ADDRESS. 41 



would not be sufficient to cause any liquefaction of the nitrogen and oxy- 

 gen, the pressure being so low. If we assume the mean temperature as 

 about the boiling-point of oxygen at atmospheric pressure, then a con- 

 siderable amount of the carbonic acid must .solidify as a mist, if the air 

 from a lower level be cooled to this temperature ; and the same result 

 might take place with other gases of relatively small volatility which occur 

 in air. This would explain the clouds that have been seen at an elevation 

 of fifty miles, without assuming the possibility of water vapour being 

 carried up so high. The temperature of the upper air must be above that 

 on the vapour pressure curve corresponding to the barometric pressure at 

 the locality, otherwise liquid condensation must take place. In other 

 words, the temperature must be above the dew-point of air at that place. 

 At higher elevations, on any reasonable assumption of temperature distri- 

 bution, we inevitably reach a temperature where the air would condense, 

 just as Fourier and Poisson supposed it would, unless the temperature is 

 arrested in some way from approaching the zero. Both ultra-violet 

 absorption and the prevalence of electric storms may have something to 

 do with the maintenance of a higher mean temperature. The whole mass 

 of the air above forty miles is not more than one seven-hundredth part of 

 the total mass of the atmosphere, so that any rain or snow of liquid or 

 solid air, if it did occur, would necessarily be of a very tenuous descrip- 

 tion. In any case, the dense gases tend to accumulate in the lower strata, 

 and the lighter ones to predominate at the higher altitudes, always as- 

 suming that a steady state of equilibrium has been reached. It must be 

 observed, however, that a sample of air taken at an elevation of nine miles 

 has shown no difference in composition from that at the ground, whereas, 

 according to our hypothesis, the oxygen ought to have been diminished 

 to 17 per cent., and the carbonic acid should also have become much 

 less. This can only be explained by assuming that a large inter- 

 mixture of different layers of the atmosphere is still taking place at this 

 elevation. This is confirmed by a study of the motions of clouds about 

 six miles high, which reveals an average velocity of the air currents of 

 some seventy miles an hour ; such violent winds must be the means of 

 causing the intermingling of different atmospheric strata. Some clouds, 

 however, during hot and thundery weather, have been seen to reach an 

 elevation of seventeen miles, so that we have direct proof that on occasion 

 the lower layers of atmosphere are carried to a great elevation. The exist- 

 ence of an atmosphere at more than a hundred miles above the surface of 

 the earth is revealed to us by the appearance of meteors and fireballs, and 

 when we can take photographs of the spectrum of such apparitions we 

 shall learn a great deal about the composition of the upper air. In the 

 meantime Pickering's solitary spectrum of a meteor reveals an atmosphere 

 of hydrogen and helium, and so far this is corroborative of the doctrine 

 we have been discussing. It has long been recognised that the aurora is 

 the result of electric discharges within the limits of the earth's atmosphere, 

 but it was difficult to understand why its spectrum should be so entirely 



