STRENGTH OF MATERIALS. 43 



of pitch, required 1400 pounds. On this principle de- 

 pends the efficacy of those substances which are used 

 for cementing broken vessels. 



The most perfect artificial polish which can be given 

 to hard metals is still so rough as to prevent the faces 

 from coming mto close contact ; hence they must be 

 either melted, or softened like iron when it is welded. 



The diiferent degrees of cohesion which take place 

 between the particles of various soils, to reunite them 

 after they have been crumbled asunder, occasion the 

 main difference between light and heavy soils. When 

 a light soil becomes soaked with water, the particles 

 adhere in a very slight degree ; and hence, when it be- 

 comes dry again, it is easily worked mellow. But if 

 it be of a clayey nature, too much moisture softens it 

 like melted wax : the particles are thus brought into 

 close contact, and strong adhesion takes place ; hence 

 the hardness and difficulty of working such soils when 

 again dried. This adhesion is lessened by applying 

 sand, chip-dirt, straw, yard-manure, or by burning the 

 earth, but more especially by thorough draining, which, 

 preventing the clay from becoming so moist and soft, 

 lessens the adhesion of its parts. . 



Different substances are hard, soft, brittle, or elastic, 

 according to the different degrees or modes of action 

 in the attraction of cohesion. 



STRENGTH OF MATERIALS. 



It is a matter of great utility in the arts to determ- 

 ine the different degrees of cohesion possessed by the 

 different substances; or, in other words, to ascertam 

 their strength. This is done by forming them into 



