SCIENCE 



NEW YORK, JULY 31, 1891. 



THE UPPER STRATA OF THE ATMOSPHERE. 



At a meeting of the Geographical Society of Berlin 

 on May 2, 1891, Professor Forster read a paper on "The 

 Upper Strata of the Atmosphere," a report of which is given 

 in the Proceedings of the Royal Geographical Society for 

 July. Professor Forster began by saying that the earlier 

 conceptions of the height of the earth's atmosphere were 

 based mainly upon obser\rations as to the duration of the 

 twilight, and as to the extent to which the light of the heav- 

 enly bodies was refracted. On the basis of such observa- 

 tions, the height of the atmosphere was estimated at from 

 forty to fifty miles; it was not, however, by any means 

 thought that, above these altitudes, there were no other 

 strata belonging to the earth, but only that the density of 

 the latter was too small for them to produce the optical ef- 

 fects just specified. The discovery of a means of determin- 

 ing the existence of such extremely thin strata beyond a 

 height of fifty miles dates from the end of the eighteenth 

 century, when attempts were first made to measure, accord- 

 ing to Chladni"s principles, the heights at which the first illu- 

 mmation of falling stars takes place. 



A specially comprehensive investigation with reference to 

 these heights was carried out at the instance of the Berlin 

 Observatory in August, 1867, by means of simultaneous ob- 

 servations in the neighborhood of Berlin, with the result that 

 not one of the altitudes at which illumination commences, 

 and which were measured with sufficient accuracy, was 

 found to exceed practically a hundred miles. These results, 

 however, possess only a relative value, being valid only for 

 the falling stars of the month of August, the so-called Per- 

 seides; for it is evident that illumination will arise earlier 

 or later, and at different altitudes, according to the varying 

 velocities with which the small heavenly bodies penetrate 

 the atmosphere. Illumination will take place earliest in the 

 case of those falling stars which move in a directly opposite 

 direction to the movement of the earth, which travels at 

 about nineteen miles per second. These heavenly bodies, 

 possessing a velocity of their own of about twenty-six miles 

 per second, consequently enter the earth's atmosphere with 

 a velocity of forty-five miles; while in the case of those 

 bodies which tend to be overtaken by the earth in their 

 movement round the sun, the velocity can, in the most ex- 

 treme case, only be equal to the difference between the two 

 velocities above-mentioned, viz., seven miles. 



The altitudes at which extinction, that is to say, the almost 

 complete dissolution of these heavenly bodies, commences, 

 vary very much, because the rapidity of the extinction is 

 dependent upon the size and composition of the bodies them- 

 selves. The Berlin observations of 1867 gave for this an 

 average height of about fifty miles. From these observa- 

 tions as to falling stars it is also supposed that the boundary 

 between the strata which participate in the earth's movement 

 and those which resist it should be fixed at least at some 

 miles higher than a hundred miles. It is here also that the 

 bodies become heated prior to their illumination. 



The polar lights extend to still greater altitudes ; their 

 height, at the time of their greatest development, when they 

 are visible as far as the tropics, would be from 300 to 375 

 miles, while in the polar regions they spread themselves, as 

 a rule, at a height of only a few miles, indeed quite close to 

 the earth's surface. But there remains the question whether 

 at those altitudes there are still strata which follow the 

 movement of the earth round the sun ; for it is possible that 

 the phenomena of the electric glow, which the polar lights 

 may be considered to be, radiate from the earth into the 

 heavens, follow also the earth's movement round the sun, 

 but at the same time extend beyond the strata belonging to 

 the earth into the strata of extremely rarefied gases, which 

 in all probability fill up the space between the planets and 

 the sun. This space maybe designated as the " Himmels- 

 luft," and is not to be confounded with the so-called " ideal 

 medium," viz., ether, in which luminous phenomena are 

 supposed to occur. 



Evidence in support of the existence of such a " Himmels- 

 luft" is to be found in the conditions existing on the sun, 

 which are gradually becoming more completely known. On 

 the sun, gases are continually being developed and given off 

 as the result of explosive processes as well as of the dissolu- 

 tion and volatilization of the numerous small meteoric bod- 

 ies which are incessantly hastening to the sun. Further, 

 the movement of Encke's comet, which, in its return, occur- 

 ring in periods of twelve hundred days, remains longest in 

 the vicinity of the sun, has furnished important evidence of 

 the obstructive effect of a so-called " Himmelsluft." The 

 movements of other comets and of the planets have not yet 

 afforded evidence of such an influence, but it must be borne 

 in mind that the perceptibility of such an effect depends not 

 only on the density, which increases towards the sun, hut 

 also on the proportion which the surface of the heavenly 

 body in question bears to its mass. This proportion is very 

 much greater in the case of comets than in the case of plan- 

 ets, and may also in one comet be much greater under certain 

 conditions than in others. 



Indications of the counter-influence of the relatively quiet 

 " Himmelsluft " as compared with the earth, which rushes 

 through it with a velocity of about nineteen miles per sec- 

 ond, can be recognized in the highest strata in the case of 

 the movements of the luminous tails and clouds of light 

 which many falling stars and fireballs leave behind them 

 along their flying course, that is, when these remain visible 

 for some minutes. The changes of position and form, which 

 proceed apparently very slowly in these luminous forms, 

 due regard being paid to their great height and distance from 

 the observer, are supposed to be executed with a velocity of 

 more than sixty yards a second. The movements which 

 take place in these meteoric tails are, according to all ap- 

 pearances, not so simple that they can be explained merely 

 as being the result of the highest strata being left behind in 

 consequence of the velocity with which the observer on the 

 earth's surface is being whirled along, and which at the 

 equator amounts to seventeen miles a minute, and at our 

 latitudes to about eleven miles a minute. The very consid- 

 erable alterations of form which these tails undergo in shift- 



