386 SCIENTIFIC RECORD FOR 1884. 



individual determinations range from 12 to 18. These observationss 

 properly refer to the upper limit of what von Bezold calls the first pur- 

 ple light, and which Jesse has observed as being the limit of the earth's 

 shadow. In an appendix Jesse adds to the preceding observations, 

 which were taken between November 1863, and March 1884, a few 

 taken in the latter part of March with better ai^paratus, and which give 

 results varying between 7 and 12 kilometers, as though a decided dim- 

 inution had taken place in the altitude of the dust, which was great- 

 est in January, and least at the end of March. (D. M. Z., i, p. 127.) 



404. All the phenomena in connection with the green sun, as re- 

 corded in India, have been collected by Prof. C. M. Smith, of Edin- 

 burgh, from which he concludes that the green sun must be distin- 

 guished from the remarkable sunsets of 1883-'84. His own view is 

 that there is some definite evidence to show that the phenomenon is 

 due to aqueous vapor, for it is not so uncommon as is generally sup- 

 posed. {Nature^ xxx, p. 347.) 



405. The Krakatoa eruption has also given rise to an interesting 

 series of studies into a so-called wave of pressure that seems to be 

 traceable from that region outward throughout the whole globe. Gen- 

 eral Strachey {Nature^ xxix, p. 181) seems to have first called attention 

 to the fact that a series of remarkable barometric fluctuations recorded 

 by self-registers in various parts of Europe harmonized with the hy- 

 pothesis that a wave of atmospheric compression, starting from Kra- 

 katoa and moving outward in all directions, after concentrating at the 

 antipodes returned to Krakatoa, making a complete circuit of the earth 

 in from thirty-four to thirty-seven hours ; after crossing at Krakatoa, 

 this again made the circuit of the earth in a second period of thirty-six 

 hours. Four such circumnavigations are claimed by some of those who 

 have studied the subject; where self recording registers are not availa- 

 ble, attempts have been made to utilize the automatic registers of press- 

 ure kept in connection with the city gas-works. 



406. Eykatchefl", of St. Petersburg, from a study of all known data 

 on the velocity of the Krakatoa atmospheric wave, fixes it at 327,9 

 meters per second, corresponding to the velocity of sound at the tem- 

 perature of — 10 C. , which temperature is attained at an altitude of 4,000 

 meters in the atmosphere when the temperature at the earth's surface 

 is -fl2 C, so that we may conclude that these great waves of pressure 

 move with the velocity of sound. The amplitude of this wave for Euro- 

 pean stations was 1.3 millimeters, but it was almost twice as great, 

 namely 2.5, at the island of South Georgia. [Z. 0. G. M., xix, p. 431, 

 and Mature, xxx, p. 135.) 



407. General J. T. Walker, of the India Geological Survey, gives the 

 results of observations of the earthquake ocean wave. {Mature, xxix, 

 p. 376.) 



