CHAPTER XX 



SCIENCE UNDERLYING MECHANICAL DRAWING 

 SUPPLIES 



270. Mechanical Drawing Supplies Required. Me- 

 chanical drawing plays a large part in directing the perform- 

 ance of all industrial operations. It is the guiding hand, so to 

 speak, which directs the erector in the shipyard, the machinist 

 in the shop, and the builder of bridges at the river. Therefore, 

 a knowledge of the principles of science underlying its applica- 

 tion is as important as a knowledge of the science of the trade. 



In making a mechanical drawing, certain supplies are 

 necessary. The first requisite is a pencil, properly made; 

 the second, a paper of suitable quality for the work in hand ; 

 and the third, an eraser of just the right degree of hardness. 

 With these simple yet important tools, together with a 

 compass and ruler, the draftsman makes his working draw- 

 ing. The tracing of the pencil-made drawing is the next 

 step in the process. For the purpose of tracing, tracing 

 cloth and paper are necessary, as well as a special kind of 

 ink, called India ink. After a tracing lias been completed, 

 the making of the blue-print comprises the final step. A 

 blue-print is used because if it is lost, another one can easily 

 be made, whereas an original drawing can be made only at 

 the cost of much time and money. 



271. The Lead Pencil. A lead pencil consists of a stick 

 of graphite in the center of a cylindrical piece of red cedar 



* 5 225 



