158 



SCIENCE 



[N. S. Vol. XLIX. No. 1259 



longer indicated a decrease of temperature 

 Avitli increase of elevation, but often even a 

 slight increase! Tliere were but two possible 

 conclusions. Either the apparatus had devel- 

 oped, in actual use, faults that the cross ques- 

 tioning of the laboratory had failed to reveal, 

 or else the upper atmosphere really was in a 

 most unorthodox thermal state. However, 

 numerous records obtained with sounding 

 balloons at different places, by different people 

 and with different apparatus all showed the 

 same thing, namely, that the temperature of 

 the upper atmosphere, though varying slightly 

 from day to day, is, at any given time, sub- 

 stantially the same at all levels, as illustrated 

 by Fig. 1. 



Here, then, was a conflict between observa- 

 tional evidence and tradition. Actual meas- 

 urements had declared the upper atmosphere 

 to be essentially isothermal — declared it in 

 the face of a tradition to the effect that the 

 temperature of the atmosphere must steadily 

 decrease to, or very nearly to, the absolute 

 zero. The name of the joker who first per- 

 petrated this scientific hoax may be lost to 

 fame, but the worst of it is we physicists 

 thoughtlessly jjerpetuated it. The qualifica- 

 tion, thoughtlessly, is used advisedly, for it 

 seems impossible than any process of reason- 

 ing could have led to such an erroneous con- 

 clusion. If the surface temperature of the 

 earth is maintained, as we know it is, by the 

 absorption of solar radiation, it is equally 

 certain that in turn the temperatures of ob- 

 jects in the full flood of the necessarily equiv- 

 alent terrestrial radiation can not drop to 

 zero ; nor, therefore, can the air, generally, cool 

 by convection to a lower temperature than 

 that which this radiation can maintain. 

 These ideas, so simple that they seem hardly 

 worth expressing, embody the fundamental 

 explanation of why the upper atmosphere is 

 essentially isothermal. 



In addition to being exposed all the time 

 to earth radiation the upper air is also ex- 

 posed much of the time to solar radiation, but 

 there is abundant evidence that the atmos- 

 phere at all levels is far more absorptive of 



the relatively long wave-length terrestrial 

 radiation than of the much shorter wave- 

 length solar radiation. Hence in computing 

 from a priori considerations the probable tem- 

 perature of the isothermal region, or strato- 

 sphere, as it generally is called, it is sufficient, 

 as a first approximation, to consider the effect 

 of only the outgoing radiation, which, ac- 

 cording to the work of Abbot and Fowle, of 

 the Smithsonian Institution, is approximately 

 equal in quantity and kind to that which 

 would be emitted by a black surface coincident 

 with the surface of the earth and at the tem- 

 perature of 259° A. As a fiirther simplifica- 

 tion the surface in question may be regarded 

 as horizontal and of infinite length and 

 breadth in comparison to any elevation at- 

 tained by sounding balloons, and, therefore, 

 as giving radiation of equal intensity at all 

 available altitudes. 



N'ow consider two such surfaces, parallel 

 and directly facing each other at a distance 

 apart small in comparison to their width, and 

 having the absolute temperature T„, and let 

 an object of any kind whatever be placed at 

 the center of the practically enclosed space. 

 Obviously, according to the laws of radiation, 

 the final temperature of the object in ques- 

 tion will also be approximately T„. If, now, 

 one of the parallel planes should be removed 

 the uncovered object would be in substan- 

 tially the same situation, so far as exposure 

 to radiation is concerned, as is the atmos- 

 phere of the isothermal region in its exposure 

 to radiation from the lower atmosphere. Of 

 course each particle of the upper air receives 

 some radiation from the adjacent atmosphere, 

 but this is small in comparison to that from 

 lower levels and may, therefore, provisionally 

 be neglected. Hence the problem, as an ap- 

 proximation, is to find the temperature to 

 which an object, assumed infinitesimally small, 

 to fit the case of a gas, will come when ex- 

 posed to the radiation of a single black plane 

 at a given temperature, and of infinite extent. 



But whether an object lies between two 

 planes of equal temperature, as above as- 

 sumed, or, like the upper air, faces but one. 



