314 



♦ KNOAVLEDGE 



[April 10, 1885. 



■mental equation P = M/, which becomes W = M7 in the case of a 



body whose weight is W and mass is M, falling freely by gravity'. 



P W 



From thepe equations we get /= — (1) and g = — (2). 



Equation (I) expressed iu words means acceleration generated 

 Pressure producing motion 

 *■ ^ iiass moved. 



Now, let two weights \Y and W be connected, as iu Atwood's 

 machine, by a string hanging over a fixed pulley; let M and M' bo 

 their masses ; T the tension of the string. 



First consider the weiglit W, and suppose it to begin to move 



downwards. The pressure producing mttion is the excess of W 



over T, or W — T ; the mass moved is M. Apply equation (1) ; we get 



W — T W — T 

 ( f )^ — — : = g, by substituting the value of M from 



equation (2). 



Next consider the two weights W and W. Then the pressure 

 producing motion is W — W. The mass moved is M + M'. Apply- 

 Vr-Vi" W-W 



ing equation (1), we get /= 



JI + M' W + W 



- . 3 by substituting for 



M + M' from equation (2). As to W g and W ' g, there are no such 



W-W 



expressions in the equations before us. 



is a factor which 



gives the ratio / to ; 

 Atwood's machine. 



W + W 



Whence 7 is determined — the great object of 



J. M. B. 



LETTERS RECEIVED AND SHORT ANSWERS. 



Senex. Undoubtedly, what you have observed has been the 

 Zodiacal light ; but I never remember to have seen its outline so 

 ill-defined as it has been during its recent apparition. It is getting 

 full late now, though, to see it at all. Unless my memory is at fault, 

 I have myself had a prolile view uf a central mountain iu Pytha- 

 goras much as you draw it. I cannot recollect the little crater in 

 its summit, though. With regard to your second letter, the writer 

 on whom you comment is not in this country at all. Scientific men 

 are nearlj' all agreed that no evidence exists of spontaneous gene- 

 ration, or of the production of life from anything but pre-existing 

 life noic ; but that is a very different thing from the definite pre- 

 dication that it never had, so to speak, an inorganic origin in the 

 days when our world was solidifying from the primal fiery mist. 

 With reference to the age of the planets, you are confusing their rela- 

 tive and actual ages. A dog at sixteen is very old indeed. When 

 you were that age you were a boy. Assuming that all the members 

 of the solar system pass through the same sequence of changes, 

 the moon would be (viewed in the light of having accomplished 

 them) of quite hoary antiquity as compared with Jupiter, which is 

 still apparently red-hot, albeit Jupiter may have actually come 

 into existence millions of years before our satellite. Read the 

 article "Nebular Hypothesis," in " Nichol's Cyclopaedia of the 

 Physical Sciences." — A Puzzled Student. Get a piece of 

 string 8 ft. long, and describe a circle by its aid of that radius. 

 Call such a circle the earth's orbit. Now measure off two inches 

 on the circumference of your circle ; this will approximately repre- 

 sent the motion of the earth round the sun in a fortnight. At each 

 end of your arc, and at as many intermediate points as you can get 

 them in, describe circles, with a radius of i in., having their centres 

 npon it. These will picture the moon's orbit. Put dots on those circles 

 in proper progression, so that the moon they stand for shall go half 

 round the earth while the latter moves through l-l" (your two 

 inches) of her orbit. If you will join all these dots by a continuous 

 line you will soon see that the moon's path must be concave as 

 referred to the sun only. " The general form of the orbit " as 

 referred to the earth is highly complicated. — Georgicm Sidus. 

 *' Twenty-seven " was carelessly copied from 2'7. The quantity 

 given on p. 80 is probably more correct ; but nothing really is 

 known definitely on the subject.— A Salvation Army Sergeant. 

 Your letter is of too purely a theological character for admission. — 

 J. C. M. DucRoz. I have entirely lost sight of " Polyglot." In 

 reading Darwin, begin with his " Origin of Species by means of 

 Natural Selection," then go on to the " A'ariation of Animals and 

 Plants under Domestication," and from that to his "Descent of 

 Man." After this you may take the rest of his books in any order. 

