182 ASTRONOMY ON MONT BLANC. 



At the time of this last visit Janssen made measurements to deter- 

 mine what, if any, movements of the observatory buikling had oc- 

 curred since its erection. It proved that there had been a slight 

 movement in the direction of Chamonix, but according to one of the 

 builders this movement took place in 1893 and 1891, and afterwards 

 ceased. There appears to have been no very appreciable amount of 

 settling, and at all events there are means provided for correcting 

 this if it should occur; so that the fears and doubts expressed by 

 Alpinists as to the safety of the observatory have proved unfounded. 

 The architect Baudouin, who visited the observatory on July 9, 1906, 

 found it buried in snow on the south side, but this had occasioned no 

 injury to the construction, so that the floor of the observing room, 

 which was dressed off in 1901, is still perfectly true. It seems there- 

 fore that the question of construction on a snow foundation at high 

 altitudes is well settled. 



III. 



Since his last ascent M. Janssen has returned to Chamonix each 

 year to direct the work which the younger men have carried on at 

 Grands Mulets and Mont Blanc. The observatory has been made 

 sufficiently comfortable to allow of observing visits of even a fort- 

 night duration. There is no lack of subjects for researches, for the 

 study of the planets Venus and Mercury, the solar and stellar spectra, 

 the chemical and heating efl'ects of the radiation of celestial objects 

 engage the attention of astronomers, while meteorology and physi- 

 ology offer problems equally interesting for solution at high altitudes. 

 M. Janssen has frequently communicated to the Academic des Sciences 

 short papers summarizing the results obtained. 



We may mention in the first place the researches on the heating 

 effect of solar radiation, which forms one of the objects of the observa- 

 tory of Mont Blanc. A principal aim of such researches, which are 

 performed by aid of instruments called actinometers or actinographs, 

 is to fix the value of the " solar constant." This is the number which 

 gives in calories per square centimeter per minute the heating powers 

 of the sun's rays before their entrance into our atmosphere. In the 

 older treaties on physics there is assigned a value less than 2 calories 

 (1.763), which comes down from the researches of Pouillet with his 

 pyrheliometer. But the tendency was later to augment this value. 

 M. Violle, having made in August, 1875, an ascent of the Bosson 

 glacier and of Mont Blanc, estimated 2.54 calories. M. Crova, the 

 able physicist of Montpellier, finds 2.83 calories from observations 

 on Mont Ventoux. In 1881, Langley, having observed near the sum- 

 mit of Mount Wliitney, which has an altitude of 4,460 meters, inferred 

 from his experiments a value exceeding 3 calories. Savelieff, at 



