Henry George from the pages 

 of this Journal. Those who credit the conductor of Knowledge 

 with the authorship of everything that appears in it may note, once 

 for all, that J/r. I'roclor signs every single arlicle, letter, and 

 paragraph tliut emanates from Ins pen. What is not so distinctly 

 signed in this magazine is none of his. — E.xcelsior. Doubtless 

 Professor Harley has been misreported. Both Deimos and 

 Phobo? travel round Mars in the fame direction. What, perhaps, 

 he did say — although this is only a conjecture — was tliat, as Phobos 

 goes round tlie planet in a little more than 7h. 39m., and Mars 

 himself takes 21h. 37m. 23s. to complete one rotation on his axis, 

 this inner satelUto must rise in the West and set in the East, 



while the rising and setting of Deimos, the outer satellite, is 

 normal. — F. C. Thompson'. See reply to J. Beid below. — 

 D. H. If the construction you put upon the passage in Norie's 

 " Navigation " be the correct one, Norie is wrong — that's all. 

 Astronomers universally regard Dec. 31 21h. COm. as 21b. 00m. 

 after tlie beginning of Dec. 31, and would hence call Dec. 31 2lh., 

 Jan. 1 Oh. But I must reiterate (and once more refer you I<> 

 p. 480 of the " Nautical Almanac" for the present year for con- 

 firmation of such reiteration) that up to the 1st day of IS'85 tie 

 astronomical day began at noon on the civil day. In poii.t 

 of fact, it does so begin in that very volume itseif. 

 I am igrorant of the address of the maker of the 

 " Hatherley Steps." — A No.\-Y. Z. Yes, entirely wrong. 

 Read the " Pneumatics," in " Weale's Series," or any other ele- 

 mentary book. Your theory that earthquakes have their oiigin in 

 the admission of water into the molten interior of the earth, and 

 its conversion into super-heated steam, is (pray forgive me for 

 saying) " as old as the hills." — Commext.4tor. The writer is in the 

 United States. The parallax of the Pleiades is quite insensible. 

 In the existing state of our knowledge, they are at an infinite 

 distance. No doubt the individual members of the group are at 

 stupendous distances (in comparison, that is, with mean distance of 

 Neptune from the sun) from each other. I do not see why thero 

 should not be "night tkies" to inhabitants of worlds circulating 

 round individual suns in such a group. A\'ritiDg which words 

 suggests to me to add that these suns are already formed. What 

 we see in the heavens is nebula? in all stages, from the mass of 

 luminous gas, through a central'y-condensed spherical aggrega- 

 tion, to the incipient stars or suns. You are under an utter 

 mistake about the resolution of the nebula in Orion. Our greatest 

 living English spectroscopist. Dr. Huggins, has shown it to consis.; 

 of a mass of glowing gas; so Brewster was simply talking nonsense. 

 There are numerous undoubtedly gaseous nebuhe in the sky. Dici 

 you ever hear of spectrum analysis ? The bigger a telescope, the 

 more light it grasps, and the smaller the stars it renders visible. 

 It does not follow, though, by any means, that the smallest stars 

 are the farthest off. We have this curious jnoof that many of the- 

 groups of stars in the heavens are physically connected : that they 

 are all moving in the same dirc-tiou in space. A striking example 

 of this is seen in LTrsa Major. We can only reason from analogy 

 as to the habitability of the members of our own solar system. We 

 can, and do, know less than nothing of the millions of worlds re- 

 volving round other suns than ours. — Poingdestee-Mesnier et Cie. 

 Should hare been addressed to the advertising department. — 

 J. C. S. What you describe seems to be simply a waking dream, 

 started by excitement of the retina. It scarcely possesses suificient 

 scientific interest to warrant its insertion. — Akshay KiMAR Das. 

 Forwarded to the j-ublishcrs. — F. W. H. Will be printed, subject to 

 the excision of the theological paragraph. — J. Reid. Forgive me if 

 I say that I cannot give up the space to your long defence of your 

 hypothesis. Doyou know that water is found in small cavities in the 

 ([Uartz of granite, iSrc. ? Cast iron mmj, like bismuth and antimony, 

 tjuite possibly undergo slight expansion at the very instant of its 

 piassage from the liquid to the solid state ; but this is really irrelevant 

 in connection with your argument; because, for example, granite — of 

 which I have just spoken — can never have been actually molten. — 

 H. Hanks. A patent was granted, but if is now void — W'. G. WooL- 

 cojiiiE. I fancy that the questions which have been set; are pub- 

 lished, but I am ignorant by whom. The head office is at South 

 Kensington, London, S.W. Try Williamson and Tarleton's- 

 " Dynamics," published by Longmans & Co. — A-nthropological 

 Institi'te. Crowded out unfortunately by press of matter. — G. H. 

 DuN.v asks for authenticated instances of hybrids breeding. Our 

 correspondent is dissatisfied with Hacckel's illustration of the hare- 

 rabbit. I can supply none myself, and would only remark that 

 Daiwin attributes the seeming non-fertility of hybrids to the fact 

 that in experimenting they have been mostly bred in and in. — 

 J. S-MITH. You have get hold of a Gregorian reflecting telescope. 

 Remove your red glass, turn the open end of the tube towards the 

 shipping in the Downs, look through the " microscope," and focus 

 by the arrangement which shifts your "small reflector."— S. E. 

 Phillips. Forwarded to the publishers. — M. Segond. Can only 

 appear as an advertisement. — Com.mentator. Marked f i r insertion. 

 — TnoM.AS Ayers. I have read your letter and your article in the- 

 newspajier, neither of which give (or, in fact, can give) any indi- 

 cation of the way in -n-hich heat produced by concussion or com- 

 pression can, in any .sense, have its origin in " processes of chemical 

 change." Pray obtain and carefully read through Clerk Maxwell's. 

 " Theory of Heat," in Longmans' " Text-books of Science." 



The dividend of the Telegraph Construction and Maintenai.ee 

 Company was in all 20 per cent, for last year. 



