8 



THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



and here, as also at the two intermediate places just mentioned, whirls 

 are formed which set in rotation the little vanes placed thei c The phe- 

 nomena agree with those that one observes in streams and rivers in 

 front of and behind sharp protruding rocks or piers. 



It must still be mentioned that neither water nor air rebounds like 

 elastic spheres from the obstacle against which it strikes, as is fre- 

 quently assumed. Even strong streams of water that I allowed to play 

 against the plates did not rebound, but continued their onward path 

 close to the obstacle, producing a strong current there. 



I had instituted these experiments in order to see in what manner 

 the resistances originate that the liquid experiences in such deviations 

 and which cause the pressure against the opposing plate. However, I 

 thought it was allowable to assume that when the plate is itself moved 

 through stationary water or air the ratios remain nearly the same and 

 that similar currents of the fluid occur in its neighborhood. The pres- 

 sure that the plate experiences in this latter case is the object of the 

 following investigation which is moreover confined to plane disks 

 moved through the air in a direction perpendicular to their planes. 



Already 40 years ago I 

 had occupied myself with 

 the same problem,* but the 

 apparatus used at that time 

 was too imperfect to give 

 useful results. In essential 

 points I have retained the 

 earlier arrangements, but 

 many changes have been 

 made in order to remove the 

 defects. The accompanying 

 plate shows the apparatus 

 now used by front and side 

 elevations (see figs. 1 and 2), 

 as also by a horizontal sec- 

 tion (see tig. 3) through the 

 line A B, of fig. 2. 



Two thin arms of straight- 

 grained pine wood which are 



JYff. 7 



*Some of the series of observations made at that time are communicated as exam- 

 ples of the application of the method of least squares in the first edition of the 

 Grundzuge der Wahrscheinlichkeits-Rechnuug." 



