248 



NATURE 



{Jan. lo, 1889 



meaning than a " triangular square," Martins divides his 

 " brotnllards sees" into four classes, viz. volcanic ashes as seen 

 in the year 1783 ; smol<e from turf or stubble fires ; callina or 

 qobar ; and a fourth kind established on negative evidence which 

 seems untrustworthy. 



W. Clement Ley has described quite well the hues of <7o/'a;-. It 

 is light buff when near or slight. Otherwise, its colour is a 

 lurid gray verging to blackness. Whatever may be its connec- 

 tion with cumuli in England, I could detect nothing of the sort 

 in Ethiopia, where I have watched qobar for whole weeks 

 without any ensuing rain or even cloud. 



Four years ago the French transit of Venus expeditions agreed 

 to investigate the amount of carbonic acid gas in the air of their 

 several stations. Mine was in Hayti, where qobar was rife, and 

 while observing for many consecutive hours the passage of air 

 through caustic potash in prepared tubes, I regretted their not 

 being made to receive plugs of loose cotton in order to collect 

 smoke, dust, or microbes. All the'tubes having been subsequently 

 tested in Paris by Prof. Miintz, he obtained the unexpected result 

 that air contains more carbonic acid in the southern hemisphere 

 than on the north of the equator. Those tubes inclosed also 

 fragments of pumice-stone previously steeped in sulphuric acid in 

 order to collect moistv:re. With a little care and trouble next 

 summer in a spot of Southern Europe where qobar abounds, 

 meteorologists might soon get an insight into its true nature. 



January 3. Antoine d'Abbadie. 



Several communications have appeared in Nature on the 

 subject of atmospheric haze. It would be interesting to know 

 whether the writers consider the haze which they have described 

 as identical in substance with that which I would call ordinary 

 atmospheric haze. The haze of these writers is a haze taking the 

 visible form of layers or bands. The haze to which I refer has 

 under ordinary circumstances no visible form at all. We are 

 conscious of its presence by its effect in diminishing the trans- 

 parency of the air. Everyone knows that, quite apart from fog, 

 or smoke, or dust, or low cloud, or falling rain, the transparency 

 of the air varies very greatly at different times. In our climate 

 there is nearly always more or less of atmospheric haze, the rare 

 exceptions proving the rule ; and the haze may be so dense as to 

 render terrestrial objects invisible at a distance of a very few 

 miles. Celestial objects may also be obscured by the same 

 cause. Not to speak of the varying brightness and varying colour 

 of the sun at sunset (in the production of which effects another 

 cause may co-operate), there are occasions on which the sun long 

 before sunset is shorn of his beams through the intervention of a 

 low general haze, the hygrometric conditions at the time being 

 such as to preclude the idea of fog, to which indeed the haze 

 referred to bears little resemblance. 



On July 24, 1868, I witnessed from the summit of Snowdon a 

 curious effect of this diffused haze. The day was cloudless. 

 Overhead the sky was clear and blue, but at lower altitudes it 

 was hazy, and the haze gradually thickened towards the horizon, 

 where it terminated in an opaque brown ring, which encircled 

 the mountain and shut out from view all objects beyond a distance 

 of about 15 miles. 



The nature of atmospheric haze has not, I think, hitherto been 

 satisfactorily elucidated, and it is much to be desired that advant- 

 age should be taken of some occasion when the haze is excep- 

 tionally dense, for the application of the various methods of 

 research which modern science has rendered possible. 



Clifton, December 25, 1888. George F. Burder. 



On the Use of the Words "Mass" and "Inertia" — 

 a Suggestion. 



As a teacher of dynamics to Engineer Students, I followed 

 with interest the discussions in Nature, as to the use of dyna- 

 mical terms, that have taken place within the last two years, j 

 and have recently re-read the whole correspondence with care. 

 Two points seem to me to have been not quite sufficiently 

 brought out. 



