226 
James Small. 
observations are not sound is amply proved by the cases recorded 
by Treub (36), Penzig (29), Ernst (15), Gates (19), Vogler (38-40) 
and others (see Sect. A), and the argument given by Wallace makes 
it clear that negative evidence in this matter has very little value. 
Further evidence of the transporting power of wind is given 
by Udden (37), who showed that particles between -001 mm. and 
•04mm. are apparently completely borne up by a wind of 8 m.p.h. 
Thoulet (35) states that a hailstone 5 mm. in diameter could be 
sustained in the air by a wind of about 22 m.p.h. and one of 10 mm. 
diameter by a wind of about 33 m.p.h. After summing up the 
available evidence on the dispersal of dust particles Evans (16) 
says, “In any case, it is sufficiently demonstrated that particles of 
detritus up to about 100 microns in diameter,. . . , are capable of 
remaining suspended in moving air for an indefinite period. A very 
moderate wind will carry them along with it, and a slight 
unevenness of the surface is sufficient to give rise to an upward 
current of the air, which will raise its freight of minute mineral 
particles high above the ground.” 
Since dust particles of the size mentioned have a rate of fall 
which is two to four times that of the average pappose fruit, it is 
not unreasonable to suppose that moving air will have an effect on 
such fruits equal to its supporting effect on more or less spherical 
particles which are not so well adapted for dispersal by wind. 
It has been calculated by Udden (see Evans, 16 p. 254) that in 
the west of the United States an average of about 850,000,000 tons 
of dust is carried 1,440 miles in each year. It is not unreasonable 
to suppose that such a weight might include some pappose fruits, 
which have an average buoyancy more than equal to the buoyancy of 
the larger particles of dust. Evans also refers to observations of par¬ 
ticles of sand as much as "2 mm. in diameter being blown from the 
Sahara as far north as Hamburg. As the rate of fall of such particles 
is considerably greater than that of most pappose fruit this is further 
evidence for the probable dispersal of such fruits to considerable dist¬ 
ances. Spherical fruits such as those of Leontopodiuni alpinum with a 
diameter of-25mm would be transported long distances at least as 
easily as grains of sand '2 mm. in diameter. 
Praeger (30, p.72) also mentions that “thistle down was watched 
half a mile from the shore blowing seaward, and in every case its 
course was practically a straight line.” It is quite clear that the 
theoretical view adopted by Praeger, that particles in moving air 
acquire the same velocity as the air, agrees neither with the 
