Maech 16, 1883.] 



KNOWLEDGE 



167 



tion, Mr. Davis only obtains a velocity of 290,000 miles per day, 

 whiTcas velocities of at least live or six million miles u day have to 

 be accounted for. But tliero is no diSiculty in conceiving that a 

 minute precipitated particle while in the region of paseous bombard- 

 ment arounil the nucleus may receive and throw off again towards 

 the .sun a continuous stream of matter many times its own weight 

 per hour. I hope that Mr. Davis will not imagine that because I 

 have ventured to make this criticism, I do not consider his numerical 

 evaluation of the repulsive force due to evaporation to be an 

 important contribution to the subject ; and I am sorry that he 

 should feel hurt at my not having mentioned his discussion, but I 

 had already far ei^ceeded the limits usually allowed to papers in 

 Knowledge. A. CowrEB Ranvard. 



COMETS' TAILS. 



[751] — Mr. Eanyard's interesting suggestions as to the forma- 

 tion of comets' tails in Knowledgk, No. 68, p. 100, may, I think, 

 be supplemented by further considering the effect of different 

 Tclocities among gaseous molecules. Supposing a comet to be a 

 swarm of stones coaling occluded gases under the sun's heat, the 

 molecules of the gas at the moment of escape are closely packed 

 together, and are, therefore, moving in all directions with indivi- 

 dual velocities varying from the point of momentary rest to a 

 (peed many times greater than the mean, and in this condition 

 they pass at once into that "fourth state of matter" in which 

 each molecule becomes a separate projectile, discharged in some 

 direction and with its own proper velocity into space. As the 

 comet's mass is too small to bo effective, the path of each mole- 

 cnlo will depend on its own motion and the attraction of the sun. 

 Those moving directly towards the sun will fly straight on with 

 accelerated speed ; those moving at any angle to this line of direc- 

 tion will take carved paths, diverging from each other. But those 

 moving directly away from the sun will pursue the same straight 

 line with retarded speed ; each one flying to a distance determined 

 by its own initial velocity and then falling back along the same 

 line. The greatest distance to which any of them can fly will be 

 that due to the greatest velocity acting against the sun's gravity at 

 the starting point, and may evidently bo vcrj- far in excess of any- 

 thing we could infer from the mere velocity of the gas. On their 

 return, the falling molecules will meet the ascendiug ones, their 

 encounters being more frequent as the nucleus is approached. 

 There will obviously be a stream of them extending as far as the 

 greatest distance of projection in a line away from the sun, and a 

 gradual condensation of this stream from its extremity towards the 

 nucleus. 



We seem here to have at least one eflicient cause for something 

 Tery like a comet's tail. Albekt J. Moir. 



March 5, 18S3. 



VALCE OP GOLD. 



[752] — About a week ago, Mr. Goschen, in the Honse of Commons, 

 mmed the members that before the Land Bill conld be properly 

 Tiewed, it would bo necessary to bear in mind (and the fact was 

 directly accepted by Sir Stafford Northcote on his authority) that 

 gold had risen in value 18 percent, within the last dozen years. 

 Now, if this means anything, it means that the purchasing power 

 of gold is greater by 18 per cent.; and that thus the farmer is 

 paying 18 per cent, more rent than he was a dozen years ago. I 

 abonld esteem it a favour on behalf of many of your readers if you 

 would elucidate this matter for us, first, by telling us how any in- 

 crement or decrement in gold value is felt and arrived at by 

 financiers, and, secondly, by what links in a chain of cause and 

 sBect it is felt by the farmer. You solve puzzles for us, and some 

 of ns are more than vague hero. Kcsxicus. 



[I leave this question for those better able to deal with it. Mr. 

 Goschen's statement is vague, but probably referred only to the 

 relation between gold and land in a particular place. — R. P.] 



VIBRATIOXS OF STRETCHED STRING. 



[753] — If a string, say 10 feet long, vibrate ten times per 

 second, and when 8toppe<I off by half its length vibrates twenty 

 times per second, and when stopped off by three-fourths of its 

 length vibrates forty times per second, by how much must it be 

 stopped off to vibrate fifteen times per second ? H. G. A. Brown. 



[The law indicated is that the number o{ vibrations is inversely 

 as the length of the unstopped portion. Wherefore wo must 

 , ' 10 20 feet 

 nave length unstopped for fifteen vibration = p: length = — g 



= 6J feet ; or 3^ feet must be stopped off. — R. P.] 



OPTICAL KNOWLEDGE OF THE ANCIENTS. 

