204 , BURTON E. LIVINGSTON 



problem thus brought forward required considerable attention 

 before a workable solution was reached. 



To avoid the possibility of any very general or extensive 

 drawing-away of the soil from the porous porcelain surface, it is 

 only necessary to enlist the force of gravitation to maintain the 

 desired contact. This can be done by giving a horizontal or 

 oblique position to the porous surface with which the soil is to be 

 kept in contact. If this surface is horizontal or oblique, and below 

 the soil to which water is to be supplied, the shifting of the soil 

 grains is generally so controlled by gravitation that the soil tends 

 always to slip downward, filling any considerable openings that 

 may occur. Consequently, contact of the soil with a horizontal 

 or oblique surface upon ivhich the soil rests is usually much more 

 perfect than it is with a vertical surface against ivhich the soil lies. 

 Only a small component of the gravitational force is effective to 

 press the soil laterally against a vertical porous cylinder embedded 

 in it, and the soil tends to settle downward past the porous cup 

 rather than against it. The desired end is partially attained by 

 placing the porous cup with its long axis horizontal or oblique, but 

 this method introduces other difficulties. In the first place only 

 a portion of the upper half of the cup's cylindrical surface is now 

 below the soil with which it is supposed to be in contact, for the 

 lower half of this surface is directed toward the soil below. It is 

 clear from this that the whole cup must be free to move downward 

 in such a case, if good contact with the soil is to be maintained 

 both above and below, but even this fails very largely, for the 

 soil movements in question are not generally uniform throughout 

 the soil mass and contact may still occur at certain places while 

 withdrawal of the soil from the cup surface may occur at others. 

 Fu"rthermore, it is impossible to place a cylinder in any position 

 so that some portions of its surface are not vertical, and vertical 

 surfaces are to be avoided. 



A horizontal plane porous surface situated below the soil mass 

 to be supplied with water operates satisfactorily, but difficulties 

 are met with when one attempts to arrange such a surface in a 

 potted mass of soil. In such a case the porous plate supplying 

 the water should be the bottom of the soil container, which neces- 



