378 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 4 



the gas follows the laws of diffusion to the extent that the rate of escape 

 is directly proportional to the differences in density. All the data at 

 present at hand accords with the assumption with the limits involved; 

 it is probably a very close approximation to the facts. 



The amount of leakage may be stated in terms of percentage of the 

 area of the surface ; this is readily determined b}^ finding the area of 

 an orifice which will permit the flow of air at the same rate as through 

 a portion of the canvas tested. The ratio of these two areas gives 

 the percentage of leakage. For practical purposes it is better to 

 determine the leakage of a doubled canvas and use this figure since 

 when this is done one can easily make determinations at any part of 

 the tent. The average leakage of doubled fumigation tents is .25% 

 and the common range from .1% to A%. depends on the character 

 of the canvas and on variations in weather conditions. 



The density of the gas depends upon the relation of the dose to the 

 volume of the tent. According to the universal practice of fumiga- 

 tors, the density used, will be greater in small tents and less in large 

 tents. If the dose were made exactly proportional to the area then the 

 density would be represented by the ratio ^ and be therefore inversely 

 proportional to the diameters in trees of similar shapes. Thus it will 

 be seen that while the area basis of scheduhng may give a rough 

 approximation of the dosage of the average leakage .25%, it will 

 clearly not apply even roughly to tents showing any other per cent of 

 leakage. 



The effect of leakage upon the density in a small tent is greater than 

 in a large tent because each unit of surface covers less average depth 

 of gas ; the quantity escaping per minute depending upon the density. 

 It is evident that if a gram escape from each unit of surface the first 

 minute from each of two tents, the remaining gas is a larger percentage 

 of the original amount in the case of the larger tent. The volume 

 behind each unit of surface of tent is indicated by the ratio I and for 

 tents of uniform shape is directly proportional to the diameters. 



Stated in other words the time required to reduce the gas to 50% 

 of its original strength is twice as long in the case of a tree 20 ft. in 

 diameter as would be required for a tree 10 ft. in diameter when each 

 is covered with a tent showing the same percentage of leakage. 



The per cent of leakage determines the time required to reduce the 

 density to any given percentage. Very evidently the size of the orifice 

 in the escape either of a liquid or of a gas determines the rate of escape. 

 Two openings will cause it to pass out in half the time and so will 

 one orifice with twice the effective area. The density of the gas in a 

 tent of .2% leakage ^vill require twice as long to drop to 50% as it 

 would in the same sized tent having .4% leakage. 



