32 



MECHANICS. 



in diameter {a, fig. 15,) it will rise a quarter of an inch; 

 if but the twenty-fifth of an inch in bore, as b, it will rise 

 half an inch ; but if only a fiftieth of an inch, the water 

 will rise an inch. This ascent of the liquid is caused by 

 the attraction of the inner surface of the tube, until the 

 weight of the column becomes equal to the force of the 

 attraction. Capillary attraction may be also exhibited by 



Capillary attraction in tubes. 



Capillari/ attraction heticeen two panes of 



si ass. 



two small plates of glass, placed with their edges in wa- 

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

 other side, as in fig. 16. As the faces of the plates ap- 

 proach each other, the water rises higher, forming the 

 curve, a. 



Capillary attraction performs many important oftices 

 in nature. Tlie moisture of the soil depends greatly upon 

 its action. If the soil is composed of coarse sand or grav- 

 el, the interstices are large, and, like the larger glass tube, 

 will 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 sufiiciency of moisture. 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 