(i) Physicists and teachers of dynamics, however careful they 

 may desire to be, use the word "mass" in two senses : (i) in 

 the old, non-scientific, (Johnsonian) sense of a "lump of matter," 

 and (2) in the precise scientific sense of the "inertia" of a lump 

 of matter. Indeed, I suppose that no scientific man would i 

 hesitate to speak of "the inertia of a mass of matter." { 



The phrase "attracting mass " is universal among scientific 

 men, when attracting "lump" would do just as well. Thus, 

 in Prof. MacGregor's very carefully written "Kinematics and 

 Dynamics," we find, in Art. 290, "mass" carefully defined (in 

 the sense of inertia) as the value of a certain ratio, and in the 

 next article the use of the word in the sense of quantity of mat- 

 ter is deprecated ; yet, in Art. 355, we have " attracting mass " 

 where attracting " inertia" would not do, followed, a few lines 

 further on, by " a particle of unit mass" where "unit inertia" 

 would serve as well. 



It is this double use of the word that, I think, sometimes 

 escapes Engineers. 



Each of the words "mass" and "weight" is used in two 

 senses, one of which is common to both, but the other not. 

 The fact confirms very strikingly Prof. Greenhill's contention 

 that the scientific man is unwise to attempt to limit for his own 

 purposes the signification of a word already well established in the 

 language. For it shows that he cannot even keep straight himself. 

 I think myself that the scientific man ought to back out with 

 as much grace and celerity as may be, and determine for the 

 future to say " inertia " when he means " inertia," and to use 

 for its numerical representation the symbol " z " (or perhaps 

 "j" — sluggishness) rather than the symbol " w." The symbol 

 " I " might still be used for moment of inertia. Such an expres- 

 sion as a "mass of 20 pounds " would still mean exactly what it 

 does at present, and nothing already written would be affected 

 by the change. 



(2) The second point that I have to mention is purely a 

 question of procedure in teaching. 



The whole subject of dynamics might well be termed the 

 study of the inertia (the " sluggishness ") of matter. This is 

 the one new property whose existence, signification, and measure- 

 ment has to be brought home to the student. Now, I would 

 urge that it does not seem reasonable to ask the student 

 simultaneously to comprehend a new property of matter and 

 to alter his unit of force by defining it with reference to the new 

 property. Do what we will, our students before they begin 

 to learn dynamics will be familiar with the notion of " force" 

 as a " push " or a "pull," and measured in terms of " pounds " 

 and "ounces." 



I think it would be far the best plan to define the British unit 

 of force as the weight in London of the standard pound lump, 

 and the unit of inertia as that of the mass or lump on which this 

 force generates the unit acceleration of i foot-per-second per 

 second. 



Thus the unit of inertia would be that of 32"I9I2 standard 

 pounds, the number 32 "191 2 being, for brevity, throughout the 

 teaching, written "^." 



This would be to adopt with careful definition, by which it is 

 rendered perfectly precise, the Engineers' unit of inertia for pur- 

 poses of instruction in dynamics. It means empoying a force- 

 time-length system of units instead of an inertia-time-length 

 system. 



Such a system would be in harmony with the order of our 

 experiences and of our ideas as we grow in intellectual stature, 

 and with the history of human thought as written in our 

 language, and it is unwise to wage war with our own past 

 even under the encouraging leadership of your correspondent 

 "P. G. T." 



Perhaps I may be allowed here to deprecate the somewhat 

 misleading effort now being made by some chemists and 

 physicists to substitute the word "mass" for "weight" where 

 no question of inertia is involved or dreamt of, as, for instance, 

 in the definition of specific heat, by reference to equal masses, 

 instead of equal weights, as if the idea of quantity of matter 

 had not been attained quite independently of the conception of 

 inertia, and were not in the case in question always determined 

 by weighing. A. M. Worthington. 



Royal Naval Engineer College, Devonport, 

 December 30, 1888. 



Eight Tiue Ribs in Man. 

 In the number of Nature which appeared on November i, 

 1888, there is a notice to the effect that " at one of the meetings 

 of the Anatomical Society, during the session of the Medical 

 Congress in Washington, Dr. Lamb, of the United States 

 Army Medical Museum, spoke briefly of a singular phenomenon 

 he had observed in his examination of human breast-bones. It 

 was the occurrence, in a number of specimens, of an eighth rib. 



