MECHANICAL DRAWING SUPPLIES 229 



run out (Fig. 98). A cup is hung at the bottom of the tree 



and gradually the milky sap runs into it. The contents of 



a number of these cups are then poured into a large vessel. 



A wooden paddle is dipped into 



the sap and when withdrawn 



is held over a fire made from 



palm nuts. The heat from the 



thick smoke hardens the sap. 



This process is repeated many 



times until a ball, called a bis- 



. , /TV rv\\ - r j TO. FIG. 99. Rubber Biscuit, 



cuit (Fig. 99), is formed. The 



paddle is then withdrawn from the biscuit and the biscuit 

 is ready for market. After coming to the market as balls 

 or biscuits, the rubber is purified and made into sheets. 

 Because of its softness and sticky nature, this crude rubber 

 is useless for erasing and consequently must be subjected to 

 a hardening process called vulcanization. This process 

 consists in subjecting the rubber to supreme heat. After 

 being vulcanized the rubber is suitable for erasers and other 

 commercial products. 



274. The Working Drawing. The drawing from which 

 the blue-print is made is called a working drawing. The 

 method of preparing it is simple. The draftsman merely 

 looks squarely at the object and draws the outline of it. 

 By changing the point of view, different views of the object 

 may be obtained. The views usually drawn are of the front, 

 top, and side. To show the interior, additional views may 

 be drawn of the object in section. 



The front view of an object is the view obtained by looking 

 squarely at it from the front. This view is often called the 

 elevation. The top view is the view obtained by looking 



