THE I'OUNG SCIENTIST. 
39 
the various shades of yellow are suitable 
to metal-cutting tools ; those correspond- 
ing to the reds and browns are suitable 
i'oL- tools for cutting wood. The purples 
are for edge tools and drilling purposes, 
and the blues for metal- 
cutting chisels or steel to 
luive great elasticity, as in 
the case of carriage springs. 
Two examples of the man- 
ner in which steel tools are 
used to cut iron are given 
in the accompanying en- 
gravings ; the first is a tool 
for that king of all metal- 
cutting machines, the lathe. 
The hook end of the tool in 
Fig. 1 is hardened, and the 
faces B and d are ground 
flat on a grindstone, giving 
t o the tool, cuMing edges n.t 
F and Ct. In Fig. 2 the tool is 
shown in action ; the work is 
revolved towards the tool, and the tool is 
slowly moved in the direction of the length 
or axis of the work, the result l)eing 
that it removes a spiral ribbon of metal 
termed the cutting or chip, whose form 
is very accurately shown in the engrav- 
ing. 
Fig. 3 shows the method of cutting fiat 
surfaces. The work is heh] in ii vise fas- 
tened to one part of the macliine, while the 
In other kinds of tools the metal is sev- 
ered by sheer pressure, as in the case of 
holes that are punched or i)lates of iron 
that are cut by powerful machines, whose 
action corresponds to a pair of common 
scissors, only that in the machines one 
jaw only moves, while in scissors both 
move. All the difference that the iiard- 
ness of the metal to be cut makes is that 
it must be cut slower, and the tools must 
be harder and stronger. 
Then either 
work or the 
tool is held by another part, 
the tool is pushed over the 
work is pulled beneath the tool, and in 
either case the tool takes off a cutting very 
similar to that shown in Fig. 2. 
Practical Hints on the Construction of 
the VioIin-IV. 
E promised in our last 
article, as the readers 
of the Young Scientist 
will remember, to de- 
scribe the manner of working 
the inside of the violin-plates 
and of regulating their thick- 
ness. 
We will suppose that Mir out- 
sides of the plates have been 
carefully finished and well mod- 
eled. This being done, the work- 
man turns his plate on the worlc- 
bench, as shown in the figure, 
and whittles out the wood with 
a gouge until the plate is of a uni- 
form thickness— say a quarter of an incli. 
Let us now imagine that we are finish- 
ing the back. The backs of all good 
violins are found to be twice as thick in 
