Jmie 30, 1887] 



NATURE 



197 



This account is written by someone to whom the dynamical 

 jiroblem is a reality and no theoretical abstraction : he employs 

 throughout the gravitation measure of force, and to an engineer 

 there is no ambiguity in his use of the words pound and ton 

 sometimes in the sense of mass or weight and sometimes in the 

 sense of force. 



Prof. A. 15. W. Kennedy, in his " Mechanics of Machinery," 

 pp. vii. and 222, has called attention to the same ambiguity of 

 language, and points out that the word pound is used in two 

 senses (three senses when pound-sterling is to be distinguished), 

 which he proposes to distinguish as weight-pound and force- 

 pound ; not, observe, mass-pound and weight-pound as the 

 mathematician would have us call them. 



An article in the current number of Wiedemann' s Annalen, 

 No. 6, 1887, by A. Oberbeck, " Ueber die Bezeichnung der 

 absoluten Maass-systeme," shows that a similar controversy on 

 dynamical terminology is now going on in Germany. 



The mathematical definition that " the weight of a body is the 

 force with which the earth attracts the body " must disappear and 

 be replaced by ' ' the attraction of the earth on a pound, a ton, or a 

 kilogramme,is called the force of a pound.a ton,or a kilogramme;" 

 these are gravitational units of force, derived originally from 

 statical considerations, but used in pr.-ictice for dynamical problems 

 also ; but inasmuch as the magnitude of these units depends on 

 the local value of }^, they are unsuitable for astronomical or 

 electrical purposes, and are now replaced in such cases by the 

 absolute units of force, the poundals, dynes, &c. 



The defect of modern dynamical teaching is the unreality of 

 its applications; it is too much the "dynamics of a particle." 

 Were the student accustomed to examples taken from the 

 magnificent problems presented by the latest industrial develop- 

 ments, he would become accustomed to the use of gravitational 

 and absolute units of force, and recognize their respective 

 advantages. A. G. Greenhill. 



Woolwich, May 30. 



Upper Cloud Movements in the Equatorial Regions 

 of the Atlantic. 



Recent communications to your paper have given the motion 

 of the upper clouds from the eastward in the equatorial regions 

 of the Atlantic. My observations (the result of having passed 

 through these regions sixteen times in sailing-ships) give the 

 motion of the upper clouds from the westward ; and the motion 

 of the intermediate cloud layers, consisting of the high low-level 

 stratiform clouds (cirro-cumulus and such like), from a point 

 somewhat to the north of east on the north side of the equator. 

 Intermediate clouds are rare in the equatorial regions south of 

 the Line. The high low-level clouds are constantly being 

 confounded with the true high clouds. 



There is another source of error in noting upper cloud 

 movements ; little attention has been paid to a movement of 

 props^ation. So marked is this at times, that they are propa- 

 gated over the sky quicker than they are moving, this movement 

 being frequently at right angles to the direction of motion. 



David Wilson-Barker. 



The Shadow of Adam's Peak. 



The shadow of Adam's Peak, to which Mr. Abbay refers in 

 his letter to Nature, vol. xxxvi. p. 152, is certainly not the 

 kind of shadow that I witnessed, but that which is only seen in 

 the clearest weather and without the intervention of mist. This 

 is mentioned in one of the last paragraphs of my paper. 



Nevertheless, I cannot think that mirage has anything to do 

 with that shadow. When the Observatory was first established 

 on Pikes Peak, the observers used to see the shadow of the 

 mountain rising against the sky on the far distant horizon. At 

 first they thought this very curious, but sojn found that the 

 appearance was always there in very fine weather. 



Further observation showed conclusively that the appearance 

 was simply the ordinary earth shadow of sunrise projected so 

 clearly against the sky, that an irregularity such as a sharp peak 

 could be distinguished on the edge of the generally circular 

 shadow. 



I do not think that mirage has anything to do with this anti- 

 crepuscular shadow, but no doubt there are abundant thermo- 

 metric observations in America for anyone who wishes to 

 investigate the subject further. Rai.pk Aiiercrombv. 



21 Chai^el Street, London, June 17 



Temperature and Pressure. 



