Nov. 



1881.] 



* KNOWLEDGE 



TO 



lttUt& to ti)t eUitor, 



[I^* EJUor does not hvU kimtelf reppontihU for iheopinion» of his correspondents. 

 Be cannot undertake to return manuscripts or to correspond with their trrtters. He 

 requests that all communivatiotu should be as short as possible, consistently with full 

 and clear statements of the tcriter'a meanitiff.'] 



All eommunications should be addressed to the Editor qf Knowledge, 71, Great 

 Queen-street, W.C. ^ , 



All Cheques and Potl-OJice Orders to be madt payable to Messrs. Wyman 4- 



SOHS. 



•^* All letters to the Editor will be Numhered. For convenience of rtference, 

 correspondents, vhen referring to any letter^tcill oblige by mentioning its number 

 and the page on which it appears. 



AU Letters or Queries to the Editor irAiV A require attention in the current issue of 

 Kjrowi.BDQB,»/iouW reach the Publishing OJice not later than the Saturday preceding 

 ike day <f publication. , 



**Iii knowledge, that m»" only is to be contemned and despised who is not in a 



BUte of transition Nor is there anyihing more adverse to acturacy 



than fixity of opinion." — Faraday. 



"There is no harm in making a mistake, but ^rreat harm in making none. Show 

 me a man who makes no mistakes, and I will show you a man who has done 

 nothing.' ' — Liebig. ,, 



(9ttv CorifSpontfitrt Columns. 



TO OUR EEADERS.— NOTICE TO CORRESPONDENTS.— 

 THE SUN'S HEAT.— WASTE OF ENERGY.— THE EARTH'S 

 REVOLUTION.— THE FIFTEEN PUZZLE.— FLIGHT OF 

 BIRDS.— CORRECTED SUN-DIAL. 



We begin to see more clearly than we did at first the lines on 

 ■which our Correspondence columns will have to be conducted, 

 though we shall always be glad to haye hints and suggestions from 

 our readers towards the improvement of this section of Knowledge. 

 Dropping the editorial "we," let me note that from the beginning 

 I have proposed to take my share in the Correspondence, not only 

 in answering queries, explaining dilEculties, &c., in subjects I am 

 able to deal with, but also in asking questions, inviting explana- 

 nations, and so forth. It will be found, also (indeed, I think the last 

 and present numbers give some illustration of the fact), that space 

 will as readily be found for corrections of the mistakes I make 

 myself, as for those into which others may fall. In the two first 

 numbers I appended, in several cases, my replies to the respective 

 letters to which they related ; but hereafter I propose to reply each 

 week to letters and queries falling into my department which have 

 appeared in the previous week. Others have promised me their 

 assistance in dealing similarly with other inquiries. But I hope 

 many who have information to give on subjects of interest respecting 

 which inquiries may be made, will do so. as occasion may suggest. 

 We wish to leave as few inquiries unanswered as possible. 



I take this opportunitj' of reminding correspondents that their 

 letters should reach the oflice by Saturday at latest, if they are to 

 appear in the number for the following week. Already, owing to 

 the unexpectedly rapid growth of the correspondence, it has been 

 found necessary to arrange that no letters received after Monday 

 shall be even read till the matter for the following week has been 

 made up ; and, of course, in making room for letters, the rule " fii'St 

 come, first served" must be attended to, not indeed with absolute 

 Stringency, but in the main. We must also beg our correspondents 

 to bear with us if we are obliged to abridge some letters. There 

 shall be no favouritism, either in selection, or in inserting more or 

 less from letters which reach us ; in every case we shall be guided 

 by our view as to the wishes and requirements of our readers. Nor 

 need correspondents be surj^rised, any more than we are, if everj- 

 letter they may send us, or every part of each letter, should not be 

 thought precisely what would suit the plan and purpose of Know- 

 ledge. 



