THE YOUNG SCIENTIST. 
7 
A 
Fig. 2. 
EDGE OF NEWLY-GKOUND KNIFE, AS SEEN UNDER THE MICEOSCOPE. 
that we were to hold one side of the blade to 
the stone, in the position shown in Fig, 2, with 
the plane of revolution of the stone as denoted 
by the marks, and the other side with the 
plane of revolution in the direction denoted 
in the same figure by the line E F; then the 
marks on the facets and the indents at the edge 
will cross one another, leaving it less ragged 
than would be the case if both sides were 
ground as in Fig. 1, and since smoothness at 
the edge is desirable, we have arriv,ed at the 
best position in which to hold the blade upon 
the stone. 
If, after having ground the two faces, in 
either position, we put the blade under a mi- 
croscope, we shall find that it appears as shown 
in Fig. 3, the edge being elongated beyond its 
natural length and turned to one 
side. The reason of this is that the 
metal, being weak, bends and gives 
way to the pressure, turning away 
from the face that is in contact with 
the stone, and it is obvious that the 
greater the pressure with which the 
Fig. 3. blade is pressed to the stone, the 
more the metal will bend away. 
It follows therefore that though the blade 
may be pressed firmly to the stone during the 
early part of the grinding, so as to remove the 
surplus metal as quickly as possible, it should 
be held lightly during the latter part, to avoid, 
as far as is practicable, the formation of this i 
undesirable web, which is termed a /"eai/ier edge, 
because of its weakness and ragged appear- 
ance. As soon as the knife edge is put to cut- 
ting duty, the feather edge breaks off, leaving a 
flat place at the thin end of the wedge, and to 
that extent impairing its sharpness. At what 
precise depth it will break off depends upon 
Fig. 4. 
the j)ressure applied and the direction in which 
it is applied; but as a rule the harder the pres- 
sure, the deeper it will break off, and hence the 
more it will impair the edge. One of the best 
methods of removing it is to draw it lightly 
across a piece of soft wood, first lengthways 
and then across the grain. 
