OCTOBKR 1, 1890.] 



KNOWLEDGE 



239 



centimetre in area. The flask is inverted, and the table is 

 placed exactly one centimetre from the inverted bottom, so 

 that the contents of air right above the table are one cubic 

 centimetre. The observing table is divided by a fine instru- 

 ment into 100 equal squares. The silver plate is then very 

 highly polished ; but the burnishing is done all in one 

 direction, so that during the observations it appears dark 

 when the fine mist-particles glisten opal-like with the 

 refieeted light, in order that they be the more easily 

 numbered. The tube to which the silver mirror is 

 attached, is connected with two stop-cocks, one of which 

 can admit filtered air from one vessel, and the other can 

 admit a small portion of the air to be examined. The 

 other tube in the flask is connected with an air-pump of 

 150 c.c. capacity. Over the flask is placed a covering, 

 coloured black in the inside. In the top of this cover is 

 inserted a powerful magnifying-glass, through which the 

 particles on the silver table can be easily counted. A little 

 to the side of this magnifier is an opening in the cover, 

 through which light is concentrated on the silver mirror. 



To perform the experiment, the air in the flask is ex- 

 hausted by the air-pump. The flask is then filled with the 

 pure filtered air. By a nicely-designed apparatus, one 

 exact cubic centimetre of the common dusty air is intro- 

 duced. After one stroke of the air-pump, the air — mix- 

 ture of 450 c.c. of filtered air, and 1 c.c. of common air — 

 is made to occupy an additional space of 150 c.c. ; and the 

 exhaustion so produced chills the air, and causes condensa- 

 tion to take place on the dust particles. The observer, 

 looking through the magnifying-glass upon the silver table, 

 sees the mist-particles fall like a shower on the table. 

 These particles last long enough to be numbered. The 

 observer then counts the number on a single square in 

 two or three places, and strikes an average. Suppose the 

 average number upon one of the squares of the silver table 

 were one, then on the table there would be 100 ; and the 

 100 particles of dust are those which floated in the cubic 

 centimetre of mixed air right above the table. But as 

 there are 600 c.c. of mixed air in the flask and the barrel, 

 the number of dust particles in the whole is 000 times 100 

 = 00,000; that is, there are 00,000 dust particles in one 

 cubic centimetre of the common air which was introduced 

 for examination. But as 1 c.c. is nearly equal to '06 cubic 

 inch, one cubic inch of that common air would contain no 

 less than one million of dust particles. 



Mr. Aitken has by this process counted 7| millions of 

 dust-particles in a cubic inch of the ordinary air of 

 Glasgow. I was with him when he was numbering the 

 dust-particles in Edinburgh eighteen months ago. In the 

 air outside the Royal Society rooms we counted four mil- 

 lions in the cubic inch. We counted the same number 

 inside, at four feet from the floor ; but near the ceiling, 

 after the gas had been burning for some time, no less than 

 49 millions were counted in the cubic inch. After the two 

 hours' meeting of the Fellows, the numbers increased to 

 Oi millions and 57t\ nnllions resi)c'ctively. He counted in 

 a cubic inch of air immediately above a Bunsen flame no 

 less than 489 millions of dust-particles. Of course, when 

 the air is very dense with dust-particles, a fraction of a 

 cubic centimetre of the air is introduced into the flask for 

 the experiment ; and, when the air contains fewer par- 

 ticles, more than one cubic centinu'tre is introduced. 



Tlie air of Colmonell, in .\yrshire, has been found to 

 contain from 8,000 to 155,000 inutieles in the cubic inch. 

 At Hyeres, in the south of France, he found from 50,000 

 to 400,000, according to tiie direction of the wind. At 

 Lucerne, in Switzerland, the specimens of air tested were 

 remarkably free from dust, some even as low as 3,500 

 in the cubic inch — the lowest observation he has yet made. 



. The question next arises as to whether 3,500 particles 

 in the cubic inch of air is the lowest limit which the 

 atmosphere ever attains to. Even away from the contami- 

 nations of smoky towns and villages, the air contains 

 cosmic dust. There is always dust in the upper atmo- 

 sphere, for without the dust no clouds could be formed; 

 and of cosmic dust, there must always he a considerable 

 quantity in the air produced by the waste from the mil- 

 lions of meteors that daily fall into it. 



" The gay motes that people the sunbeams " are not, 

 therefore, as Milton considered, " numberless." They 

 have been enumerated with marveDous accuracy. 



THE FACE OF THE SKY FOR OCTOBER. 



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



EVIDENCES of renewed solar activity are in- 

 creasing, a very tine double group of spots 

 being on the sun's disc at the time of -nTiting 

 these lines. Conveniently observable minima of 

 Algol occur on the 9th at 12h. 43m. a.m., on the 

 11th at 9h. 34m. p.m., on the 14th at 6h. 23m. p.m., and 

 on the 31st at llh. 16m. p.m. Mercury is a mornmg 

 star throughout October, and is very favourably situated 

 for observation. On the 1st he rises at 5h. 49m. a.m. 

 with a southern declination of 2° 53', and an apparent 

 diameter of 10". On the 15th he rises at 4h. 38m. a.m., 

 or Ih. 47ra. before the sun, with a southern declination of 

 0" 17', and an apparent diameter of G]". On the 31st he 

 rises at 5h. 50m. p.m., or Ih. 4m. before the sun, with a 

 southern declination of 9° 55', and an apparent diameter 

 of 5". He is stationary on the 8th, and at his greatest 

 western elongation (18°) on the 15th, and will appear at 

 his brightest on the mornings of the 21st to 20th. He 

 is in Virgo throughout the month, but does not approach 

 any conspicuous star. 



Venus is an evening star, but is badly situated for 

 observation in these latitudes owing to her great southern 

 declination. She sets on the 1st at Oh. 42m. p.m., Ih. 

 5m. after sunset, with a southern declination of 23°, and 

 an apparent diameter of 27". On the 31st she sets at 

 5h. 38m. P.M., 111. 4m. after sunset, with a southern de- 

 clination of 27° 57', and an apparent diameter of 42". 

 She is at her greatest brightness on the 30th, but, as she 

 is almost mvisible in the greater part of England, no 

 further details need be given. During the month she 

 passes from Libra into Scorpio. Mars is visible, bi\t at a 

 great disadvantage, as an evening star. He sets on the 

 1st at 9h. 16m. p.m., with a southern declination of 

 25" 52', and an apparent diameter of 9:J". On the 31st 

 he sets at 9h. 8m. p.m., with a southern declination ol 

 22' 56', and an apparent diameter of 7i-". t)u the 31st 

 his brightness is only one-sixth of what it was at opposi- 

 tion ; and in view of liis increasingly diminishing diameter. 

 we shall not continue om- ephemeris of him after the pre- 

 sent mouth. He is in Sagittarius during October, but 

 does not approach any conspicuous star very closely. 



Jupiter is an evenmg star, setting on the 1st at llh. 

 50m. P.M., with a southern declination of 20" 25', and an 

 apparent ecjuatorial diameter of 42'". On the 31st he 

 sots at lOh. 2m. p.m., with a southern declination of 

 20' 0', and an apparent equatorial diameter of 38 J". He 

 is in quadrature with the sun on the 27lh. The following 

 phenomena of the satellites occur while the planet is more 

 than 8° above, and the sun more Uian 8' below, the 

 horizon. .\ transit mgress of the shadow of the third 

 satellite at 7h. 12m. p.m. on the 1st. A transit egress of 

 the shadow of the fourth satellite at Oh. 30m. p.m. on the 



