126 THE YOUNG 
self-acting lathes and planing machines, 
in which cast iron, steel, and brass, is 
turned, or planed, steel tools when pro- 
perly hardened are best fitted to withstand 
the duty required of them, no tempering 
or modification of their original hardness 
being requisite or desirable. 
In order to "harden " such tools, all that 
is requisite is to heat the operative part to 
just such a degree of red heat as that when 
plunged into cold water they shall just 
become hard— any degree of heat beyond 
this will prove seriously injurious to the 
capability of the cutting edge in preserv- 
ing its keenness and power of resisting the 
strain of the cut. Such a degree of red 
heat as will be barely visible in daylight, 
is ample to cause steel to acquire the 
maximum of hardness when plunged into 
cold water. 
Want of due care and attention to these 
conditions in hardening steel is a more 
fertile source of failure and disappoint- 
ment than many are aware of. If those 
who have to do with practical manage- 
ment of machines requiring the use of 
steel tools would carefully attend to the 
above observations, the results would well 
repay them for the special attentions given 
to the subject. 
In the case of such tools as are em- 
ployed in the generality of wood-turn- 
ing, and for screw-taps, drills, chipping 
chisels, and the like, the process of " tem- 
pering" is requisite. 
Tempering in this case, as the word sig- 
nifies, is the modification to a greater or 
less degree of the original hardness and 
brittleness, due to and natural ;o the pro- 
cess of hardening. Tempering, in fact, is 
the process whereby we exchange some of 
the original hardness in order to gain a 
corresponding equivalent of toughness, 
and is attainted by re-heating to certain 
degrees the original hardened tool. 
The mode by which this tempering pro- 
cess is most conveniently attained is by 
grinding the part of the tool we desire to 
temper so as to render it bright, and then, 
by resting it on a piece of red-hot iron, 
we thereby communicate to our hardened 
tool such a degree of heat as shall modify 
or "let down" the hardness, and induce 
such a degree of toughness as will best 
SCIENTIST. 
suit the requirements of the case. Th& 
progress of re-heating effected in this- 
manner is easily observed by the bright- 
ened surface of the tool acquiring in suc- 
cession a series of prismatic colors. Thus 
1st. Pale straw color. 
2d. Brass color. 
3d. Deep brass color. 
4th. Keddish purple. 
5th. Purple. 
6th. Blue. 
7ih. Slate blue. 
Beyond slate blue all remaining hard- 
ness, such as is required for the generality 
of purposes, terminates, and the steel re- 
turns to softness. 
According to the purpose to which the 
tool is to be applied, the degree of temper 
or modification of the original full, hard 
condition may be arrested by plunging it 
into cold water the instant the color in- 
dicating the desired " temper " has come- 
on. 1, 2, 3, as from the above list, serve 
well for most wood-turning tools. 3 and 
4 for screw taps and drills, and for chisel& 
for chipping iron and brass. 5. 6, 7, for 
springs or such purposes where a certain 
degree of elasticity may be required in 
conjunction with a very moderate degree 
of hardness. 
To say nothing as to the importance ta 
practical mechanics of a thorough know- 
ledge of the art of hardening and temper- 
ing steel, on which the final efficiency of 
all our lathes, planing and other metal 
working machines depends, we would 
earnestly recommend to the amateur 
workman to devote a few hours to acquire 
the art of hardening or tempering steel 
in accordance with the brief practical 
hints we have endeavored to give in th& 
preceding remarks on the subject. 
Drawing Lessons— III. 
BY JOHN CLARK CENTER. 
■ Drawing Models. 
AS it may be difficult for some of our 
readers to procure " The Geometri- 
cal Models," as subjects for study, we 
furnish by our full-page illustration a 
grouped collection of the most important 
ones used in art educational schools. 
The five primary solids are the cube, the 
