GEOMETRICAL AND ENGINEERING DRAWING. 87 



to teach this subject clearly. I have always found it is the 

 elementary portion of descriptive geometry which really 

 is hard for beginners to understand ; but once that is 

 passed there is little difficulty. The real object of learning 

 descriptive geometry is to study designing ; and I thought 

 it as well, in putting together all these models, to add 

 besides a few examples of German and other models of 

 surfaces, illustrating those last mentioned, in plaster of 

 Paris. All these are simple intersections of cylinders, 

 prisms, cones, and some of these peculiar surfaces with 

 their generating lines upon them. As you see them 

 there, so they occur in space ; but once the student has 

 learned these in his elementary course, he should think 

 no more of them, except to be able to apply them when 

 required. Unfortunately, in my experience of a good 

 many years spent in teaching these subjects, I have found 

 students are extremely backward in understanding descrip- 

 tive geometry, and I should impress upon you that it is 

 absolutely essential, not only to know some plane geometry, 

 but to know also a good deal of solid, and some analytical 

 geometry. 



