142 
THE YOUNG SCIENTIST. 
in order to judge of the trueness to which 
the surfaces are being reduced. Wlien 
any one of three prepared surfaces will lie 
on the prepared surface of either of the 
other two without allowing any light to 
pass through the line of junction, the 
edges may be considered sufQciently true 
to admit of their being used in the produc- 
tion of a metallic straight edge. To this 
end three similar strips of steel, of the size 
desired, are smoothed or cleaned upon 
their sides on a grindstone or with a file. 
They are then laid one upon the other, and 
a hole drilled at each end, a rather tight- 
fitting pin or rivet being run through each 
hole to keep the three bars together. In 
this state they will appear as one thickish 
bar. The compound bar being placed in 
the vice and clamped on each side with 
sheet lead or zinc, the edges are filed level, 
beginning first with a rough file, and 
gradually increasing in fineness. Every 
now and then the edge being produced is 
tested against one of the wooden edges 
above described, which should be previ- 
ously rubbed over with red chalk, etc., to 
render prominences visible. When the 
eye no longer detects any differences in 
level on the application of the wooden 
straight edge, the steel pieces are to be re- 
moved from the vice, and pins extracted. 
They must now be tested against one an- 
other, until, by careful filing and repeated 
comparison with one another, it is found 
that the edges of all three will unite closely 
without any irregularity being perceptible. 
A good way of ascertaining whether any 
such exists, is to place two edges in con- 
tact and rub them together with some 
force ; the prominent portions will by this 
treatment be somewhat burnished, and 
will render themselves apparent by their 
superior lustre. The reason why three 
edges should be prepared simultaneously 
will be sufficiently evident on refiection. 
It will be readily understood that if A and 
B were two strips of steel, that A might 
be slightly concave and B correspondingly 
convex, without the eye being able to de- 
tect any fault, as no light would pass : but 
if a third strip, C, having the same con- 
vexity as B, were applied against the lat- 
ter, the fault would immediately become 
apparent, and on correcting the faults of 
B and C, and applying them to B, the con- 
cavity of this latter would also be rendered 
visible. 
• 
How Insects Breathe. 
THE fact that we ourselves, and all the 
larger land animals with which we are 
acquainted, breathe through the mouth 
and nostrils, has led most people to think 
that insects too breathe through the 
mouth. Ask those who have not made a 
special study of insects, how the fly 
breathes, or, rather, at what point of its 
body it takes in the air, and it is ten 
chances to one that you will be told that 
it breathes through the mouth. The in- 
sect, however, does not breathe through 
the mouth, but through a series of open- 
ings placed along the side. In many flies 
and small insects these openings are in- 
visible to the naked eye, but in some of the 
larger beetles and in most caterpillars 
they can be readily distinguished without 
the aid of a microscope. The common 
cockchafer or May-bug in the larval or 
caterpillar state shows these breathing 
pores or spiracles very clearly. In the 
white caterpillar they appear as a row of 
dark brown dots down the side. When 
viewed with a low power (25 diameters) 
these dots are seen to be oval openings, 
partially shielded from dust, etc., by min- 
ute hairs. In the beetle their position is 
shown in the engraving, the hard wing- 
cases having been removed so as to un- 
cover them. They can be clearly seen 
with a pocket magnifier. 
BREATHING POKES OP BEETIiE. 
These spiracles form very curious and 
interesting objects when mounted for the 
microscope. The skin of the caterpillar 
may be soaked, in weak potash solution so 
as to remove the grease and render the 
substance transparent. It should then be^ 
washed in water, afterwards in the strong- 
est alcohol, and after that it should be 
soaked in spirits of turpentine. When 
