240 



PROGKESS OF METEOROLOGY IN 1889. 



between 8 and 9 p. M. Hours of miuimum atnouuts of rain-fall are 4 

 to 5 A. M. and 11 A. M. to noon. For the results of the spring, summer, 

 and autumn months, which by themselves are somewhat irregular, a 

 periodic formula of two terms is computed, and from examination of 

 the computed results the essential periodicity of the various groups is 

 discoverable. For all three seasons the minimum of rain-fall is between 

 9 A. M. and noon, the maximum in spring falls from 10 to 11 P. M., that 

 in summer from 7 to 8 p. m., and in autumn from 2 to 3 A. m., with a 

 secondary maximum from 8 to 9 P. M. The late hour of the maximum 

 is in coutrast with the general assumption that it occurs iu the early 

 afternoon. 



Secular variation of rain-fall— Frofessov Frank Waldo gives three 

 tables containing the mean residuals of five-year periods from the mean 

 rainfall. The first is taken from Dr. Wild's "Eegen Verhiiltnisse des 

 Kus. Eeiches;" the second is from Dr. Lang's article, "Dersaculare 

 Verlauf der Wetterung als Ursache der Gletcherschwankungen iu den 

 Alpen," and the third is compiled from American observations. The 

 writer finds evidence in these tables of a long-period inequality having 

 the following periods ; 



The maximum at one station is often found to occur at the time of a 

 minimum at another.* {Am. Meteor. Journal,, v, p. 412.) 



Causes of raw.— Professor von Bezold, in a paper before the Berlin 

 Meteorological Society, discussed the manner of formation of precipita- 

 tion. The mixing of warm moist air with cold air by which the tempera- 

 ture falls to the mean of the two can but seldom produce an appreciable 

 precipitation. Precipitation occurs only when a mass of moist air is 

 directly cooled, as in nature, chiefly by radiation and ascension. Clouds 

 are most dense in the center of a cyclone where the pressure is a mini- 

 mum, and are progressively less dense toward the periphery. 



Mr. H. F. Blanford, in a letter to Nature, discusses th& humid climate 

 that fosters the raidi exuberance of the Aruwhimi forests traversed by 

 Stanley's expedition. He believes the excessive rainfall is due to the 

 equatorial position of the Aruwhimi basin where ascending convection 

 currents prevail on a gigantic scale. By dynamic cooling these currents 



*This is different from the result derived by Dr. E. Bruckner. 

 G. E. C. 



(Ante, pp. 217,218.) 



