102 
THE YOUNG SCIENTIST. 
soft, medium and hard, the shape of 
teeth should be as near to those shown as 
possible. 
The larger back-saws are often used for 
sawing tenons, and therefore have to per- 
form the services of a rip-saw. In hard- 
wood work the ordinary back-saw tooth 
Fig. 2. 
will rip pretty fairly, but it does not work 
so satisfactorily in soft wood ; still, in ama- 
teur work it will be found better to em- 
ploy the back-saw for ripping tenons than 
a rip-saw, as the inexperienced workman 
will find it so much easier to follow the 
lines with the former, and, though he 
may not do the work so rapidly, it will be 
better done. 
If the blade of a back-saw should get 
slightly " buckled " or crooked, it maybe 
straightened — in a great many cases— by 
tapping the back gently with a hammer, 
but care must be taken not to overdo it. 
A little practice in this matter will soon 
teach the operator how to obtain the best 
results. 
In the common hand-saw it is necessary 
that the line of teeth should be contin- 
uous or straight to insure the efficient 
working of the instrument, and this be- 
comes more necessary in a back-saw, for 
if the teeth are not in line or straight, 
both on their line of points and on their 
sides, the saw will make terrible work on 
line shoulders, dove-tails, or mitres, the 
raggedness and tearing of the fibres 
showing more plainly on fine work than 
on work done with the coarser saws. 
Another thing : when the saw is used in 
a mitre-box, if the teeth are not jointed 
on the sides with an oil-stone, the project- 
ing points will soon destroy the truth of 
the mitre cuts, and render it almost im- 
possible to cut a nice clean mitre joint, 
thus causing a great deal of unnecessary 
trouble in fitting the joint with a block- 
plane. 
Joints cut with these saws should be 
good without any repairing; if they do 
not fit, something is wrong— the saw is out 
of order or the workmanship is clumsy and 
careless. In cutting shoulders for tenons, 
it is always better, when practicable, to 
rip down the tenons first, leaving the 
shoulders uncut until the last thing. 
This saves the shoulders from getting 
braised or damaged in any way by falls 
or with having other stuff thrown against 
them. A little oil or grease should occa- 
sionally be applied to the blades and 
backs of these saws, to keep them from 
rusting or getting covered with a sticky 
film, which is sure to occur if they are 
only used once in awhile, and then used 
in pine. Of course, very little oil or 
grease must be used on the blade, for too 
much would scrape off in sawing joints 
and spoil the stuff for gluing, besides dis- 
coloring the joint and unfitting that part 
for taking varnish or polish. 
If my amateur friends will again read 
the papers in the February and March 
numbers of the Young Scientist on this 
subject, and connect them with the pres- 
ent paper, I think they will have no diffi- 
culty in thoroughly understanding the 
methods necessary for filing and caring 
for the three classes of hand-saws dis- 
cussed, namely — cross-cutting saws, rip- 
ping-saws, and back-saws for general pur- 
poses ; and feeling that this is the case, I 
leave them now, and will in my next take 
up a few of the other sorts of saws that 
amateurs are sometimes obliged to use. 
Cheap, Swift and Safe Canoes. 
BY W. L. D. o'GRADY. 
WISE man may pick up some 
things worth knowing almost 
anywhere. Many of our most 
■ ' wonderful inventions sprang 
from very humble begin nings. Perhaps I 
can give a 'hint or two picked up in India 
which may add to the enjoyments of the 
readers of the Young Scientist. 
Many boys love sailing, rowing and 
paddling, all of them manly and health- 
ful exercises, yet cannot afford boats 
