396 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 3 



to give measurements of pressure when connected with various 

 experiments, it was found very difficult to secure absolute tightness 

 even when the interior was thoroughly coated with glue and the 

 exterior hkewise covered with paraffin. 



The apparatus used in making the tests here reported was designed 

 to give comparative results rather than an absolute measurement of 

 the porosity of a given area in the materials tested. The idea involved 

 is certainly not new and has, I think, been employed by Dr. A. W. 

 Morrill and Mr. W. W. Yothers in tests of tent materials in white 

 fly investigations in Florida. The accompanying illustrations (Plate 25, 

 figures 1 and 2) give a fair idea of the apparatus used. A large aspirator 

 bottle was elevated to such a height that a column of water varying 

 from 17 to 27 inches in height, could be utihzed. Steam valves arranged 

 to make air-tight connections through rubber stoppers were fitted to 

 both inlet and outlet openings. The outlet pipe opened under water 

 to prevent air being drawn back into the apparatus when a partial 

 vacuum was produced. This was arranged, however, so as to overflow 

 and not affect the uniformity of pressure at the outlet. The samples 

 tested were placed in the union coupling at the upper left hand corner 

 of the apparatus, where they were clamped between two metal sur- 

 faces. With the upper valve closed, and the lower valve open, water 

 could escape until equilibrium was established. The height of the 

 water surface in the bottle was then read upon the scale and the upper 

 valve quickly opened. The amount of water which then ran out in a 

 given period of time depended entirely upon the amount of air which 

 could enter through the material placed in the union. If the material 

 were air tight, no water could escape. If the material were very 

 porous, water would escape rapidly. In the comparison of materials, 

 the percentage of porosity is based upon the ratio of the inlet of air 

 through the material under test as compared with the inlet of air 

 through the apparatus with no material enclosed. That is, with the 

 union joint entirely free and open, the percentage was taken as 100. 

 The area of the samples as used was actually .44 of a square inch, but 

 this area does not affect the comparison made. 



In the table given below, the materials have been arranged in order 

 of the increasing percentage of porosity ranging from those which were 

 air tight to those which retarded the inlet of air but very little. 



