28 MECUAIS'ICS. 



coming into close contact; hence tliey must "be either 

 melted, or softened like iron when it is welded. 



The different 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 becomes 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 diffi- 

 culty of working such soils when again dried. This ad- 

 hesion 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 be- 

 coming 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 OP MATERIALS. 



It is a matter of great utility in the construction of 

 machinery to determine the different degrees of cohesion 

 possessed by different substances ; or, in other words, to 

 ascertain their strength. This is done by forming them 

 into rods of equal size, and applying weights to their 

 lower extremities sufficient to break them, by drawing 

 them asunder. The amount of weight shows their rela- 

 tive degrees of strength. The following table gives the 

 weights required to break the different substances, each 

 being formed into a rod one quarter of an inch square : 



