48 MECHANICS. 



weight of the column hecomes equal to the force of the 

 attraction. Capillary attraction may be also exhibited 

 by two small plates of glass, placed with their edges in 

 water, in contact on one side, and a little open at the 

 other side, as in Fig. 16, p. 47. As the faces of the 

 plates approach each other, the water rises liigher, 

 formmg the curve, a. 



Capillary attraction performs many important offices 

 in nature. The moisture of the soil depends greatly 

 upon its action. If the soil is composed of coarse sand 

 or gravel, the interstices are large, and, like the larger 

 glass tube, wiU not retain the rain which falls upon it. 

 Such soils are, therefore, easily worked in wet weather, 

 but become too dry in seasons of drought ; but when 

 the texture is finer, and especially if a due proportion 

 of clay be mixed with the sand, the interstices become 

 exceedingly small, and retain a full sufficiency of moist- 

 ure. If, however, there is too much clay, the soil is 

 apt to become close and compact, and the water can 

 not enter until it is broken up or pulverized. It is for 

 this reason that sub-soil plowing becomes so eminently 

 beneficial, by deepening the mellow portion, and thus 

 affording a larger reservoir, which acts lilce a sponge 

 in holding the excess of falling rains, till wanted in the 

 dry season. For the same reason, a well-cultivated 

 soil is fomid to preserve its moisture much better dm*- 

 ing the heat of summer than a hardened and neglect- 

 ed surface. 



If capillary attraction should cease to exist, the 

 earth would soon become a barren and uninhabitable 

 waste. The moisture of rains could not be retained 

 by the particles of the soil, but would immediately 



