12 Trans. Acad. Sci. of St. Louis. 



reading of train speed, as shown on the dial of a Boyer speed 

 recorder. 



The work was all done on the Illinois Central Railroad 

 between Champaign and Centralia. The car was the second 

 car from the locomotive in the fast freight trains running 

 between Chicago and New Orleans. The start from Cham- 

 paign was made at 5 : 35 a. ra. and Centralia would be 

 reached at 11:50 a. m. Sometimes we made the return trip 

 in the afternoon, starting from Centralia at 2:45 p. m. and 

 reaching Champaign again at 9 :00 p. m. The work dur- 

 ing the latter part of this run was done by the light of 

 caboose lamps. The double run proved too much for our 

 endurance, and we were generally satisfied with the single 

 run. The speeds varied from 20 to 50 miles per hour, as 

 shown by the recorder. The indications of the recorder were 

 checked by watch observations on the mile posts. We found 

 a small index error which could easily have been corrected by 

 an adjustment of the driving pulley of the instrument, but 

 we preferred to make no change. 



The experiments were begun on June 26, 1897, and were 

 continued every day untilJuly 20, although part of this time 

 was devoted to work not within the scope of this paper. 



About 1,500 independent measurements were made upon 

 the pressure board. It was decided to make a very thorough 

 determination of pressures along the middle lines of the 

 board. Such observations were made along the horizontal 

 line of squares from 1 to 12, e. Figs. 3 and 4, and along the 

 two vertical rows a to ^, 6 and 7. In addition the half of the 

 board furthest from the axis was well explored, and observa- 

 tions were made at a few symmetrically located points in the 

 other half of the board in order to detect any substantial dif- 

 ference which might exist. It was to have been expected that 

 slight flexures of the board might result in some diflerences 

 in the lateral halves, although no appreciable difference was 

 found. It was however found that the dragging of air along 

 with the train caused the pressures on the front side to be 

 greater at the top than at the bottom of the board. This 

 effect was least when strong winds blew across the trains. 

 It was also found that the rarefaction was orreater near the 



