ATMOSPHERE. 



101 



lems have come to be prosecuted, both in our 

 own and in other countries, and in the import- 

 ant information, not only meteorological but 

 physiological also, which is being acquired, that 

 the inculcation of principles such as those above 

 referred to has not been thrown away. To 

 man, as inevitably a denizen of this so-much 

 forgotten ocean which we call the air, and as 

 by the laws of nature drawing the very condi- 

 tions and energies of his life from its conditions 

 and its activities about him and upon him, 

 every thing calculated to throw light on the 

 constitution, the limits, the perturbations, the 

 materials, actions, and influences of this atmos- 

 phere, must necessarily be of the highest im- 

 portance and interest. 



The quite definite and constant ratio of dimi- 

 nution of density of our atmosphere with in- 

 crease of elevation above the sea-level, as ob- 

 served in ascents of high mountains and in 

 balloons, and a consideration of the opposing 

 action of the elastic force of the air on one hand 

 and of gravity on the other, had led to the con- 

 clusion which was for many years generally ac- 

 cepted, that our atmosphere must entirely ter- 

 minate and of course, however attenuated its 

 density may have become, still with a definite 

 fluid surface at a height of from 45 to 50 miles 

 above the sea. The rapid and also very regular 

 diminution of density of the air with increase 

 of elevation, is still attested through Mr. 

 Glaisher's balloon ascents, to heights until his 

 time unprecedented ; and from such data, the 

 conclusion that the atmosphere must altogether 

 terminate at no very great distance above the 

 earth would seem to be inevitable. When, 

 therefore, Prof. Loomis places the lower limits 

 of auroras at from 46 to 50 miles, and their 

 upper limits at those of 495 to 534 miles, we 

 are naturally disposed to question if, as is gen- 

 erally supposed, the aurora requires a material 

 medium of some kind for its manifestation 

 whether there may not be, in the apparent posi- 

 tion and plane of such auroras, and in the pos- 

 sible appearance of different auroral fields to 

 different observers at the same time, circum- 

 stances which mislead the judgment, and cause 

 a phenomenon at no great distance from the 

 earth to be optically, and by "trigonometrical 

 measurement also, referred to spaces much more 

 remote. 



But there are, on this head, a great number 

 of independent facts which require attention. 

 Meteors and meteoric stones in their various 

 forms, it is known, quite suddenly acquire at 

 some distance above the earth an incandescent 

 state : they begin, continue briefly, and then 

 cease, to emit light, and perhaps also to throw 

 off* luminous fragments. And it is equally cer- 

 tain that this behavior can only be explained 

 by one circumstance; namely, that the tem- 

 porary incandescence and light exhibited by 

 these bodies must be due to their plunging for 

 the first, at some distance or other above the 

 earth, into a material medium dense enough to 

 heat them, by friction and retardation of move- 



ment, to whiteness ; so that they become totally 

 consumed and dissipated in this medium, or 

 through it reach and bury themselves in the 

 earth. And here, observers differ as to the 

 fact of the height at which meteors begin to 

 grow luminous ; though a large proportion of 

 the observations, and especially of those more 

 recently made, place this height above the long- 

 admitted limits of our air. Thus, Mr. A. S. 

 Herschel, 1864, concludes, from a comparison 

 of observations, that the heights of shooting 

 stars at Rome are sensibly the same as over 

 places where they have been most observed in 

 northern Europe the limits respectively of first 

 appearance and of disappearance being 73 miles 

 and 52 miles above the sea, with a probable 

 error of not more than 2 or 3 miles. The aver- 

 age velocity of shooting stars, calculated from 

 the same observations, he makes about 35 (more 

 accurately 34.4) miles per second. Prof. Phil- 

 lips, in his address before the British Associa- 

 tion, 1865, says: 



" Other information bearing on the constitu- 

 tion of the atmosphere comes to us from the 

 auroral beams and other meteoric lights known 

 as shooting stars. For some of these objects 

 not only appear at heights of 10, 50, and 100 

 or more miles above the earth, but at the height 

 of 50 miles it is on record that shooting stars or 

 fire-balls have left waving trains of light, whose 

 changes of form were in seeming accordance to 

 varying pressure in the elevated and attenuated 

 atmosphere." 



A table of altitudes of shooting stars, sup- 

 posed to include nearly all the instances pub- 

 lished, from September llth, 1798, to November 

 13th, 1863, inclusive, and prepared by Prof. H. 

 A. Newton, appears in the Amer. Jour, of 

 Science, July, 1864. Taking the more trust- 

 worthy cases, the estimated heights, for first 

 appearance, range from 4 up to 216 miles a 

 large proportion of those observed since 1856 

 being at elevations of between 40 and 90 miles ; 

 while the estimated heights at disappearance (a 

 few instances of heights of the middle point of 

 the path included) range from 3 to 166 miles 

 a large proportion of those since 1856 being at 

 elevations between about 30 and 75 miles. 



For the shooting stars observed at Washing- 

 ton, Philadelphia, and other places, on the 

 night of November 13th-14th, 1863, Mr. New- 

 ton finds a mean altitude at first appearance of 

 96.2 miles ; at extinction, of 60.8 miles. For 

 those observed at New Haven, Hartford, and 

 elsewhere, August lOth-llth, 1863, he finds 

 the respective altitudes 69.9 and 56 miles, in 

 this agreeing quite nearly with Mr. A. S. Her- 

 schell's computations, based in part on the 

 same instances. Thus, the November meteors 

 would seem to occupy a region fifteen or twenty 

 miles higher than those of August ; and it is 

 suggested that the former may be the more in- 

 flammable. Prof. Newton doubts whether any 

 meteors really appear at a height greater than 

 125 or 150 miles, supposing that heights beyond 

 these are assigned only through errors of obser- 



