(52 



the ])er cent of porosity of the noil, i. e., the 8] .ace which, in the (hy soil, is occui.ied 



by air. 



The vohniie weight of a soil varies witli tlie amount of ]>acking. A freshly plowed 



soil is much lighter per cubic foot than the same soil packed by rains or by tramping. 



In other \\ ords, soil has an api>arent and a real specific; gravity. Average field soils in 



good tilth have an apparent specific gravity 

 of alxHit 1.2, and when entirely free from' 

 air, a real specific gravity of about 2.5. 



The compacting machine referred to 

 al)Ove was designed to pack all the soils 

 into the tubes uniforndy and thus elimi- 

 nate, in a large degree, the error due to 

 unecpial packing in different tubes when 

 making comparisons of apparent S2:)ecific 

 gravity of different soils. The machine 

 does not do the work willi absolute exact- 

 ness, but seems to be a decided improve- 

 ment over the uncertain method of tilling 

 by hand, which at best gives very unsatis- 

 factory results. 



J'lrjicriiiinif Ni>. 3. 



TJIE I'OWKIJ OK LOOSIO SOILS TO UKTAIN 

 MOIST L'KK. 



l^se soils Nos. 2, o, 4, and 5 in this ex- 

 periment. I'lace disks of damp cheese 

 cloth in the bottom of tlie tubes (iig. 13) 

 and then weigh the tubes carefully on the 

 torsion balance. Fill the tubes up to the 

 mark, 1 inch from the toji, by pouring the 

 soil in gently, leaving the soil in tlie tubes 

 in a verv loose condition, with much air 

 space throughout the mass. Weigh the 

 tilled tubes. l*lace the filled tubes in the 

 emjity galvanized iron box. Pour water 

 in the box until the water level almost 

 reaches the tops of the tubes, thus allow- 

 ing the Avater to percolate up through the 

 soils. When the vi^ater level in the tubes 

 comes up to the level of the water in the 

 box remove the tuVjes and place them in 

 the frame, where the water is allowed to 

 percolate out of them. Glass plates should 

 be placed over the tops of the tubes to 

 prevent evaporation. The tubes should 

 be weighed from day to day until the 

 minimum weight is reached — until perc()- 

 lation ceases. 



The difference in weight between tlie tubes tilled with dry soil and the wet soil will 

 be the amount of water retained by the loose soil. In order to get the total water 

 content of the wet soil, it is necessary to add to this the weight of hygroscopic water 

 which the dry soil contained. The hygroscopic water of the dry soil should be 

 determined with a special sample taken at the time the tubes are filled. 



Fig. 12.— Sdil-eompacting machine. 



