172 
THE YOUNG SCIENTIST. 
Fig. 5 sliows the form of teeth suitable 
for these saws ; they should be fine and 
filed square across, and the backs of the 
teeth should be filed square also. The 
teeth must be jointed all to one length 
and filed up to sharp i>oints, and must 
stand square out on the edge of the blade. 
Fig. 5. 
It is better not to joint saws of this sort 
on the sides, as tlie edges left on the teeth 
by the file help the saw to cut the metal. 
I have seen a piece of an old scythe do 
good service as a hack-saw^ when the lat- 
ter was not at hand. The edge of the 
scythe was nicked with a cold chisel, and 
then touched up wii li an old file, and the 
3'esults obtained from using the hn- 
proiiiptu saw were quite satisfactory, A 
hack-saw is simply a very thin file, with 
the cutting teeth all on one edge. 
Brass, silver, zinc or gold may be easily 
cut with a hack-saw having fine teeth and 
a narrow blade, for inlay work or for 
metal ornaments. If the saw is adapted, 
it may be used in a foot-power scroll-saw, 
and, with patience, may be made to per- 
form excellent work, 
Eor cutting ivory, pearl, bone or shell, 
the teeth shown in Fig. 2 are the most 
suitable, but they require to be very fine 
and in good order. 
Hack-saws, or metal-cutting saws for 
scroll or inlay work may be made from 
watch springs, and if the teeth are pro- 
perly formed and made even and regular, 
tbey may be made to do good work. I 
have often made saws for cutting scroll- 
work ,in hard wood of Avatch springs, 
when I could not get the kind of saws I 
wanted without going to considerable 
trouble. When well made they will out- 
last anything that you can purchase, but ^ 
they are very difficult 1o put in order, 
when once they require it. 
I think, from what I have said in the ^ 
papers on the subject of saws, that the 
amateur who has followed me closely will ' 
Ibe able to keep his saws in tolerable good 
order, and repair those belonging to 
neighbors who have not been so fortunate 
as to get the Young Scientist regularly. 
In a future paper I will endeavor to illus- 
trate and describe some of the aids and 
appliances connected with amateur saws 
and their uses. 
Amateur Boat-Building— II. 
N making a set of designs for a 
boat or a ship, the object pro- 
pose4 is analogous to that en- 
tertained in the preparation of 
drawings for a house or a steam-engine. 
This object is the representation on a 
plane surface of the thing to be con- 
structed as viewed from at least three po- 
sitions—or in elevation, in plian, and in 
section or sections. In the construction 
of vessels also, as in regard to the house 
or the engine, the preparatory drawings, 
must be made to scale, that is to say, that 
in all the set of designs a certain definite 
and unchanging proi)ortion to the dimen- 
sions of the thing to be made shall be ob- 
served, as, for instance, one-eighth of an 
inch in the drawing shall stand for one 
inch in the boat or machine, or one inch 
in the former shall be equivalent to one 
foot in the latter, and so on. The larger, 
as a general rule, the scale upon which 
any working drawing is made, the better. 
In a shipbuilder's yard the lines of a pro- 
jected vessel are frequently laid down on 
the floor of what is termed the " mould- 
loft," one-quarter, one- half, or even the 
full size of the vessel to be built. 
For the purposes of our amateur boat- 
builder, however, a less ambitious scale 
will suffice, as very large drawings for 
his presumably small craft may be incon- 
venient. Speaking generally, a scale of 
one-eighth will be large enough. This 
has the further recommendation that, as 
the workman must provide himself with 
a rule on which the inches are divided 
into eighths and sixteenths, he can read- 
ily abstract his measurements therefrom. 
It is not advisable to adopt a less scale 
than one-eighth, however, and if the boat 
is not a long one it will be better to de- 
sign it upon a scale of one-fourth the ac- 
tual size of the proposed craft. 
In the illustrations, Fig. 1 is the " sheer 