 [754] — The collection of gems, Assyrian, Egyptian, and Grecian, 

 in the British Museum — ask any expert whotlier the majority of 

 these could have been executed witliout tho aid of magnifying 

 power H What credit is to bo attached to tho statement that Archi- 

 medes invented a machine of glass which faithfully indicated the 

 motion of the heavenly bodies, or that ho set firo to the Roman 

 ships by means of burning glasses ? Tho tirst determination would 

 go far towards settling the statement concerning tho Syracusan 

 geometrician and engineer. W. Cave Thomas. 



RELIGION AND CLASSICS. 



[755] — First, Why do you continue insidiously to carp at religion, 

 when you profess to have nothing to do with it ? 



Secondly, Why do you rail against classical education, when tho 

 ivisest sentiments of all ages (mixed, it is true, with much foolish- 

 ness and ignorance) are enclosed within the boards of classical works, 

 and no one can bo a good scholar without classical education ? 

 subscribe to your work, viz., Knowleuoe, with much misgiriny. 



Rev. Urban Smith. 



[Mr. Smith's first question reminds me of tho question shouted 

 to a candidate for election to Parliament, " Answer, Yes or No ; 

 have you left off beating your grandmother ? " I cannot answer 

 u'hij I continue insidiously carping at religion, for tho simple reason 

 that I have never begun carping at religion. Tho follies of re- 

 ligionists are not religion. I suppose Mr. Smith refers to the 

 remarks in "' Science Gossip " on certain priyers characterised as 

 akin, when rightly apprehended, to profanity ; but exception was 

 taken to profanity, not to prayer j'^r >")■ That no profanity was 

 intended is, of course, obvious enough. Not a reversal of the laws 

 of nature, but a change to bo brought about by those laws was 

 asked for. The difficulty to the student of nature is to determine 

 where any chansre can be wrought without an interruption of natural 

 laws. When William IV. was asked to sanction prayers for drier 

 weather, he said "They ought to pray for a change of wind," 

 knowing, as a sailor ought to know, that drought and wet depend 

 on wind. Science sees a natural cause lying behind tho change of 

 wind : and suspects a natural cause — not necessarily a sun-spot 

 change — behind that again. The more we know, the farther back we 

 can trace the chain of natural causation. If we knew tho whole series 

 of relations, asking for a change of weather would be at once seen to 

 mean either, (1) asking for what was hound to happen in conse- 

 quence of the natural relations involved, or else (2) asking for 

 what could not happen without a miracle. The only diff-rence 

 between asking for such a miracle and asking that — say — a shipful 

 of souls should be saved (when water had filled the ship) by a 

 temporary cessation of the action of gravity, is that in one caso 

 we ask for what would not be obviously miraculous, though really 

 60 ; in the other we ask for what would bo very obviously a 

 miracle. There is no connection between this question and religion, 

 properly understanding the word. Every reasoning mind recog- 

 nises that it is tho mental attitude in prayer that is de- 

 sirable, rather than any belief in the direct influence of 

 prayer; and there is as much of a religious nature in patient 

 submission as in earnest appeal. I know several persons eminently 

 religious in the truest sense of the word, self-sacrificing and de- 

 voted, faithful and zealous in the discharge of every duty, persons 

 whose whiile life is a religious lesson, who yet — even in the time of 

 direst trouble and anxiety, w.itching by the bed on which a much- 

 loved child lies seemingly near death, longing for the safe return 

 of those dear to them from some most dangerous journey, or them- 

 selves suffering intense bodily torture, — would feel ashamed to use 

 any form of prayer except that found in the last part of tho appeal 

 of Gethsemane. I am not concerned with tho rightness or wrong- 

 ness of these views, only with the consideration that the question 

 (as touched on in the note referred to) has, in reality, nothing 

 to do with religion, still less with any form of dogmatic 

 theology. Tho most depraved and brutal savage prays when ho 

 corses the beneficent rays of tho sun because they are too 

 warm for his personal comfort, or when he asks the ]>articuiar god 

 of his tribe to scatter the enemies of his chief, confound their 

 politics, frustrate their knavi.sh tricks, and make them fall; while 

 one whose whole life and thought are pure and unselfish, may 

 refrain, through purely religions motives, from all prayer for what 

 he most desires. Religion, which is tho quality holding a man 

 back from wrong-doing and selfishness, and binding him to duty 

 and loyalty and devotodness, tells nothing either way, and was 

 in reality not thought of when that note was penned. 



On Mr. Smith's second point, I note simply that it is hecauxe so 

 ranch of wisdom is enclosed in classical works that many object to 

 what is called — absurdly enough — a classical education. How much 

 of the wie'lom of tho ancient Greek and Latin writers, how much 

 of tho grand stories some of them have told, do nine hundred and 



