MOUNTAIN OBSERVATORIES. 397 



they have doubtless much to tell, were their message only legible by 

 us. It has not as yet proved so ; but the characters in which it is 

 written are being earnestly scrutinized and compared, with a view to 

 their eventual decipherment. The prodigious advantages afforded by 

 great altitudes for this kind of work were illustrated by the brilliant 

 results of Professor Young's observations in the Rocky Mountains 

 during the summer of 1872. By the diligent labor of several years 

 he had, at that time, constructed a list of one hundred and three dis- 

 tinct lines occasionally visible in the spectrum of the chromosphere. 

 In seventy-two days, at Sherman (8,335 feet above the sea), it was ex- 

 tended to two hundred and seventy-three. Yet the weather was ex- 

 ceptionally cloudy, and the spot (a station on the Union Pacific Rail- 

 way, in the Territory of Wyoming) not perhaps the best that might 

 have been chosen for an "astronomical reconnaissance."* 



A totally different kind of solar research is that in aid of which 

 the Mount Whitney expedition was organized in 1881. Professor S. 

 P. Langley, Director of the Alleghany Observatory in Pennsylvania, 

 has long been engaged in the detailed study of the radiations emitted 

 by the sun ; inventing, for the purpose of its prosecution, the " bo- 

 lometer,"! an instrument twenty times as sensitive to changes of tem- 

 perature as the thermopile. But the solar spectrum as it is exhibited 

 at the surface of the earth is a very different thing from the solar 

 spectrum as it would appear could it be formed of sunbeams, so to 

 speak, fresh froin space, unmodified by atmospheric action. For not 

 only does our air deprive each ray of a considerable share of its energy 

 (the total loss may be taken at twenty to twenty-five per cent when 

 the sky is clear and the sun in the zenith), but it deals unequally with 

 them, robbing some more than others, and thus materially altering 

 their relative importance. Now, it was Professor Langley's object to 

 reconstruct the original state of things, and he saw that this could be 

 done most effectually by means of simultaneous observations at the 

 summit and base of a high mountain. For, the effect upon each sepa- 

 rate ray of transmission through a known proportion of the atmos- 

 phere being (with the aid of the bolometer) once ascertained, a very 

 simple calculation would suflSce to eliminate the remaining effects, and 

 thus virtually secure an extra-atmospheric post of observation. 



The honor of rendering this important service to science was ad- 

 judged to the highest summit in the United States. The Sierra Ne- 

 vada culminates in a granite pile, rising, somewhat in the form 



* R. D. Cutts, "Bulletia of the Philosophical Society of Washington," vol. i, 

 p. VO. 



f This instrument may be described as an electric balance of the utmost conceivable 

 delicacy. The pi-inciple of its construction is that the conducting power of metals is 

 diminished by raising their temperature. Thus, if heat be applied to one only of the 

 wires forming a circuit in which a galvanometer is included, the movement of the needle 

 instantly betrays the disturbance of the electrical equilibrium. The conducting wires or 

 "balance-arms" of the bolometer are platinum-strips yl^y of an inch wide, and Tj-sitru of 



