through Porous Materials. 359 



using the apparatus the tube, T, would be disconnected from 

 the source of compressed air, and one of the sizes of shot, B 

 for example, poured into it till the desired length of shot 

 column had been added. The tube would then be reconnected 

 to the compressed air-pipe, and the cock, S, turned till the 

 manometer indicated the desired difference in air pressure 

 between the two ends of the column of shot. The rate of flow 

 of air through the shot would be measured by noting with a 

 stop-watch the time required for a definite volume of air, 

 usually one cubic foot, to pass through the meter. The data 

 recorded would be the size of shot, A, B or C ; the length in 

 centimeters of the column of shot in the tube ; the difference 

 in pressure between the two ends of the column, given by the 

 manometer in centimeters of water, and the time in seconds 

 required for a given volume of air to flow through the shot. 



The first experiments were for the purpose of ascertaining 

 the relation between the resistance offered by a granular mate- 

 rial to the flow of air and the length or thickness of the mate- 

 rial used. The length of a column of shot, C for example, was 

 made equal to 10, 20, 30, etc. centimeters successively, and the 

 difference in pressure between the two ends of the column was 

 made respectively 1, 2, 3, etc. centimeters of water. Under 

 these conditions the times required for a given volume of air 

 to flow through the different lengths of the material were 

 found to be equal. 



For convenience in stating the results of these experiments, 

 the term pressure-gradient will be used to designate the 

 quotient obtained by dividing the difference in pressure between 

 the two ends of a column of shot by the length of the column. 

 It represents the difference in pressure per unit length of the 

 material. Using this term, the results of the above experi- 

 ments may be stated as follows : 



For a given size of shot and a given pressure-gradient, the 

 rate of flow of air through the shot is independent of the length 

 of column used. This is equivalent to the statement that, other 

 things being equal, the resistance offered by one of these mate- 

 rials to the flow of air through it, is directly proportional to 

 the length or thickness of the material used. 



Experiments were conducted with pressure-gradients rang- 

 ing from 3L to about -§- of a centimeter of water per centimeter 

 length of material, and the columns of shot were varied in 

 length from 5 to 70 centimeters. All three sizes of shot were 

 experimented with, and no marked deviation from the above 

 law was found within these limits. 



The next experiments were for the purpose of studying the 

 relation between the increase in difference of pressure between 

 the two ends of a column of shot, and the increased flow of air 



