318 NINETEENTH REPORT. 



(28 ft.) marks were found to contain spores. The distribution of spores 

 was verj^ uneven^ but the fact that some fell into the dishes at the 

 farthest end of the tube, is a positive indication that they were carried 

 at least 14.6 m. (48 ft.). Minute black deposits of spores were also found 

 scattered along the board upon which the dishes were placed. While 

 it is possible that the spores remained suspended in the air over the 

 entire length of the tube it is more likely that they fell to the bottom 

 of the tube several times while in transit, to be lifted again by the wind 

 and carried farther. 



The Rate of Fall of Spores: 



This assumption is not based entirely upon the uneven distribution of 

 spores within the tube but also upon the results of the following experi- 

 ment, whose purpose was to determine the velocity of spores falling in 

 still air. For this purpose a glass tube 1.5 meters long and 4.8 cm. 

 inside diameter was used. This tube was set up in a vertical position 

 so that the lower opening rested about 1 cm. above the top of the table. 

 The windows and doors of the room were closed, so as to exclude, as 

 far as possible, any disturbing draughts. A petri dish containing agar 

 was slipped under tlie lower opening of the tube. Dried spores were 

 then released at the upper opening. Eight seconds after the release of 

 the spores, the agar dish was quickly removed and another immediately 

 slipped into its place. This second dish was no sooner in place than it 

 was replaced by another. The second was removed 10 seconds, the 

 third 12 seconds, the fourth 15 seconds and the fifth 20 seconds, after 

 the release of the spores. Other dishes were placed under the tube at 

 five-second intervals after the 20th second. Only in the first two dishes 

 were spores found, — a large number in the first, and only a few in the 

 second. It required, therefore, a maximum of about 10 seconds for the 

 spores to fall a distance of 1.5 meters, — that is, the minimum velocity 

 of a falling spore is roughly, about .15 meters per second. A velocity 

 of 6.268 km. per hour is equivalent to 1.74 meters per second. In the 

 14.6 meter tube used in the preceding experiment about two seconds 

 would therefore be required for the spores to fall from the top to the 

 bottom, — a distance of 30 cm. A wind with a velocity of 1.74 meters per 

 second would carry a spore a maximum horizontal distance of 2 x 1.74 m. 

 or 3.48 m. before it fell to the bottom of the tube. Once having fallen 

 there is no reason why a slight upward trend of the wind could not 

 again lift the spore and carry it farther, provided of course, it were not 

 already trapped or impeded by some obstacle. 



