182 



KNOW^LEDGE 



[Mahcii 23, 1883 



" Let Knowledge grow from more to more." — Ai.frkd Tennyson. 



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COMETS' TAILS. 



[763] — With respect to Mr. A. J. Mott's suggestion in the last 

 tinmber of Knowledge, I would remark that the mean velocity of 

 evaporated molecules must evidently be greater than the mean 

 velocity of the molecules of the occluded gas before evaporation, 

 for the swiftest-moving molecules of the occluded gas will be the 

 first to free themselves from the attractive forces of the molecules 

 near to the surface of the solid lx>dy in which the gas is occluded. 

 Thus a selective action will be set up which will cause the mean 

 velocity of the molecules of the gas left behind to be less than the 

 mean velocity of the molecules which escape. In other words, 

 evaporation tends to cool the body from which evaporation takes 

 place. 



Thus the mean velocity of the molecular bombardment I referred 

 to will evidently be much greater than the mean velocity of the 

 molecules of the evaporating gas when reduced to the temperature 

 of the solid body from which the evaporation is taking place. 

 For comets near the sun the velocity of bombardment may be 

 quite sufficient to carry such molecules as are not projected into 

 the sun outside the action of the solar system ; but the fiercest 

 evaporation will evidently take place towards the sun, and even if 

 the individual stones of a meteoric swarm all revolve rapidly about 

 their own axes, we should not expect a large proportion of the 

 evaporation to take place- in the direction in which comets' tails 

 form — that is, away from the sun. 



But there are other reasons for believing that the tails of comets 

 are not merely gaseous appendages. When we consider that the 

 orbit of a comet is not sensibly affected by the matter driven off 

 from the nucleus to form the tail during the perihelion passage, 

 we may safely assert that the weight of the tail cannot be great 

 compared with the weight of the nucleus, and that, consequently, 

 the immense space occupied by the tail (or, rather, by the many 

 successive tails driven off during the perihelion passage) cannot be 

 filled with gas sufficiently dense to disperse the amount of light 

 observed. A comparatively dense gas when freed in the laboratory 

 from the minute particles floating in it, appears almost absolutely 

 black when a beam of electric light is passed through it ; but the 

 moment that dust-laden air is admitted, light is dispersed in every 

 direction, and the dispersed light is polarised in a manner which 

 corresponds with the polarisation of the light derived from the tails 

 of comets. That is, the intensity of ]iolarisation increases as the 

 angle between the direction of illumination, and the line drawn 

 from the observer to the illuminated particle increases towards a 

 right angle. A. Cowper Eanyabd. 



TRICYCLES. 



[76-i] — In reply to your correspondent, D. M., I would beg to 

 point out that 1 have not in any way fallen into "the common 

 confusion of engine cranks with tricycle cranks." 



Three years since, I drew the attention of several of my friends 

 to the fact that I could start any tricycle provided with cranks from 

 the position known as dead centres on a fairly level road in good 

 condition ; but I should call the moving a tricycle forward under 

 such conditions (with a backward pull of one foot and a forward 



thriist of the other, and the action of the knee and ankle exerting a 

 slight influence overthc pedals), "not having complete control over 

 them." To explain my meaning more clearly, your correspondent 

 should use one crank and not two, and then see wliether he could 

 start the machine with the crank at the bottom of the throw. By 

 controlling a pedal, I meant that if a machine were going uphill 

 along a rough road with no momentum, each foot would have 

 sufficient control over one pedal or the other to keep the machine 

 going, and in this sense my statement would, I believe, be understood 

 by most readers. 



A correspondent, Mr. Edwin Wells, has written and asked me to 

 state my opinion in Knowledge, " Whether the backward pedalling 

 of the ' Sterling Tricycle' is in conformity with the proper motion 

 of the muscles of the leg ? " 



This seems to me an important question. If my correspondent 

 means, does the motion of the leg in pedalling the "Sterling" 

 resemble the action of the leg in walking as much as pedalling an 

 ordinary fonvard-pedalhng machine, I should answer at once, cer- 

 tainly not ; but the leg has more power over the crank in the back- 

 ward action than in the forward, and has strong control over the 

 crank through a greater portion of a revolution, and, although other 

 muscles than those used in walking or in driving ordinary machines 

 are brought into play, yet I can answer for it that the motion is 

 not more tiring : from my own experience I should say less so. 



John Browning. 



THE WEATHER FORECASTS. 



[765] — I may be wrong in my ideas of fairness, but I am deci- 

 dedly of opinion that when an individual desires to show the use- 

 lessness of the weather forecasts he should certainh- not attempt 

 to do so by means of "mental observations taken at various times 

 diuing different times of the year" (letter 749, p. 153). Neither 

 do I think readers will be impressed with the fairness of a system 

 which compares the forecasts and the weather, and takes note of 

 every time the two disagree, but omits to mention when they agree 

 {vide Sir E. Beckett's letter, where he says " R^hen H omits the 

 wind it agreed with the prediction"). 



[Well, but surely this is indicating when they agree. — R. P.] 



After reading the letter on p. 53, I determined during the forth- 

 coming month to institute a rigid comj^arison between the actual 

 state of the weather and the Meteorological Office's forecasts, and 

 the plan I adopted was the following: — I carefully noted the direc- 

 tion and force of the wind (1 to 12) at 9 a.m., 12, 3, and 6 p.m., 

 and made rough notes at intervals of about two or three hours of 

 the state of the weather. In the evening I brought all my notes 

 before me, and from them compiled an accurate record of the wind 

 and weather of that day. This I entered in my day-book, and then 

 (and til ?io case before then) consulted the forecast and entered it 

 below the history. At the end of the month I summarised the 

 comparisons, and obtained the following results : — On fourteen 

 days the wind agreed both in force awd direction with the fore- 

 cast. On nine it agreed with one or the other, and on one day only 

 the forecast was entirely wrong. The weather forecast was quite 

 correct on thirteen days, partly correct on eight, and altogether 

 wrong on three. This is the result I have obtained, and it is one, 

 I think, of which the Meteorological Office have no occasion to be 

 ashamed. 



With regard to the weather warnings my experience tallies with 

 that of Mr. Mattieu Williams, i.e., that the change comes later than 

 was expected. This was well exemplified on the 19th and 20th. 

 The forecast for the 19th ran ; " Light N.W. breezes, and fine, cold 

 weather ; wind subsequently backing, and weather becoming cloudy 

 and unsettled." During the whole of the day, however, the weather 

 was settled and the wind constant from the N.W., but on the 20th 

 it backed to S.S.W., and we had a storm of rain and wind. 



If this letter serves no other useful end, I shall be content if it 

 draws from Mr. Mattieu Williams the explanation he promised 

 (page 72) if his prophecy jiroved correct, which it has. 



J. W. Stanifobth. 



RATIONAL DRESS. 

 [766] — During my absence from home, the committee of the 

 Rational Dress Society has, I am infonned by the Viscountess 

 Hurberton, appointed a paid secretary in my stead. Being anxious 

 to continue the work of dress reform with the power which com- 

 bination gives, I am now forming another society, to be called the 

 Rational Dress Association, composed of both men and women. 

 As I daily receive letters of inquiry as to whether the Exhibition of 

 Rational Dress Is to take place, I should be glad to be allowed t'^ 

 state that it most certainly >rill take place ; and that it is only 

 owing to my recent serious illness, and to the many vexations 

 obstacles which have been thrown in my way, that the necessary 

 an-angements for it have not yet been completed. 



