194 Trans. Acad. 8ci. of St. Louis. 



and tested similarly. The first pieces came flying off at velo- 

 cities of about 50 miles per hour, and from then on large 

 masses were torn away. By calculations it was found that the 

 wind velocities represent the following pressures per square 

 centimeter: — 



49 miles per hour = 2.738 gr. per sq. cm. 

 52 *« fi " = 3.425 " " " " 

 56 •« " " = 3.930 " " " " 

 70 " " " = 6.105 " " " " 



It appears from this that a wind velocity of at least 50 miles 

 an hour would be required to tear away pieces of the thallus. 

 That this figure is much too high will be evident to one 

 who knows how frequently the lichen is torn, and an inspec- 

 tion of the accompanying table recording wind velocities along 

 the Atlantic and Pacific coast shows that the mean velocities 

 nowhere approach this. The seeming error lies here. In the 

 first place when the moist thallus was introduced into a 

 stream of air, the drying action of the latter was felt im- 

 mediately, so quickly, in fact, that after one minute's expo- 

 sure the thallus was almost dry, i. e., no longer in its weakest 

 condition. In its native habitat no drying takes place, even 

 with high wind velocity, because of the atmospheric moisture. 

 Again, granting that a velocity of 50 mile3 tore the mass, it 

 is not by any means necessary to assume such velocities for 

 winds acting for periods longer than one minute. In this 

 respect the experiment was a failure, that it gave only an 

 extreme wind velocity. The relation found between tension 

 and pressure was unexpected. The wind pressure per square 

 centimeter at the period of greatest velocity, *. e., 70 miles, 

 was 6.1 gr., while at the tearing point it was but 3.+ grs. 

 per sq. cm. The lowest breaking strength of the thallus 

 threads however was found to be at least 23 gr. 



