Nipher — Method of Measuring Pressure on a Structure. 7 



With a view of making a further test of the disk collector 

 of Fig. 1, it was decided to make a study of the distribution 

 of wind pressure over a large pressure board, carried above 

 the roof of a railway car. The President and the General 

 Superintendent of the Illinois Central Railroad readily con- 

 sented to co-operate with us, and furnished a car which was 

 changed to adapt it to the work. The shops of the road at 

 East St. Louis and Centralia, Illinois, were practically placed 

 at our disposal, and the apparatus, which had been con- 

 structed in the shop of the Physical Laboratory of Washing- 

 ton University, was sent to the railroad shops, and mounted 

 upon the car. 



The pressure board was three feet wide in vertical dimen- 

 sion, and four feet long. It was made of dry pine seven- 

 eighths inch thick, the wood having been boiled in oil, in 

 which it was allowed to cool. In this way it was hoped to 

 prevent warping due to sun and rain. The boards were 

 grooved together, and were bound at the top and bottom by 

 cross pieces of wood in the plane of the board. The board was 

 provided with two strap hinges of iron, one-fourth of an inch 

 thick, four inches wide, and crossing the entire board. They 

 covered the horizontal strips marked b and h in Figs. 3 and 4. 



The hinges were each bolted by twelve f inch bolts at the 

 center of the four inch squares shown in the figures.* The 

 hinges extended beyond the board and locked to a vertical 

 iron pipe one and five-eighths inch external diameter which 

 passed down through the roof of the car to the floor. The 

 board was mounted upon the pipe somewhat as a flag is 

 mounted upon its staff. The distance from the edge of the 

 board to the center of the pipe was six inches, the clearance 

 between board and pipe being intended to isolate the board 

 from the pipe. The hinges were only an inch wide in vertical 

 dimension where they crossed this interval, but were made 

 broader laterally. The lower edge of the pressure board was 

 one foot above the center of the roof. 



The pipe turned on ball bearings at the bottom, capable of 

 taking up any thrust in a vertical or a horizontal direction. 



* The pressure board is not shown to scale In these figures, since the 

 small areas are not squares. 



