228 



APPLIED SCIENCE 



rollers which compress and squeeze out more of the water 

 from the creamy substance, making the sheet of paper less 

 spongy. After the pulp has been pressed into a sheet, the 

 screen passes over hot rollers for the purpose of drying the 

 wet sheet of paper. The distance which the paper pulp 

 travels on the screen before it is transformed into paper 

 is 100 ft. or more. The thickness of the paper depends 

 upon the rate at which the pulp is allowed to flow on the 

 traveling screen. "Hot pressed" paper is paper to which 

 the extreme pressure is applied while the pulp is still hot, 

 while "cold pressed" paper is not subjected to pressure 

 until the pulp is cold. The former type of paper is of the 

 highest grade. 



273. Rubber Erasers. Rubber erasers are used exten- 

 sively in drawing to remove pencil and ink marks. They 



are made of rubber combined 

 with sufficient sulphur to 

 give the proper hardness. 

 Other materials are added in 

 varying proportions to give 

 different degrees of softness 

 and suppleness. It is by 

 these qualities that the dif- 

 ferent grades of erasers are 

 distinguished. 



The process of obtaining 

 the rubber used for erasers, 

 and for many other purposes 

 as well, is a most interesting 

 one. Rubber is obtained from the sap of certain tropical trees. 

 A series of slanting cuts made in the bark allows the sap to 



FIG. 98. Tapping Rubber Trees. 



