Nov. 22, 1888] 



NATURE 



79 



But, however far the supply exceeds the demand, there is 

 always room for what is thoroughly good, especially if it 

 has improvements that its predecessors lack. Though 

 every author is apt to think his pet methods are the very 

 best, and more or less inclined to regard his fads as steps 

 towards perfection, if not indeed its full realization, there 

 are a few who take a sounder view of things, and care 

 nothing for novelty for its own sake. The author of the 

 volume before us has shown that he is one of the few. 

 This book is of sterling value, and will be welcomed by 

 the teacher of elementary chemistry as a guide for his 

 students that he will have pleasure and full confidence 

 in placing in their hands. The volume is well got up, 

 printed in clear type, and illustrated with a sufficient 

 number of excellent diagrams, many from original draw- 

 ings made by the author. Its 272 pages are not crowded 

 with information or anything else, but the facts included 

 are clearly described in a readable and concise manner. 

 In scope, the book includes the principal non-metallic 

 ■elements and their chief compounds, followed by the 

 more important metals and their salts. The selection is 

 good and not novel. The periodic law is briefly referred 

 to, and the last eighteen pages are occupied with the 

 chemical physics that it is usually considered well for 

 elementary students to master, such as the relation 



|>etween specific heat and atomic weight, critical tempera- 

 ure, diffusion of gases, effects of temperature and pressure 

 ipon gases, and so on. 

 LETTERS TO THE EDITOR. 

 The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take to return, or to correspond with the writers of, 

 rejected manuscripts intended for this or any other part 



The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 tdke to return, or to correspond with the writers of, 

 rejected manuscripts intended for this or any other part 

 of Nature, No notice is taken of anonymous communi- 

 cations.^ 



Alpine Haze. 

 Prof. Tyndall has done good service by drawing atfention 

 to Alpine haze, and is quite right in adding that it appears in 

 horizontal layers. Such is its common form, but I have also 

 observed a vertical part of it connecting two horizontal striae 

 rendered conspicuous by concealing portions of a setting sun, 



Setting sun observed February i6, i 846, at Garuqqe, in lat. 8^ 06, long. 

 36° 51', at an altitude of 2175 metres. 



just as thick boards might do. On another occasion I saw a 

 rough column of it towards the north-west at a supposed distance 

 of three or four kilometres. A few hours later, while I was 

 noting liown the phenomenon, a native exclaimed that it had 

 changed its posiUon, and on looking north-west I could see no 

 trace of it, a column similar in size and distance being then in 

 the north-east. It towered above my level on a rolling plain 

 2300 metres above the ocean. In another place I have seen it 

 at a height of 3600 metres. 



Although generally overlooked by meteorologists, this pheno- 

 menon has a special name in warm countries. Portuguese call it 

 neblita ; in Spain it is mentioned as callina or calina, and Basques 

 name it lai'toa. Nowhere have I seen it so frequent and thick as 

 in Ethiopia, every different language of that wide region having 

 a special word to express it. The Amara call it tigag ; the 

 Oromo, or Galla, qayota ; the Tigray, taqa ; while old Ethiopia 

 employs the term qobar. I have used the latter in my published 

 accounts, because brume in French and haze in English are 

 generic and not specific names. 



Qobar is gray, and of a livid hue when intense, verging some- 

 times to blackness. The Gascon-speaking population in the 

 Pyrenees call it bruma terranera, i.e. earth-haze. Its edges 

 are not jagged, like those of clouds, but quite smooth. At 



Quarala, in 1845, when I was at the level of Lake Tana, the 

 Island of Daga, which rises suddenly 140 metres above the 

 water at an angular height of 16' and a distance of 11 6 miles, 

 was visible only by 4' or 5' of its upper part, the lower 11' or 12' 

 being concealed by qobar thicker than usual, and seemingly 

 spread on the lake. I have seen it often on the Red Sea, and 

 sometimes even here in the Bay of Biscay towards the north. 



Qobar is the surveyor's foe, and has made me lose several im- 

 portant bearings. It blurs the landscape, diminishes estimated 

 distances, and in Ethiopia is often so thick that no feature of a 

 country is visible beyond the space of a mile. 



Fifteen years ago I published in my " Physique du Globe " all 

 that I know about qobar. In Germany it goes by the name of 

 Heiderauch, or by six other words all ending in " rauch." Ethio- 

 pians also compare it to but distinguish it from smoke. When 

 commenting on chapter x. of Exodus, their native professors say 

 that the darkness mentioned in verse 22 was an iiitense ^^^Z^^r, and 

 go on to explain that the light enjoyed by the Children of Israel 

 is fully borne out by the fact of qobar being sometimes prevalent 

 in one place, yet absent in its neighbourhood. I have notetl 

 several instances of this partial occurrence. Without quoting 

 them, I may mention that, according to my working hypothesis, 

 qobar is only dry air, visible because in large quantity. On the 

 other hand, astronomers well know that very moist air is the most 

 transparent. 



Natives are swarthy in countries where qobar abounds. Does 

 it darken man's skin ? At all events it is worth while to draw 

 some hundred litres of it through suitable reagents. Chemists 

 could thus test Kaemtz's notion that it is always smoke. 



Antoine d'Abbadie (de I'lnstitut). 



Abbadia, Hendaye, November 10. 



P. S. — I forgot to mention that, after crossing the three layers 

 shown by the figure, the setting sun crossed two other layers, 

 and finally disappeared behind the lower stratum of qobar, then 

 3' or 4" above the horizon. 



Rankine's Modification of Newton's Investigation of the 

 Velocity of Sound in any Substance. 



Prof. Everett's letter (Novembers, p. 31) calls attention 

 to a difficulty which is apparently felt by students over the 

 attempted elementary method of deditcing the general expres- 

 sion for the velocity of sound given in Maxwell's " Heat." Ad- 

 vanced students need feel no diffictilty of the kind, because they 

 arrive at it by another path ; but inasmuch as the Rankine 

 method seems the easiest available to intermediate students, it is 

 desirable as a matter of pedagogy to put it in its simplest form ; 

 and so I venture to quote here the plan I have for some time 

 adopted. 



First lead up to the .subject by considering the velocity of a 

 htimp on a stretched string. Explain the plan of imagining the 

 string to move along at the same pace as the hump, but in an 

 opposite direction, so as to keep the hump stationary in space, 

 obtaining the velocity necessary to do this by equating the 

 normal compound of the tension to the centrifugal force — 



ry ds _ \ds . n"^ 





where T is the tension, and \ is the linear density of the string ; 

 and then actually show the experiment — running a light loose 

 flexible endless cord on a pulley, and making a hump on it. 

 The tension in a loose whirled endless cord free from gravity 

 being that due to the centrifugal force only, viz. — 



T = ^A-^ . ''- = Kv\ 

 r ds 



it follows that 2) = u, and so the keeping of the hump still is 

 automatic, except for a slight interference by the weight of cord 

 hanging below the hump. This interference being less and less 

 notable as the hump is initially made nearer the bottom of the 

 loop of cord. 



Next explain, and illustrate by moving diagrams, the simple 

 harmonic motion of the particles of a medium conveying sound- 

 vibrations. 



Then proceed to conskler a longitudinal pulse travelling along 

 a substance contained in a tube of unit area, and imagine a wind 

 of the sttbstance blowing through the tube in the opposite 

 direction with such a velocity, U, as just to keep the pulse 

 stationaj7 in space. 



