316 



NA TURE 



^August 3, 1 1 



could not be asserted (as it has been of cases at like 

 altitudes in the Alps), that reflection from the snow had 

 anything to do with the state of our very literally " burned " 

 faces. 



In view of the lateness of the season it was decided to 

 make the permanent camp at an altitude of rather less 

 than 13,000 feet, rather than wait and make a road to the 

 peak, which rose 2000 feet above us, and was daily climbed 

 for observations with portable instruments. 



The sky was perpetually fine, of a deeper violet than I 

 have observed elsewhere, even on Etna, and the dryness 

 extreme, though water from the snow-fields above was 

 abundant. An equatorial telescope of 5^ inches aperture, 

 which was kindly loaned by Prof. E. C. Pickering, it was 

 found too late could not be well used to determine the 

 quantity of the "seeing," owing to a maladjustment of 

 the eye-pieces, which it was necessary to hold in the 

 hand. Under these circumstances a critical estimate of 

 the definition under high powers could not be made, but 

 enough was seen to make it evident that it was generally 

 excellent, and that such a site possessed advantages for 

 an astronomical, as well as for a meteorological station. 



It is greatly to be desired that it should be occupied, 

 with the protection of a permanent building, adapted to 

 either object, and it is probable that such provision will 

 be hereafter made. 



For us, however, there was no other shelter than our 

 tents, and the high wind, the cold, and the mountain 

 sickness consequent on the rarefied air, made the con- 

 tinuous observations which were kept up synchronously 

 with that at the camp at Lone Pine, a matter of difficulty. 

 These observations were persistently maintained, how- 

 ever, but we were not sorry on September 1 1 to break up 

 our wintry camp and to descend to summer again. We 

 resumed our journey across the desert, and then across 

 the continent, reaching Pittsburgh on September 28, 1SS1. 



The reductions of the observations are still incomplete, 

 but some conclusions of interest may already be indi- 

 cated. 



It has been said that the determination of the amount 

 of heat the sun sends the earth is the fundamental 

 problem of meteorology, since on this all the phenomena 

 which that science contemplates depend. Accordingly 

 the observations were directed first to this primary object, 

 chiefly through methods which involved the study of the 

 phenomena of selective absorption (so intimately con- 

 nected with it), and secondarily to these phenomena 

 considered in themselves. 



The final result may be affected by some still imper- 

 fectly determined corrections, and it will be sufficient to 

 here give an approximative one. 



It appears probable that the true solar constant is one- 

 half greater than that determined by Pouillet and by 

 Herschel near the sea level, and even greater than the 

 recent values assigned by M. Violle. Thetrue value, it is 

 believed, will be shown by the data when published to be 

 larger than those hitherto accepted. 



Of more general interest, perhaps, is the conclusion as 

 to the limit of that cold which increases under full sun- 

 shine as we ascend above our atmosphere. "What," it 

 may be asked, " would the temperature of the soil be on 

 a mountain top rising wholly above the air, or what the 

 temperature of the sunward hemisphere of the earth, if 



the present absorbing atmosphere were wholly with 

 drawn?" The personal experiences already alluded to 

 may prepare the general reader for the paradoxical result 

 that if this atmosphere were withdrawn, the temperature 

 would greatly fall, though under a materially greater 

 radiant heat. 



The student of the subject is aware that this conclusion 

 follows from the fact that the loss by radiation into space 

 as the atmosphere is withdrawn is much more rapid than 

 the gain by direct solar heat, but even lie may not per- 

 haps be prepared for the extent of the fall. 



The original observations, which will be given at length, 

 lead, in the writer's opinion, to the conclusion that in the 

 absence of an atmosphere the earth's temperature of 

 insolation would at any rate fall below —50° F.,by which 

 it is meant that (for instance) mercury would remain a 

 solid under the vertical rays of a tropical sun were radia- 

 tion into space wholly unchecked, or even if the atmo- 

 sphere existing, it let radiations of all wave-lengths pass 

 out as easily as they come in. Remembering, then, that 

 it is not merely by the absorption of our air, but by 

 the selective quality of this absorption, that the actual 

 surface temperature of our planet is maintained, we see 

 that without this comparatively little-known function, it 

 appears doubtful whether, even though the air supported 

 respiration and combustion as now, life could be main- 

 tained upon this planet. 



These conclusions do not, in the writer's opinion, 

 depend upon the Mount Whitney observations alone, but 

 exist implicitly in the results of previous observers who 

 have, however, not apparently drawn them, with the 

 exception, perhaps, of Mr. Ericsson, who has observed 

 that the surface of the airless moon must remain cold 

 even in sunshine. 



We see, if these results be true, that the temperature of 

 a planet may, and not improbably does, depend far less 

 upon its neighbourhood to, or remoteness from, the sun, 

 than upon the constitution of its gaseous envelope, and 

 indeed it is hardly too much to say that we might 

 approximately indicate already the constitution of an 

 atmosphere which would make Mercury a colder planet 

 than the earth, or Neptune as warm and habitable 

 a one. 



It must at the same time be admitted that our informa- 

 tion as to the special constituents of our own air, which 

 are chiefly here concerned, is still imperfect, though the 

 observations made at Mount Whitney upon the selective 

 action of that undoubtly prominent agent, water-vapour, 

 will, it is hoped, add somewhat to our knowledge. 



In the same connection it may be added that the 

 writer's investigations have led him to the conclusion 

 that the "temperature of space," so called, must at any 

 rate be lower than that assigned by Pouillet (if we accept 

 the received values for that of the absolute zero), and in 

 this case the temperature of the earth's surface, in the 

 absence of the quality of selective absorption in our air, 

 would be yet loner than that here given, 



In view of the great importance of this quality, interest 

 will attach to the statement that the bolometer observa- 

 tions at the summit and base of Whitney show a different 

 distribution of solar energy (heat, light, or "actinism") 

 at the upper station from that at the lower, and show 

 (among other things) that without our atmosphere, the 



