80 NIPHER— ELIMINATION OF VELOCITY EFFECTS [April 20, 



it by soldered or brazed rivets, with four or five circular sheets of 

 wire cloth loosely clamped between the disks. The bundle of wire 

 cloth sheets projects to some distance beyond the metal disks. On 

 either side of the disk-collector so formed, a couple of sheets of wire 

 cloth are laid, thus hiding the metal disks in the wire-cloth bundle. 



With this arrangement, the pressure at the mouth of the col- 

 lecting tube is wholly unaffected by the motion of the current of air. 

 The compression and rarefaction around the tube are prevented 

 from affecting this pressure by the two disks. The compression 

 and rarefactions around the edges of the wire cloth bundle are also 

 eliminated by the projecting margin around the edges of the disks, 

 so that these compression and rarefaction effects do not get between 

 the disks. They do get between the disks and affect the reading in 

 a way that cannot be corrected, if the wire cloth layer does not pro- 

 ject beyond the metal disks. 



The layers of wire cloth on the outside of the disks is a recent 

 improvement, which is necessary for very high velocities. It serves 

 to smooth away irregularities in outline and prevents rarefactions 

 due to these irregularities in the projecting layer of wire cloth from 

 affecting the air pressure between the disks. ^ 



When this disk collector stands edge-wise in free air in a 

 current of air from a ten inch pipe delivering 96 cubic feet per 

 second, the gauge connected with it is not in the least affected. 



For comparatively small velocities and uniform pressures, as in 

 pipes connected with blowers, the tube piercing one disk of the 

 collector may be very small for a distance of an inch, and then widen 

 in order to give necessary stiffness. The wire cloth layer need not 

 then be over an inch in diameter, and the metal disks need not be 

 over half an inch in diameter. , 



In measuring wind pressures on buildings where the pressures 

 vary rapidly the tube should be a quarter of an inch in diameter, 

 and the disks should be three and one-half inches and the wire cloth 

 layers from five to five and one-half inches in diameter. The disks 

 are placed flatwise near the wall. 



^ Trans. Acad, of Sc. of St. Louis, VIII. : i. 



