214 



KNOWLEDGE. 



[November 2, 1891. 



than two and a half miles, so that we have considerably 

 under-estimated the actual rate of decrease of the density 

 of the atmosphere on mounting upwards. 



Let us assume that the density of the air at a height of 

 100 miles is reduced to a thousand-millionth of the density 

 of the atmosphere at the sea level, we are still very far 

 from having got rid of the atmosphere altogether. Ac- 

 cording to recent estimates, founded on curious corro- 

 borative evidence derived from more than one class of 

 physical phenomena, a cubic inch of such tenuous 

 atmosphere would contain about 350 thousand million 

 molecules of gas ; a number which is so enormous that its 

 vastness can only be partly realized by means of an 

 illustration. Let us suppose a small bulb of one inch cubic 

 capacity to be filled with air of a thousand millionth, the 

 density of atmospheric air at the sea level. If the bulb 

 were imperfectly sealed so that a thousand molecules could 

 rush into it in every second of lime, it would take more 

 than eleven years tor the pressure inside the bulb to 

 become doubled — that is, for as many more molecules to 

 rush through the leakage as were originally contained in 

 the bulb. 



If the pressure of the air continues to decrease according 

 to the same law, as we proceed upwards its density will 

 have been decreased to one milhon million millionth 

 part of the density at the sea level at a height of 200 

 miles, and a cubic inch of such air would contain only 

 about 350 molecules ; and at a height of less than 222 

 miles, a cubic inch would only contain one molecule. 



At what height should we reach the surface of the 

 atmosphere "? Many people, whose opinions are entitled to 

 respect, have thought that the air has not an upper limit. 

 Professor Young, in his "Test-book of General Astronomy," 

 in dealing with the height of the atmosphere (sec. 98), is 

 cautious, and says that it cannot " be asserted positively 

 that the atmosphere has any definite upper limit." But 

 Professor Forster, of Berlin, in a paper which he read 

 before the German Geographical Society'-' in May last, 

 brings together evidence which he thinks proves the 

 existence of a " Himmelsluft," or thin air pervading in 

 greater or less density the whole of the solar system, and 

 which he assumes is associated with the strata of 

 extremely rarefied gases, which follow the earth's move- 

 ments round the sun. He quotes, as evidence of such a 

 medium, the retardation of Encke's comet when near to 

 perihelion, and the appearance of the zodiacal light and 

 the " gegenschein," or " counterglow," which has been so 

 fi-equently observed, as well as some' other evidence which 

 appears to be very doubtful with regard to the height of 

 luminous clouds and the aurora. 



The retardation of comets near to perihelion may be 

 satisfactorily accounted for by the great extension of coronal 

 matter about the sim, and the appearance of the zodiacal 

 light and the " gegenschein " may be due to the light dispersed 

 by flights of meteors which seem to be aggregated near to 

 the plane of the ecliptic and not to be evenly dispersed 

 about the sun, as would be the case if an atmosphere tilled 

 inter-planetary space. But the kinetic theory of gases 

 seems to afl'ord evidence that the molecules of the atmo- 

 sphere do not escape from the regionof theearth's attraction. 

 According to the kinetic theory the molecules of hydrogen, 

 at a temperature of 0° Cent., move with an average velocity 

 of a little more than 6000 feet per second. Clerk Maxwell, 

 in his " Theory of Heat," 9th edit., p. 814, gives their 

 average velocity as 6097 feet per second. The atomic 

 weight of nitrogen is 14-01 ; consequently, assuming Clerk 



* An abstract of this paper is printed in the " Proceedings of the 

 Royal Geographical Society of London " for July, 1891. 



Maxwell's velocity for hydrogen, the molecules of nitrogen 

 at a temperature of 0^ Cent, will move with an average 

 velocity of 1029 feet per second, and the molecules of oxygen, 

 the other chief constituent of the atmosphere, will move a 

 little more slowly. The square of the mean velocity 

 varies as the absolute temperature ; consequently, as the 

 temperature is lowered the average velocity of the 

 molecules decreases rapidly, and we may feel quite confi- 

 dent, from observations made in balloons and at mountain 

 observatories, that the temperature near to the limits of the 

 atmosphere is far below 0° Cent. We shall therefore be 

 certainly much above the mark in assuming an average 

 velocity of 1629 feet per second for the molecules of nitro- 

 gen in the highest strata of the atmosphere, where their 

 free path becomes very long, and some of them can escape 

 upwards without sufl'ering a collision. A projectile thrown 

 upwards from such an altitude under the influence of the 

 earth's gravity, with a velocity of 1629 feet, would be car- 

 ried to a height of less than nine miles above the point at 

 which it started and would fall again into the atmosphere. 

 It would require a velocity more than twenty-two times 

 as great, or of about seven miles per second, to carry it 

 away from the earth. The velocities above referred to are 

 the mean velocities of molecules at a temperature of 0° 

 Cent. We know that the actual velocities of the mole- 

 cules will be distributed about the mean velocity according 

 to the law of probable error, and it seems very improbable 

 that in the extremely cold upper regions of the atmosphere 

 any molecules will have a sufficiently great upward velocity 

 to be carried outside the region of the earth's attraction. 



Nottcr of Boolt. 



On the Ailjustinent and Teatiny uf Tflesciiplr Ohjectivi's 

 (published and sold by T. Cooke and Sons, Buckingham 

 Works, York ; price 5.v.). — This little book will be greatly 

 welcomed by observers, as teaching them how to adjust and 

 test the adjustments of their object-glasses and mirrors. The 

 illustrations are excellent, particularly the frontispiece, which 

 shows the various appearances of star discs as seen in and 

 out of focus, with perfect and defective object-glasses, and 

 with a good instrument when badly adjusted. The author- 

 ship of the work is not stated ; it appears to be a joint- 

 production of persons possessing very considerable prac- 

 tical as well as theoretical knowledge. 



METEOROLOGY OF BEN NEVIS. 



By Dr. .J. G. McPherson, F.R.S.E. 

 [Lecturer an Meteorohgy in tlif Unirersity of St. Andrew's.) 



BEN NEVIS, in Inverness-shire, is not only the 

 highest moimtain in Scotland, but also the high- 

 est in the British Islands. It is comparatively 

 easy of ascent, and its west side is nearly precipi- 

 tous. Fort William, an old military town at its 

 western base, is on the sea coast. Some years ago this 

 mountain was considered exceptionally advantageous for 

 observations at its summit and base for the pressure and 

 temperature of the air, and for the direction of the wind at 

 a difl'erenee of above 4000 feet level. Accordingly 

 Mr. Wragge made arrangements for observations from 

 June to October during the three years 1881 — 83. Since 

 that time improvements were made in the buildings on the 

 summit ; telegraphic communication was made between 

 the summit and base of the mountain ; and Mr. Ormond 



