6 Trans. Acad. Sci. of St. Louis. 



From what has been said it is apparent that the open leg of 

 the water gauge should be connected with some standard 

 pressure, and not allowed to communicate with the air in the 

 room. The method suggested by Abbe was employed.* 



Two thin circular steel plates eighteen inches in diameter 

 were ground to a sharp beveled edge. They were clamped 

 together and held rigidly one-eighth of an inch apart, by four 

 screws near the edge having accurately turned bushings be- 

 tween the plates. A small screw was also used to clamp the 

 plates near the center, in order to prevent vibrations of the 

 plates. A face plate was screwed to the center of one plate 

 into which an iron pipe could enter, and which served as a 

 vertical support for the plates. Inside of the pipe a rubber 

 tube made connection with a small brass tube which pierced 

 the large plate flush with its surface. This device is placed 

 above the building in the undisturbed stream of air, with the 

 plane of the disks horizontal. The tube terminating between 

 the two disks was connected with the inclined leg of the pres- 

 sure gauge. The cistern of the gauge was connected with 

 the collector shown in Fig. 1. This collector was mounted 

 above the building near the other device. When the large 

 disks were as near as one-eighth of an inch it was found 

 that the two collectors balanced against each other per- 

 fectly, in the strongest winds of last winter. The conditions 

 of steady flow of a fluid between two large plates have been 

 discussed by Sir William Thomson. f Abbe states the result 

 as follows : " Steady motion becomes easy and turbulent 

 motion becomes difficult when the distance between the plates 

 is equal to or less than the diameter of the plates, multiplied 

 by the ratio — coefficient of viscosity of the air, divided by 

 coefficient of skin friction of the air on the plate. 



M The pressure of the air within the tube is then the same 

 as that between the plates, and in the free air around them." 



When the cistern of the gauge was allowed to open into 

 the air of the room, the variation of pressure within the 

 building, due to gusts of wind, was plainly shown. 



* Report of the Chief Signal Officer, 1887. 2 : 144. 

 t Phil. Mag. Sept. 1887, and verbally at the Manchester meeting of the 

 British Association. See Report of the Chief Signal Officer before referred to. 



