216 Trans. Acad, Sci. of St. Louis. 



Some years later I arranged a device with which to test the 

 conditions in this air-draught. It was built after the plan of 

 the rotating anemometer used by the Weather Bureau, except- 

 ing that the four hemispherical cups were replaced by thin 

 flat metal plates. The rotating system was very delicately 

 mounted, but in an ordinary wind, there was rotation only 

 as a result of eddy motions in the air. In the long run, the 

 slow rotations in opposite directions canceled each other. 



A top view of the rotating part is shown in the adjoining 

 figure. 



The four cross arms, having the flat plates at their outer 

 ends, were of No. 6 brass wire, and they were braced by a 

 steel wire which connected their outer ends. 

 The distance from center to center of oppo- 

 site plates was one foot, and the plates were 

 four inches square. 



When this differential anemometer is 

 brought near a building against which the 

 wind blows obliquely, it begins to revolve. 

 Fig. . ^h\^ revolution is due to the fact that the 



velocity of the air sheet sweeping over the surface, is less, the 

 nearer the face of the building is approached. When held 

 out of the window of a moving train, the system is put in 

 rapid rotation. 



The plate nearest the car is shown to be in wind of less 

 velocity, than those further out. This shows that air is being 

 dragged along by the train, and that the concentric layers of 

 air around the train are shearing on each other. 



On one occasion the apparatus was carried into the country 

 three or four miles west of Iowa City, and planted on the 

 ground near the track of the C, R. I. & P. R. R. It was 

 so placed that the fast train from Chicago, passed at 

 a distance of four inches from the vane nearest ihe track. 

 The plane of the vanes was about a foot above the bottom of 

 the car. The observer lay down near the track in order that 

 there might be no doubt about the question of actual collision 

 of the train with the apparatus. The distances were as had 

 been planned. The effect of the blow from the air when the 

 train passed was to break the small steel brace-wire, and to 