Taking first letter 2, p. 13, I note that " Anti-Guebre's " diffi- 

 culty is one which I have found, during my lecture tours in Great 

 Britain, America, and Australasia, to be more common than any 

 other whatsoever. I suppose I must have received more nearly 

 a thousand than five hundred letters presenting precisely " Anti- 

 Guebre's" difficulty, so that, I may remai-Ji in passing, Mr. Newton 

 Crosland must not regard himself as sole inventor and patentee of 

 the paradox in question. The answer is not so simple as many seem 

 to suppose. None of our correspondents have, indeed, answered 

 "Anti-Guebre's" difficulty correctly and fully. K we remember 



that the existence of snow in large quantities on mountain tops and 

 of ice particles in the cirrus clouds, implies the prior existence at 

 those heights of large quantities of the vapour of water, we shall 

 see that the asserted dryness of the higher air (and, therefore, the 

 unimpeded passage of "heat rays through such air) can hardly be 

 regarded as demonstrated. Again, to say that rarefied air has no 

 capacity for heat is, in reality, to speak in terms belonging to the 

 old and erroneous theory of heat as a sort of fluid. Many seem to 

 imagine that the mere rarefaction of a gas is a cooling process, 

 overlooking the statement which accompanies all correct accounts of 

 experiments on the subject, that it is not the rarefaction itself, but 

 the work done in expansion that causes loss of heat. In like manner, 

 when pas is compressed, it is not heated because of its greater 

 density, but because a considerable part of the extraneous work 

 applied to produce compression is transformed into heat. In one 

 case we have force obtained at the expense of so much heat, in the 

 other heat is produced by the expenditure of such and such force. 

 If the air around lofty mountain tops were simply veiy thin, and 

 there were never any interchange between the higher and lower air, 

 not a particle of snow would ever be seen on our mountain tops, 

 nor should -sve ever see the cirrus or feather)- snow clouds in 

 the upper aii-. Bat because theie is interchange, because the 

 air which rises along mountain slopes expands and docs work 

 in expanding, it is made cooler and cooler, tiU at great heights 

 it becomes altogether refrigerated, while the air which de- 

 scends to replace the air which has risen becomes warmer, 

 because in descending it is compressed by the action of gravity (an 

 extraneous force) and a portion of the force thus exerted appears in 

 the form of heat. Again, in the case of the upper feathery snow- 

 clouds, the refrigeration comes mainly from the ascent of aqueous 

 vapour. The wool-pack clouds wliich we see on a summer day are 

 the upper parts of cjlumns of ascending aqueous vapour, where the 

 vapour has bt en cjudens-nl to i\aer particles or vesicles, and so 

 appears in the form of visible cloud. At the upper surface of these 

 clouds a process of evaporation is continually taking place, and 

 currents of vaporous air are continually ascending. When these 

 ascend high enough to be sufficiently refrigerated, they form in 

 turn into water-drops or vesicles, but under different circumstances 

 from those prevailing where the cumulus or wool-pack clouds are 

 formed. The refrigeration is more rapid owing to the rarity and 

 relative dryness of the higher air, and thus the water particles up 

 there form snow-crystals, and (under the conditions stated) cirrus 

 clouds are formed. That they are not always formed is due to the 

 circumstance that these conditions do not always prevail. 



As to the waste of solar and stellar energies referred to in the 

 same letter, it is probably only the limited nature of our knowledge 

 which causes that to seem like waste which may, for aught we 

 know, involve the most perfect adaptation possible of the energies 

 in question. We may say of such inquiries, with the poet : — 

 " Reason, alas ! it does not know itself ; 



Yet man, vain man ! would with this tiny plummet 



Measure the deej) profound. 



He sees but part of the chain — the nearest links — 



His eye not reaching to the equal beam 



That poises all, above." 

 We need not infer that what we see is seen incorrectly because we 

 see not all. 



In letter 5, p. 35, Mr. Crosland uses arguments which only 

 requii-e to be understood to be refuted. What meaning can be 

 found in the statement that " light and heat are phenomenal pro- 

 ducts, caused by magnetic and electric forces, in a state of intense 

 activity, acting upon atmospheric conditions " ? Why not equally 

 well say "Magnetism and electricity are phenomenal products, 

 caused by luminous and calorific forces, acting under aqueous con- 

 ditions " ? Again : the sun might " possess the power of producing 

 the phenomena of incandescence, without being itself incandescent." 

 But, as a matter of fact, he is incandescent, if incandescence means, 

 as it does, glowing. Electricity produced in another room may 

 make a wire on the table at which I write white hot ; but when 

 looking at the wire I see it to be glowing white, and when by actual 

 tests I find it to be radiating heat, I cannot agree that it is neither 

 incandescent nor hot, because something which is neither glowing 

 nor a som-ce of radiant heat may yet produce both light and heat. 



Several correspondents think " Tyro's " letter (6, p. 35) should 

 not have appeared. It is not very pertinent, but it serves to show 

 one chief way in which paradoxes arise, viz., from want of thought. 

 " Tyro " must have seen hundreds of cases where a luminous 

 body causes a distant opaque body to appear bright, without any 

 flood of effulgence along the space between j yet he expects to see 

 that on the large scale which ho docs not see on the small scale. 

 I do not take exception to his remarks as relating really to light, 

 not to heat, because I assume he considers that where there is 

 lustre (inlierent) there is also heat. It is not always, though it is 

 generally the case. 



