30 
THE YOUNG SCIENTIST. 
impossible for him to say whether this dif- 
ference is due to actual size, or to the con- 
vergence caused by the perspective. And 
if instead of a simple pair of blocks, like 
those shown, the arrangement were a com- 
plicated machine, it would require a very 
expert draughtsman indeed to determine 
the real sizes of the different parts. This 
feature, which is very obvious when once 
pointed out, renders drawings made in per- 
spective almost useless in the workshop. 
Working drawings are therefore made 
according to the simplest possible system, 
the outlines being drawn as if the object 
were viewed from an infinite distance. A 
few illustrations will explain this better 
than a volume of words. In Fig. 2 the two 
Fig. 2. 
blocks are shown resting on each other, the 
observer being supposed to be situated at a 
great height above them. As seen in Fig. 
2, however, it would be impossible to say 
whether the lines bounding the inner figure 
denote a hole or a projection. To avoid 
«,ny such ambiguity, draughtsmen are in the 
habit of introducing what are called shade 
lines — that is to say, thick lines, such as are 
shown in Figs. 3 and 4. In Fig. 3 the out- 
lines are shaded as if the inner rectangle 
represented a hole; in Fig. 4 the shading 
Fig. 3. 
represents a projection, and the impressions 
made on the mind by such lining is fortun- 
4itely quite definite. No one could mistake 
Fig. 3 for two blocks lying one on top of 
the other, or Fig. 4 for a block with a hole 
in it. Nevertheless such shading is to a 
Fig. 4. 
certain extent conventional, that is to say it 
is followed by a sort of agreement amongst 
draughtsmen without regard to actual light 
and shade, as existing in nature, for in 
mechanical drawing the light is always sup- 
posed to fall in one direction, which is such 
that the edges bounding the right hand and 
lower surfaces on the drawing are shade lines. 
But although the reader would have no 
difficulty in determining that Fig. 3 is a 
block with a hole, and Fig. 4 a block with a 
projection, these figures give no idea of the 
depth of the hole or the height of the pro- 
jection. To determine these points in this 
kind of drawing, other figures must be in- 
troduced. These figures are known as 
elevations and sections, and will occupy our 
attention next month. 
A Good Education. 
THE late Edward Everett condensed into 
a single brief paragraph his estimation 
of what constituted a good education. Here 
it is: *' To read the English language well, 
to write with dispatch a neat, legible hand, 
and be master of the first four rules of 
arithmetic, so as to dispose at once, with 
accuracy, of every question of figures which 
comes up in practice. I call this a good 
education. And if you add the ability to 
write pure, grammatical English, I regard 
it as an excellent education. These are the 
tools. You can do much with them, but 
you are hopeless without them. They are 
the foundation; and unless you begin with 
these, not with flashy attainments, a little 
geology, and all other ologies and osophies, 
are ostentatious rubbish. " 
