MOUNTAIN OBSERVATORIES. 399 



tinct blue * — not merely bluish, but a real azure just tinted with green, 

 like the hue of a mountain-lake fed with a glacier-stream. 



Moreover, the further consequence ensues, that the sun is hotter 

 than had been supposed ; for, the higher the temperature of a glowing 

 body, the more copiously it emits rays from the violet end of the 

 spectrum. The blueness of its light is, in fact, a measure of the inten- 

 sity of its incandescence. Professor Langley has not yet ventured 

 (that we are aware of) on an estimate of what is called the " effective 

 temperature " of the sun — the temperature, that is, which it would be 

 necessary to attribute to a surface of the radiating power of lamp- 

 black to enable it to send us just the quantity of heat that the sun 

 does actually send us. Indeed, the present state of knowledge still 

 leaves an important hiatus — only to be filled by more or less probable 

 guessing— in the reasoning by which inferences on this subject must 

 be formed ; while the startling discrepancies between the figures 

 adopted by different and equally respectable authorities sufficiently 

 show that none are entitled to any confidence. The amount of heat 

 received in a given interval of time by the earth from the sun is, how- 

 ever, another matter, and one falling well within the scope of observa- 

 tion. This, Prof essor Langley's experiments (when completely worked 

 out) will, by their unequaled precision, enable him to determine with 

 some approach to finality. Pouillet valued the " solar constant " at 

 1*7 "calories"; in other words, he calculated that, our atmosphere 

 being supposed removed, vertical sunbeams would have power to heat 

 in each minute of time, by one degree centigrade, 1*7 gramme of 

 water for each square centimetre of the earth's surface. This estimate 

 was raised by Crova to 2-3, and by Violle in 1877 to 2-5 ; f Professor 

 Langley's new data bring it up (approximately as yet) to three calo- 

 ries per square centimetre per minute. This result alone would, by its 

 supreme importance to meteorology, amply repay the labors of the 

 Mount Whitney expedition. 



Still more unexpected is the answer supplied to the question, 

 "Were the earth wholly denuded of its aeriform covering, what would 

 be the temperature of its surface ? We are informed in reply that 

 it would be at the outside 50° of Fahrenheit below zero, or 82° of 

 frost. So that mercury would remain solid even when exposed to the 

 rays— undiminished by atmospheric absorption — of a tropical sun at 

 noon.J The paradoxical aspect of this conclusion— a perfectly legiti- 

 mate and reliable one — disappears when it is remembered that under 

 the imagined circumstances there would be absolutely nothing to hin- 

 der radiation into the frigid depths of space, and that the solar rays 



* Defined by the tint of the second hydrogen-line, the bright reversal of Fraunhofer'a 

 F. The sun would also seem — adopting a medium estimate — three or four times as brill- 

 iant as he now does. 



f " Annales de Chimie et de Physique," tome x, p. 360. 



X S. P. Langley, "Nature," vol. xxvi, p. 316. 



