November 2, 1891.] 



KNOWLEDGE 



213 



above the sea level ; and observations of the duration of 

 twilight show that a glimmer of light may generally be 

 traced till the sun is 18° below the horizon, which 

 indicates that there is matter capable of dispersing 

 the sun's light at an altitude of about -iO miles. On 

 ordinary occasions the air above that altitude is either too 

 rare, or too pure from foreign particles, to disperse any 

 perceptible amount of twilight. After the explosion of 

 Krakatoa the sunset colours and the longer duration of 

 twihght showed that dust was suspended in the upper 

 atmosphere for many months, at considerably greater 

 altitudes than usual, probably at a height of at least 00 

 miles. It has been suggested that the volcano threw the 

 dust to an altitude of a few thousand feet, and that it was 

 then carried into the upjser air by the rising heated atmo- 

 sphere of the tropics, which always gives rise to an upcast 

 current carrying the air from near the surface of the 

 ground to a great altitude, and dispersing it and the dust 

 it contains north and south into either hemisphere. But 

 as similar sunset effects were observed in the last century 

 extending over Europe after the great eruption of Skaptar 

 •Tokul in Iceland,'- it is probable that the heated column of 

 air over the volcano itself would be sufficient to carry dust 

 into the higher regions of the atmosphere. 



The smaUer shooting stars generally become visible at a 

 height of from 70 to 80 miles above the earth, and they are 

 usually entirely consumed before they penetrate to 50 miles 

 above the sea level, but there is ample evidence that larger 

 meteors have occasionally been observed at a height of 

 over 100 miles above the earth's surface, proving that there 

 is sufficient atmosphere at such great altitudes to resist 

 their motion and convert a part of their energy of transla- 

 tion into heat and light sufficient to render tbem visible. 



The air at these great altitudes must be very rai-e, much 

 rarer than the vacuum within the bulb of an electric in- 

 candescent lamp, for the density of the atmosphere is 

 halved at a height of about three and a half miles, and we 

 know that with an atmosphere composed of gas, which 

 obeys Boyle's law connecting the density with the pressure, 

 the density will continue to be halved with every additional 

 ascent of three and a half miles ; consequently at a height of 

 70 miles the density will have been reduced in the propor- 

 tion of 1 to 2-", that is, the atmQsphere will have less than 

 a millionth of the density of the air at the sea level, which 

 about corresponds to the density of the air within the bulb 

 of an electric lamp (usually spoken of as the vacuum). At a 

 height of 105 miles the density of the air will have been 

 reduced in the proportion of 1 to 2'", or more than a 

 thousand million times. 



The reason for this rapid decrease in geometrical pro- 

 gression of the density of the atmosphere as we ascend 

 is easy to see when we consider the pressure on each 

 stratum of three and a half miles in thickness as we rise 



* Grilbcrt White, of Selborne, iu one of his letters to the Hou. 

 Danes Harrington, describes " tlie amazing and portentous phe- 

 nomena " observed in the summer of 1785. He says the sun "shed 

 a rust-eolourcd ferruginous liglit on the ground, particularly lurid 

 and blood-coloured at rising and setting." There are many refer- 

 ences to the "red fog" which caused great alarm over Europe 

 during the whole of the sunnner of 1783. Lalande ascribed it to the 

 effect of a hot sun succeeding a long period of heavy rains, and 

 Cowper refers to it in " The 'J'ask." Book U., liiu's .5.3-65, which were 

 written in the autumn of 1783 ; he also refers to the earthquakes 

 and tidal waves of that year. Mrs. Souierville, in her " Physical 

 G-eography," traces the origin of these phenomena of 178U to the 

 great eruption of Skaptar .lokul, which broke out on May 8th and 

 continued till the end of August, sending forth imnu-nse (juantities 

 of dust as well as lava. Sir .Tohn Herschel, in his " I'hysical 

 Q-eography," states that Skaptar .Iok\d ejected 21 cubic miles of lava, 

 a quantity equal to the volume of wiilcr jiourcd by Ihc Nile into the 

 sea in a year. 



above the earth's surface. The lowest stratum, which 

 contains a half of the atmosphere, is compressed by the 

 weight of the upper half of the atmosphere that rests upon 

 it. The next quarter of the atmosphere will occupy a 

 stratum of the same thickness as the lowest half of the 

 atmosphere, because every part of it will be compressed 

 with just half of the weight which compresses a similar 

 stratum of the lowest half, and the atmosphere is composed 

 of gases which very approximately obey Boyle's law ; that 

 is, the volume varies inversely as the pressure. In the 

 same manner the third stratum of three and a half miles 

 in thickness will contain an eighth part of the whole 

 atmosphere under the pressure of the weight of the 

 remaining eighth part above it, and the fourth stratum 

 wull contain a sixteenth part under the pressure of the 

 weight of the remaining sixteenth part, and so on. 

 The suc- 



cessive steps 

 at which the 

 density of 

 the earth's 

 atmosphere 

 is halved 

 will in fact 

 b e c o m e 

 shorter and 

 shorter as 

 the temper- 

 ature falls 

 in the upper 

 air, for the 

 volume oc- 

 cupied by a 

 mass of gas 

 depends up- 

 on its tern- . 

 perature as 

 well as the 

 pressure up- 

 on it. While 

 the pressure 

 remains 

 constant the 

 volume var- 

 ies as the 

 absolute 

 tempera- 

 ture. Thus, 

 if the height 

 of the lower 

 half of the 

 atmosphere 

 at a temper- 

 ature of 32 ' 

 Fahr., or 0" 



Cent. , were exactly three and a half miles, its height at a tem- 

 perature of T degrees would be ^J;? x 3-5 miles, where T i.s 

 the absolute temperature measured in degrees Centigrade. 

 We know that the temperature falls very rapidly in the upper 

 air; thus, at the greatest height reached by Mr. Glaisher on 

 the 5th SeptemluT. lSti2. the temperature of the air as 

 registered by a miiiimum thermometer fell to — 11-0^ Fahr.. 

 at a height where the pressure of the atmosphere was only 

 reduced to about a quarter of the pressure at the sea level. 

 If at a height of 70 miles the temperature falls to -01" 

 Cent. .the height of the successive steps of the above series 

 will be reduced to two-thirds of the height of the lowest 

 half of the atmosphere — that is, the density of the atmo- 

 sphere would be halved at successive steps of a Uttle less 