I have to thank Mr. S. A. Hill for replying to my letter, 

 and it is most interesting to know that the same connexion 

 between temperature and pressure exists in India as in Jamaica 

 (Nature, vol. xxxv. pp. 437, 606). 



No doubt, as Mr. Hill observes, different localities will have 

 different values of the coefficients \ and n in the equation — 

 5 T = A 8 P + ^ (« P)2 ; 



indeed, we must expect very different values ; but still, by 

 putting 5 P = o in the equation for minimum temperatures, each 

 locality should give the same limiting temperature, which we 

 may regard as the temperature of space. 



It is of course to such concordance that we must look for the 

 determination of the temperature of space, so defined, rather 

 than to extreme care in the taking of observations in any one 

 particular part of the world. 



With reference to Mr. Hill's remarks about extrapolation, it 

 is hardly necessary for me to point out that astronomical refrac- 

 tion is caused by the whole terrestrial atmosphere, and that 

 some law between temperature and pressure must be adopted 

 before refraction -tables can be constructed ; Mr. W. H. M. 

 Christie, the Astronomer-Royal, has, in the Memoirs R.A.S., 

 vol. xlv. p. 177, recently pointed out how errors may arise from 

 this source. 



Indeed, errors must arise from this source. In Jamaica the 

 values of A and 11 are not the same for mean and minimum tem- 

 peratures, or, roughly speaking, for day and night ; neither is it 

 to be expected that they will be the same anywhere else. But 

 enough has been said to indicate the importance of the problem, 

 and the steps which should be taken for its solution, 



Jamaica, June 6. Maxwell Hall. 



British Association Sectional Procedure. 



In reference to Prof. Thompson's letter (June 16, p. 151), will 

 you allow me to say that in 1884 I went from the meeting of the 

 Association of American Microscopists at Rochester, N.Y., 

 to that of the British Association at Montreal. At the former 

 the proceedings commenced daily at 9 a.m., closing about noon, 

 and another short session was held in the afternoon. The 

 middle of the day was thus left at liberty for Committee work, 

 sight-seeing, or rest, and the greater amount of work got 

 through in the day as compared with the usual plan at our 

 Association was very striking. Alfred W. Bennett. 



6 Park Village East, Regent's Park, June 18. 



Mirage. 



This afternoon, shortly after 4.30 p.m., my attention was 

 drawn to an extraordinary and wonderfully perfect "mirage." 

 My house, situated almost on the extreme point of Hartlepool, 

 near the Heugh Lighthouse, overlooks with a south aspect the 

 Hartlepool or Tees Bay, Redcar, Saltburn, and in clear weather 

 a beautiful high coast-line stretching from Saltburn to Staithes. 

 When first seen, all the houses of Redcar, some seven miles 

 distant, -and lying almost at sea-level, were enormously elongated 

 to at least six or seven times their ordinary height, and looked 

 like high square towers with intensified colouring. I could not 

 however determine (with the aid of an opera glass) whether the 

 phenomenon was a simple elongation or whether the upper part 

 of the "mirage" was an inverted image of the houses. I am 

 inclined to think that the lower two-thirds was an elongation of 

 the buildings, while the upper third was an inverted image. 



During the height of the mirage a dark misty stratum of air, 

 bounded by a distinct upper margin parallel with the horizon, 

 and decreasing in density towards it, stretched from the western 

 end of Redcar through an arc of almost 90° seawards. I estimated 

 the height of this stratum at 35' to 36' of arc. After some 10 

 minutes the "mirage" gradually dwindled over Redcar, but 

 remained distinctly visible for a short time longer over Saltburn, 

 the coast-line, and out to sea. At Saltburn, about 114 miles 

 distant, some of the buildings were duplicated, a white house 

 being visible as two spots widely separated. The normal coast- 

 line south of Saltburn was obscured by the haze, but a beautifully 

 clear "mirage" of it was visible, taking as its horizon the upper 

 margin of the misty air stratum, the horizon being thus bodily 

 raised through some 36' of arc. Out at sea in an almost easterly 

 direction a smoking steamer was faintly visible with an inverted 



