646 Journal of Agricultural Research voi. x, no. 12 



farther. Continual collisions between splash drops and rain drops 

 occur in the field, and because of this many of the former may never 

 reach their extreme distance at the prevailing wind velocity, though, 

 without doubt, some of them will exceed the distance determined in the 

 laboratory. 



COMPARISON OF THESE METHODS WITH NATURAL CONDITIONS 



It may be true that there are points in the discussion of these experi- 

 ments where the justification for comparison is less than in others, yet 

 on the whole, the results indicate the probabilities which exist, and these 

 probabilities confirm the conclusions reached in the field where the 

 spread of the angular leafspot of cotton was observed. An analysis 

 of all the factors in their resemblance to natural conditions is made, in 

 order to remove whatever doubt may exist on this point. 



The drops of the sizes used afford a means of comparison of the effect 

 of size and volume of drop on the number of splash drops and the distance 

 of splash. It is recognized that the larger drops used are much larger 

 than raindrops, yet the smallest one comes well within the limits of 

 those in a winter rain where comparisons were made. It was found 

 that drops of the same volume falling equal distances upon dry blotting 

 paper cause wet spots of relatively equal sizes — that is, 25 of the 0.02- 

 c. c. drops falling 16 feet made spots on the blotting paper with a mini- 

 mum diameter of 14 mm., a maximum of 18 mm., and an average of 

 16.03 nini- 'I'he 0.04-c. c. drop made a spot with a minimum diameter of 

 19 mm., a maximum of 25 mm., and an average of 21.08 mm. Of 106 

 drops measured during a rain on the morning of February 20, 191 7, 

 fourteen drops were within the limits of the 0.02-c. c. drop as measured 

 and compared in the way outlined above. Those of a summer rain have 

 a greater average volume so that they more closely equal the size of the 

 0.02-c. c. drop used. It is only this drop which is compared to rain- 

 drops, though the others used show the variation in the results due to the 

 variation of this factor. 



None of the surfaces used can be closely compared with those of cotton 

 leaves, though that condition is approximated most closely by the 

 inclined glass plate. In that case the glass surface holds a film of water 

 thicker than the blotting paper does and thinner than that of a level 

 plate. There are places about the margins and veins of leaves where 

 water gathers to greater depths than over the other parts, so that a 

 variation of depth of film exists which is approximated by some one or 

 another film used. 



The stability of the surface of impact in the experiments is a factor 

 which does not exist in the field. Leaves during a storm are constantly 

 in motion and the petiole affords a springlike support which permits 

 the leaf to yield after the impact of a drop. There are numerous in- 

 stances also in which the leaves meet the falling drop while moving 



