GEOMETRICAL AND ENGINEERING DRAWING.!^ 



Lastly, I may mention another very beautiful instrument, 

 although one not of much practical use for engineering 

 purposes. It is what is called the geometrical pen. There 

 are a great variety of figures on the board on which it 

 is placed, which can be traced by it ; but they are all 

 nothing more than either circles, cycloids, epicycloids, 

 or the combination of one with the other ; the whole 

 apparatus depends on these three wheels, which are 

 convertible. According as you shift this point and as 

 you change the size of the wheels, of which you see a 

 great number here, you can produce any of these curves 

 you choose. 



Now, although I have mentioned these instruments, 

 every draughtsman should be familiar with the curves 

 which they are arranged to draw, so as to be able to draw 

 them at points of his work where it would be utterly 

 impossible to use these geometrical instruments, and in 

 any case no instrument, however perfect, will supply a 

 knowledge of freehand drawing ; for almost every archi- 

 tectural or engineering drawing contains some portions 

 which cannot be executed with the compasses or with 

 ordinary drawing pens or instruments, and for which 

 recourse must be had to the common pen. There are two 

 instruments with which I have no doubt you are familiar 

 some of the oldest and best known, and which are really 

 of great use. They are both intended for enlarging or 

 diminishing drawings. One is called the eidograph, and 

 its intention is to enlarge or copy plans. You see here 

 two plans, one large and one small, the small one being 

 diminished from the original by the eidograph. This consists 

 of two bars, which are rigidly parallel, retained in their 

 places by two steel bands. At this point there is the 

 instrument for tracing, and here is the point round which 

 the whole instrument can rotate. Here is a rod which 

 simply keeps the instrument in its place : and at the 

 other end is the pointer or tracer. The stylet, fulcrum, 

 and tracing-point are all in a straight line, and the dis- 

 tances of the tracing-point and stylet from the fulcrum 

 are in the ratio you wish to have your drawing reduced or 

 enlarged. Then here is another instrument, called the 

 pantograph, of which there are several in the exhibition, 

 and which would serve the same purpose as the eidograph. 



