THE YOUNG SCIENTIST. 
23 
H, or No. 4, and must be nicely pointed on a 
file or piece of sand-paper. 
The sliding carriages G and C are made of 
two pieces of black walnut, 2 in. x in x ^ in. 
thick, such as is used for scroll sawing, and 
two pieces | in. x i in. x 1^ in. long ; the small 
pieces are glued between the ends of the large 
pieces so as to make a block, with a hole 1 in. 
by ^ in. (the size of the rulers), running through 
it. The pieces AC and DF must now be slightly 
reduced so as to slide easily, but not too freely, 
through the holes in the carriages. Two ordi- 
nary cotton spools with one end of each sawed 
off square, just where the bevel ends and the 
straight part, on which the cotton is wound, 
commences. One of these spools must now be 
glued to the centre of one of the thin sides of 
each carriage, and the opposite side must be 
cut through one of the joints where it is glued 
to the ^ in. x I in. x 1^ in. piece, with a thin 
saw, so that when that end is pinched together 
it will bind on the ruler, where before it slid 
easily. The clamping screw is made of a com- 
mon 1 inch wood screw, inserted from the side on 
which the spool is glued, and the? head counter- 
sunk, in the centre of the width of the carriage, 
and I in. from the end, thus passing through 
the middle of the joint that was cut. 
To prevent the screw from turning and also 
from dropping out, a small wire staple, similar 
to a double-pointed carpet tack, is driven across 
the head of the screw, so as to lie in the groove ; 
and a nut is made from a piece of hard wood I 
1 in. long, ^ in. wide and f in. thick, cut to the 
shape of an oval and slightly rounded on the 
under side, so as to bind only on the bottom, as 
shown at A. In order to get a good, clean 
thread in the nut, a tap may be made by tak- 
ing a common screw of the size you are using, 
and with a saw-file making three or four 
grooves lengthwise of the thread deepest at 
the point. 
To reduce a drawing to one-half the original, 
set the instrument so that a hole at B, which is 
in line with the points of the pencils at C and 
G, is just as far from G as that point is from C. 
I always make the pencils fit tight in the spools 
by wrapping paper around them, but a screw 
may be inserted to hold 
them, but it is not necessary. 
In setting the instrument it 
is always necessary to have 
the pencils and the hole in 
which the fulcrum is placed, 
perfectly in line. For . this 
purpose a piece of wood the 
size of an ordinary lead-pen- 
cil, about four inches long, 
with a knitting needle driven 
into the end, so as to be in 
line with its centre, is very 
useful, as by placing it in the 
carriage at C, and then sight- 
ing across the point of the 
pencil at G to the hole in B, 
they may be adjusted per- 
fectly in line. The distance 
from B to C repi-esents the 
original, from B to G the 
reduced copy; by varying 
either, any amount of reduction may be r.;ade. 
Instead of two pencils a piece of hard wood 
the size of a pencil, sharpened to a point, may 
advantageously be used in C, when reducing 
and in G when enlarging. 
At A, on the under side of the piece AB, and 
just beyond the block under the screwy is glued 
another block 2^ in. long, of the form shown ; 
and at its lower extremity is fastened by a 
screw, a small wheel, readily made out of a 
coat button, and projecting below the bottom 
of the block to which it is fastened, ^th 
inch. 
When passing from point to point of the 
original, and not wishing to make a mark on 
the copy, the pencil may be raised from the 
paper by raising the joint at F a sutficient 
amount. 
