Sept. 17, 191 7 Disease Dissemination by Wind and Rain 647 



toward it in the wind and increase the force of impact, in that way 

 increasing the number of splash drops and distance of splash. 



The greatest distance of fall (16 feet) was the highest available in the 

 laboratory. The smallest drop used had probably reached its maximum 

 velocity when falling this distance. If the distance of splash as a meas- 

 ure of the force of impact is an indication of the velocity of the falling 

 drop, this is true, as is shown in Table II, where the distance of splash at 

 4, 8, and 16 feet from blotting paper and glass plates are relatively the 

 same. This conclusion supports our measurement of raindrops as com- 

 pared to the 0.02-c. c. drop, since the atmospheric resistance retards an 

 acceleration of velocity beyond a definite maximum, and both our 

 0.02-c. c. drop and the raindrops had reached this maximum when they 

 were measured. 



The influence of the wind was not further investigated because of the 

 limited apparatus available. The fan used produced a velocity of 10 

 miles per hour at a distance of 5 feet. It was thought best to keep 

 that far from the fan because of the small size of the air cone in motion 

 nearer it. At the distance used, all the air surrounding the surface 

 of impact was moving uniformly and the conditions were most favorable. 

 When it is noted that the distance of splash was increased by such a 

 wind velocity to 18 feet and that this velocity is almost a constant 

 feature of the summer thunderstorms of this region, it is readily seen 

 that this distance of splash is not to be unexpected in the field. In 

 the graphs in the article already cited ^ it is shown that — 



the wind reaches a velocity of 25, 29, and 35 miles per hour during periods in the 

 storms when rain is falling heavily and after the foliage has been wet for some time. 



While it has not been determined, it is most probable, all things being 

 considered, that the distance of splash will vary directly with the wind 

 velocity. If water is splashed 18 feet by a wind blowing 10 miles per 

 hour, it will splash approximately 50 feet at 30 miles per hour. 



The elevation of the surface of impact in the experiment in which 

 the effect of the wind was observed was 3 feet, and this is entirely within 

 the range of the height of cotton leaves, many being higher than this dur- 

 ing the latter half of the season. It is shown in Table III that the splash 

 drops from the 0.02-c. c. drop falling 16 feet reach their maximum dis- 

 tance at this elevation in a still atmosphere. 



CONCLUSIONS 



It has been shown by experiment that water is splashed by a falling 

 drop only when it falls upon a film of water, and it is the water of the 

 film which composes the splash drops. The distance of the splash varies 

 according to the size of drop, depth of surface films, elevation and inclina- 

 tion of surface of impact, and the velocity of the wind. Field condi- 

 tions have been duplicated with sufficient accuracy to justify the con- 



1 Faulwetter, R. C. Op. cit., p. 465. 



