CAUSES OF CLIMATIC CHANGES. 323 



"Evidence of the dust appeared at Bassour, Algeria, on or before June 19, 

 and at Mount Wilson, California, on or before June 21. 



"The total direct radiation of the sun was reduced by nearly or quite 20 per 

 cent at each of these stations when the effect reached its maximxmi in August. 



"In the ultra-violet and visible spectrum the effect was almost uniform for 

 all wave-lengths, but was somewhat less in the infra-red. 



"From Bassour experiments, including measurements by two methods of 

 the radiation of the sky, it appears that the quantity of heat available to warm 

 the earth was diminished by nearly or quite 10 per cent by the haze. There 

 is, however, some indication that this was in part counterbalanced by a 

 decrease in the earth's radiation to space, caused by the haze. 



"Similar periods of haze followed great volcanic eruptions in former years. 

 The influence of Krakatoa, Bandai-San, Mayon, Santa Maria, and Colima 

 seems to have been recorded by measurements of solar radiation, and caused 

 pronounced decrease in the direct solar beam from 1883 to 1885, 1888 to 1894, 

 and 1902 to 1904. 



"Evidence is presented that the dust layer of 1912 affected terrestrial 

 temperatures, especially of high stations. 



"A remarkable correspondence is found between the average departures 

 of the mean maximum temperatiu-e for 15 stations of the United States and 

 a curve representing a combination of the sun-spot numbers of Wolfer and 

 the departures from mean values of the annual march of direct solar radiation 

 from 1883 to 1904." 



Humphreys (1913 : 1) has reached the following conclusions in regard to 

 the action and effect of volcanic dust: 



"As a matter of fact, volcanic dust, at least much of it, consists of thin- 

 shelled bubbles or fine fragments of bubbles, and therefore must settle much 

 slower than solid spheres, the kind above assumed. Indeed, the finest dust 

 from Krakatoa, which reached a great altitude, probably not less than 40 nor 

 more than 80 kilometers, was from 2| to 3 years in reaching the earth, or 

 presumably, as above explained, the upper cloud levels. 



"At any rate volcanic dust is so fine, and the upper atmosphere above 11 

 kilometers so free from moisture and vertical convection, that once dust is 

 thrown into this region (as it obviously was by the explosions of Skaptar 

 JokuU and Asamoyama in 1783, Babuyan in 1831, Krakatoa in 1883, Santa 

 Maria and Pel6 in 1902, Katmai ia 1912, and many others), it must require 

 as a rule, because of its slow descent, from 1 to 3 years to get back to the earth. 

 And this clearly has always been the case, since the earth first assumed sub- 

 stantially its present condition, or had a cool crust and a gaseous envelope. (11) 



"As just stated, the total quantity of dust sufficient, as we have seen, to cut 

 down the intensity of the direct solar radiation by 20 per cent, and therefore 

 if indefinitely continued, capable, presumably, of producing an ice age, is 

 astonishingly small — only the one hundred and seventy-fourth part of a cubic 

 kUometer, or the seven hundred and twenty-seventh part of a cubic mile, even 

 assuming that the particles are spherical. (31) 



"Hence even this small amount of solid material distributed once a year, 

 or even once in two years, through the upper atmosphere, would be more than 

 sufficient to maintain continuously, or nearly so, the low temperature requi- 

 site to the production of an ice age, nor would it make any great difference 

 where the volcanoes productive of the dust might be situated, since from 

 whatever point of introduction, the winds of the upper atmosphere would soon 

 spread it more or less evenly over the entire earth. A little calculation will 



