178 



KNOWLEDGE 



[September 1, 189^. 



recording de\'ices that would be most useful in meteoro- 

 logical work. 



The efficiency of the light in different parts of the solar 

 spectrum for producing motion at the lampblaoked surface 

 in a radiometer has been found to be as follows : Assuming 

 that of the ultra red waves to be represented by 100 the 

 extreme red gives 85 per cent, of this standard repulsion ; 

 red, 73 ; orange, 66 ; yellow, 57 ; green, 41 ; blue, 22 : 

 indigo, 8 J ; violet, 6 ; and ultra violet 5 per cent. 



It has been suggested that a kind of mechanical process 

 of photography might be based upon the different repulsive 

 powers of difterent kinds of light. A still more interestmg 

 adaptation of the phenomena of radiant matter to purposes 

 connected with photography, and one that has been prac- 

 tically carried out by Prof. Crookes, is in the study of those 

 thermal changes which accompany photographic action at a 

 surface. Suppose a miniature Daguerreotype plate delicately 

 suspended in a " radiant matter bulb," the exhaustion and 

 sealing of the bulb having been carried out in non-actinic 

 light. If, when the apparatus is at rest and in thermal 

 equilibriimi in a dark room, a beam of actinic light is directed 

 upon the sensitive surface, of course a certain amount of re- 

 pulsion, due to molecular pressure, will take place ; but such 

 repulsion is a result of the conversion of light into heat. And 

 hence, for so long as a great part of the incident radiation 

 is engaged in performing the work of chemical .separation, 

 the repulsion due to that radiation must be considerably 

 below the normal. Space will not permit, or rather it 

 would be inappropriate to the subject of these articles, to 

 introduce a discussion of the results of these experiments, 

 and of others equally interesting, bearing on the absorp- 

 tion of the different coloured powders used as coating 

 upon radiometric vanes. Of course, the wave-length of the 

 radiant heat or light that is employed is an important 

 factor that has to be kept in view in the iBterpretation of 

 all such experiments. 



When radiation of very low frequency is employed, it 

 may be so largely absorbed by the glass bulb that this, in 

 consequence of the increase of its temperature, becomes 

 itself a generating surface for molecular 

 wind. Effects of this kind, which may be 

 produced also by placing a warm finger on 

 the bulb, give rise to movements of the 

 vanes that are at first sight very puzzling. 

 Again, a thin platinum wire, mounted 

 within the exhausted globe (see Fig! 2), 

 may be so heated by being made part of 

 a galvanic circuit as to generate very 

 powerful currents of radiant matter, and 

 produce mechanical effects of propor- 

 tionate intensity on a " turbine wheel " 

 radiometer. The skewed vanes in this 

 little heat engine are usually made of 

 sheet mica ; but, of course, the nature of 

 the material is unimportant, so long as 

 for the turbine wheel simply plays the 

 in relation to radiant matter that the paddles 

 of a water-wheel, or the sails of a windmill, do to currents 

 of water or air. 



Fio. 2. 



rigid and 

 same part 



light. 



THE FACE OF THE SKY FOR SEPTEMBER. 



By Herbert Sadler, F.E.A.S. 



HE solar disc shows little or no diminution in the 

 fi-equency of groups of spots and faculffi. The 

 following are conveniently observable minima of 

 Algol : September 1st, Oh. 49m. p.m. ; September 

 4th, (Jh. 38m. p.m. ; September 24th, 8h. 20m. p.m. 



T 



Mercury is a morning star throughout September, and 

 is well situated for observation during the second and third 

 weeks of the month. On the 8th he rises at 3h. 49m. a.m., 

 or Ih. 3Gm. before the Sun, with a northern declination 

 of 111° and an apparent diameter of 7^", about youths 

 of the disc being illuminated. On the 13th he rises at 

 3h. 51m. A.M., or Ih. 42m. before the Sun, with a northern 

 declination of 10'^ 46' and an apparent diameter of 6|", 

 about T%'Viths of the disc being illuminated. On the ICth 

 he rises at 4h. Om. a.m., or Ih. 39m. before the Sun, with a 

 northern declination of 9° 36' and an apparent diameter of 

 60', yr.ths of the disc being illuminated. On the 23rd he 

 rises at 4h. 38m. a.m., or Ih. 12m. before the Sun, with a 

 northern declination of 5° 22' and an apparent diameter 

 of 5i", about f'-ths of the disc being illuminated. After 

 this he approaches the Sun too closely to be conveniently 

 observed. He is at his greatest western elongation (18") 

 on the 11th, and his greatest brightness about the 17th. 

 While visible, Mercury describes a direct path through Leo, 

 being near Kegulus on the 7th'. 



Venus is still a very conspicuous object in the morning 

 sky, but both her brightness and diameter decrease con- 

 siderably during the month. She rises on the 1st at 

 lb, 24m. a.m.. with a northern declination of 17° 23' and 

 an apparent diameter of 2BJ", jf^Po^^^s of her disc being then 

 illummated, and her brightness being about the same as 

 in the middle of May. On the 30th she rises at Ih. 44m. 

 A.M., with a northern declination of 13° 6' and an apparent 

 diameter of 21", -fVoths of the disc being illuminated, and 

 her brightness having diminished to what it was at the 

 beginning of April. She is at her greatest western elonga- 

 tion (46°) on the 19th. At 3h. 50m. a.m. on the 22nd a 

 7i magnitude star will be 1' south of the planet. During 

 the month Venus describes a direct path from the boundariei 

 of Gemini through Cancer into Leo. 



Mars is an evening star, but is still wretchedly situated 

 for observation in these latitudes. On the 1st he rises at 

 6h. 10m. P.M. with a southern declination of 21;^° and an 

 apparent diameter of 22|", the defect of illumination on 

 the following limb becoming now very perceptible. On 

 the 30th the planet sets at Oh. 33m. a.m., with a southern 

 declination of 21° 11' and an apparent diameter of 17'0", 

 his brightness towards the end of the mouth being less 

 than half of what it was at opposition. During the month 

 he describes a direct path in Capricornus. The minor 

 planet Pallas will be in opposition on September 20th, on 

 which evening she souths at midnight with a southern 

 declination of 6° 8'. The present opposition is not a very 

 favourable one, the stellar magnitude of the planet being 

 only 7i. During the month Pallas describes a retrograde 

 path in Cetus. 



.Jupiteris nowbecominga magnificentobjectintheevening 

 sky, being actually visible to the naked eye in sunlight at the 

 end of the month, the coming opposition being a very favour- 

 able one. He rises on the 1st at 8h. Ira. p.m., with a 

 northern declination of 8° 1' and an apparent equatorial 

 diameter of 46i^". On the 30th he rises at Oh. 3m. p..m., 

 with a northern declination of 6° 54' and an apparent 

 equatorial diameter of 49". During the month he describes 

 a retrograde path in Pisces. The following phenomena of 

 the satellites occur before midnight, while -lupiter is more 

 than 8° above and the Sun 8° lielow the horizon. A 

 transit ingress of the second satellite at 9h. 49m. p.m., and 

 a transit egress of its shadow at lOh. 24m. p.m. on the 1st. 

 An eclipse disappearance of the first satellite at 101i.5m.26s. 

 on the 2ud. A transit egress of the shadow of the 

 first satellite at 9h. 86m. p.m., and of the satellite itself at 

 lOh. 30m. p.m. on the 3rd. An eclipse reappearance of the 

 third satellite at 9h. 49m. 483., and an occultation dis- 