 — C. F. N. alleges that the hog, having but a thin covering of 

 hair, " has developed a layer of thick fat between the skin and the 

 flesh " ; hence arguing that clothes are necessary to mankind in 

 temperate climates. For myself, I always fancied that the fat on 

 the hog was a purely artificial product, and that milk in the pig's 

 wash, together with " sharps," pollard, peas, and the like, had not a 

 little to do with it. Moreover, I seem to remember that in the 

 hottest parts of China the pigs are at least as fat as ours. — A 

 CiiKLsTiAN. I am absolutely in accord with you, but the subject is 

 one of those excluded by our rules. — Rev. Henry Briggs invites 



attention to the " Everycycle '' as "the most comfortable and easy 

 machine" he has "ever tried." Perhaps this may catch the eye 

 of Mr. Browning. — The Shade of Parallax writes to the conductor 

 of this journal from " Erewhon " anxiously anticipating the advent 

 of his (the conductor's) incorporeal essence. If Mr. Proctor can 

 by any means discover the whereabouts in space of the " Aniniula, 

 vagula, blandula" of the departed carth-llattener, though, he will, 

 as far as in him lies, himself make arrangements for going else- 

 where! — Thomas Ayees. You shoi;ld communicate your views to 

 Professor Laugley himself.- —A. K. See " The Childhood of Religions" 

 by our own esteemed contributor, Mr. Clodd. Also the paragraph in 

 capital letters which concludes those heading the correspondence 

 columns. — T. S. B. An excellent joke, but the delusion is too serions 

 a one for its main issue to be obscured by a jest, however clever. — 

 E. C. 11. I am sorry to differ with any one whose ideas are so 

 deserving of respect as those of the American aavunt whom you 

 quote ; but to pretend that (.at all events) the darker parts of the 

 moon's surface must not be highly heated by fourteen days' per- 

 jjetual, and more or less vertical, sunshine, seems to me to contra- 

 dict all terrestrial experience. Your second quotation simply refers 

 to the relative amount of heat we receive from the sun. Taking 

 it, for practical purposes, that all the solar heat passes through our 

 atmosphere, on striking the earth it becomes partly converted into 

 ihtrk heat, to which the air is largely athermanous. At night sue!) 

 of this heat as can penetrate our vaporous surrounding is radiated 

 into space. In summer, when the days are longer than the nights, 

 the terrestrial temperature does continue to increase. In winter 

 more heat is radiated than is received. — F. W. Rudleb. Thanks, 

 but the Vision-testing papers would have been of more immediate 

 interest here. 



Owing to the patriotic way in which printers have been serving 

 their country in the Weald of Sus.sex, on Brighton Downs, and else- 

 where — in fact, to Easter-holid<ayism generally— a quantity of corre- 

 spondence is unavoidably held over until next week. 



As it happened, I did not see any proof of the conclusion of this 

 " Letters Received " column, on p. 202, last week. Hence, it comes 

 to pass that my reply to Two Inquiuer.s into Knowdedge is scarcely 

 reproduced as I wrote it. The initial part of that reply should read 

 thus : " Premising that the change in the eccentricity of the 

 earth's orbit is very slow (000011 in 100 years) ; cannot continue 

 beyond 24,000 years, when it must attain its minimum of '0033 ; is, 

 in fact, caused by the attractive influence of the other planets, and 

 is confined within very narrow limits, I may proceed," &c. By the 

 insertion of a non-existent " wo " and a redundant " and " the com- 

 positor has rendered me less intelligible than I should prefer to be. 



We have received a copy of the catalogue and' descriptive price- 

 list of electrical appliances manufactured by Messrs. Weedon & 

 Irish, of the Electric Works, Sunderland. It is replete with infor- 

 mation upon all that pertains to electric lighting and other of the 

 applications of electricity. 



Society- for the Promotion of the Study of General History. — 

 The second of a course of lectures by Dr. Zerfli was delivered at 

 the Royal School of Mines, Jermyn-street, on Saturday, March 28, 

 the subject being China. The lecturer explained that this nation 

 had to be taken into consideration first, not because the ChinesT was 

 the oldest of civilisations, but because it possessed the oldest written 

 records. The most striking feature in tlie life of this nation was 

 its stationary character, it having remained in almost the same 

 social condition for thousands of years. Dr. Zerffi then proceeded 

 to discuss the causes of this strange phenomenon. The character 

 of the land in which the people lived had no doubt had some influ- 

 ence i a complete wall of well-nigh impassable mountains isolated 

 them from other races and other civilisation ; the vastness of 

 the plains and the immense length of the rivers could not but have 

 produced an effect on their ways of thinking. The chief cause, 

 however, was to be found in their stereotyped social organisation, 

 formed and worked on the patriarchal notion of government^ 

 being, in fact, but the family on a gigantic scale. He then gave 

 a brief sketch of the first five kings, and explained that the only 

 historical significance which this mythic account could have was 

 that the five kings were but the representation in traditional 

 record of five periods in the past life of the nation. He also drew 

 attention to the important part which the number 5 had taken in 

 early Chinese thought. This was followed by a rapid glance at 

 the historical dynasties up to the inviision of Kubl.a Khan, and the 

 introduction of Buddhism as the State religion in the eleventli 

 century a.d. Dr. Zerffi then passed in review the Chinese system 

 of centralisation, which, though originally founded on high moral 

 principles, had checked development and cut off all possibility of 

 real progress. The next lecture will be on the "Sacred Books of 

 the Chinese." 



