Nipher — Metliod of Measuring Pressure on a Structure. 11 



eight inches high, which was connected by a large rubber hose 

 with the Abbe collector before described. The latter was 

 exposed above the roof of the car. It was pkced symmetri- 

 cally opposite the wind vane, and 38 inches above the roof. 

 At all points where rubber tubing might become sharply bent, 

 a connection of brass pipe was inserted. This pipe was bent 

 into a smooth curve by first filling it with lead. The tubes 

 were all adjusted so that gauge columns would respond with 

 equal promptness, when they were affected by a common pres- 

 sure change. The iron tank containing the common standard 

 pressure against which all pressures were measured, was made 

 large enough, so that a rise to the gauge limit affecting only 

 one gauge would not appreciably affect the others. 



The resistance of the fine brass wire cloth in the collectors, 

 was found to be equal to that of 20 to 25 feet of the rubber 

 tubing used in connecting. This was determined by a method 

 equivalent to a Wheatstone bridge arrangement. A divided 

 circuit consisting of two equal tubes led from a common 

 source at an air blast to the extremities of a long horizontal 

 glass tube, which drooped in the middle and contained a short 

 column of water as an index. The other two arms of the 

 bridge consisted of the disk collector, and a tube whose 

 length was so chosen that the pressures on the water index 

 were balanced when an air blast was applied. These circuits 

 were "grounded" in the air. The final adjustment on the 

 car was, however, made by trial, by blowing gently for a 

 moment into the tank. This adjustment was effected by 

 means of screw clamps, which pinched the rubber tubing 

 connecting the cisterns and the gauge tubes. See Plate 1. 



The gauge readings were all made by myself. The instru- 

 ment was kept constantly leveled and steadied, and on a call 

 of '* read " from Mr. Timmerman, the three gauges were 

 simultaneously determined. The columns vibrated constantly 

 through two or three centimeters, and it was necessary to be 

 always ready to take an average reading of them all. Two 

 wooden pins were set on the divisions to be read on two of the 

 scales, and the third was read first. The other two were then 

 read later. All of the work was recorded by Mr. Louis 

 Schlossstein, of St. Louis, who also made a simultaneous 



