MEMOIR OF AUGUSTE BEAVAIS. 161 



the Cote d'Or to the Diountains of Lignria. The circumjacent monutains, 

 on the contrary, were seen with great distinctness. After throwing a 

 glance on this magnificent panorama, jMM. Bravais, Martins, and Le 

 Pileur hastened to arrange their instruments — barometer, thermometer, 

 hygrometer, psychrometer, pyrheliometer, actinometer, compass; the 

 instrument for measuring the horizontal magnetic intensity ; another 

 for measuring the inclination of the magnetic needle; another for measur- 

 ing the temperature of ebullition of water; instruments for observing 

 the tints of the sky and transparence of the atmosphere, &c., another for 

 measuring the electric intensity. The genius of I)e Saussure had sug- 

 gested a part of the same exj)eriments, but his extemporized instruments 

 were less complicated than those of to-day, whose precision is paid lor 

 by the minute precautions which their management exacts. 



During the five hours passed on the summit of Mont Blanc, our three 

 l^hysicists had time to derive from their instruments all they were ca- 

 pable of yielding, and to collect a series of measurements which left 

 little to desire. At the approach of evening, the principal experiments 

 in physics having been nearly termin.ated, M. Bravais established the 

 theodolite, and, assisted by M. Le Pileur, who wrote the angles at his 

 dictation, commenced a survey of the horizon, measuring- the angle of 

 depression of each of the mountains which formed it, and the azimuth 

 which expressed the direction in which it was seen. This circuit of the 

 Jiorizon of Mont Blanc, which had never before been made, (tor De 

 Saussure had confined himself to general remarks,) will remain a valua- 

 ble monument for geodesy and geology. The work was almost finished, 

 and nothing remained to be taken but the least interesting parts of the 

 panorama, when the process was interrupted by a phenomenon which 

 equally surprised the eight persons (guides and travelers) then assembled 

 on Mont Blanc, because, in none of the ascents j^reviously made, had 

 any one ventured to remain there till the setting of the sun. 



" At forty minutes after six," says M. Bravais, in the little work which 

 contains his tour (Thorizon, " the sun approaching the moment of its 

 disappearance, we cast our eyes on the side opposite to the luminary, 

 and saw, not without wonder, the shadow of Mont Blanc projected on 

 the snow-covered mountains in the eastern part of our panorama. I 

 took the summit of that shadow with the thedolite, and obtained the 

 depression of — 1°. A minute afterwards it was — 0° 48', ascending 

 in i^roportion as the sun declined. We still remained some ten minutes 

 occupied in packing our baggage, and rather anxious to descend on ac- 

 count of the shortness of twilight on high mountains. * * * i have 

 delineated iu the panorama the form then presented by the shadow of 

 Mont Blanc. It rose gradually into the atmosphere, as though this were 

 a canvass on which it was just portraying itself. The separation of the 

 shadov,' and the light was strongly defined in its outlines, and it thus 

 continued to ascend, rising above the mountains of the Valley of Aosta, 

 until it attained the height of 1°, still remaining perfectly visible. The 

 air above the cone of shadow was of that rose-purple tint which, in fine 

 sunsets, is seen to color the western sky, while the border of this tint, 

 along the line of separation, presented a more intense hue of red, con- 

 tributing greatly to enhance the splendor of the phenomenon. 



" Let the mountains of the great Valley of Aosta be now conceived 

 as also simultaneously projecting their shadows into the atmosphere; 

 the outline of their mighty spires distinctly visible ; dark, or rather 

 faintly green below, but soaring up into that expanse of rose-('.)lored 

 light from which they were separated by a band of deeper hue ; to this 

 be added the j)recision of the cones of shadow, and especially of the 

 11 s 



