Cipher — Frictional Effect of Railway Trains upon the Air. 221 



gauge, communicated with the air in the car. The car win- 

 dows and doors were open, and the measurements were 

 usually made at the middle of the car. In the special car, 

 the open end of the manometer was connected with a tank of 

 standard pressure, in communication with an Abbe collector 

 above the top of the car. There was no substantial difference 

 in the results obtained by these various methods. 



It is evident that if the air around the car were being carried 

 along with it at the same velocity as the car itself, the gauge 

 should show no pressure. A person standing on the ground 

 near the car would then feel a gust of wind, blowing with the 

 velocity of the car. If the air near the train is being carried 

 along by the car, but lags somewhat, the pressure measured 

 from the car will correspond to this lag, or slip of the air 

 upon the car. If the observation could be made from the 

 ground, the mouth of the collector being turned in an oppo- 

 site direction, the pressure collected would be due to motion 

 of the air, dragged along by the train. 



Let P = the pressure corresponding to the velocity of the 

 car. This would be the pressure shown by the gauge, if the 

 collector were thrust far out from the car into the undis- 

 turbed air. This distance to which the collector must be 

 thrust in order to collect the pressure P, is really infinite, 

 but at a comparatively short distance this pressure is nearly 

 attained. 



It is not uncommon to see hats blown from the heads of 

 people standing 25 feet from p LlmltiIlg Pre8BUre 



the track, by the air-draught 

 of a fast express train. 



If the collector be thrust 

 out to a distance d from the 

 side of the car, the pres- 

 sure will be some pressure 

 p, smaller than P. 



In the diagram, p, re- 

 presents the side of the car, 

 and is taken as the axis of 

 pressure. d, is the axis of distance from the side of the 

 train. The pressure at the car surface, as shown by the 



