+---------+----------+----------+-----------+
| CHAIN | ZIG ZAG | TREBLE | UNEQUAL |
|RIVETING.| RIVETING.| RIVETING.| PITCHES. |
+---------+----------+----------+-----------+
| O O | O | O O | O O O |
| | O | O | |
| O O | O | O O | O O |
| | O | O | |
| O O | O | O O | O O O |
| | O | O | |
| O O | O | O O | O O |
| | O | O | |
| O O | O | O O | O O O |
+---------+----------+----------+-----------+
In fig. 41 may be seen an example of zig-zag riveting.
[Illustration: Fig. 41.]
CAULKING.—By this is meant the closing of the edges of the seams of
boilers or plates. In preparing the seams for caulking, the edges are
first planed true inside and outside; and after the plates have been
riveted together, the edges are caulked or closed by a blunt chisel
about 1/4-inch thick at the edge, which should be struck with a 3 or
4-lb. hammer; sometimes one man doing the work alone and sometimes one
holding the chisel and another striking.
_Fullering_ a boiler plate is done by a round-nosed tool, while
_caulking_ is executed by a sharper instrument.
The thinnest plate which should be used in a boiler is one-fourth of an
inch, on account of the almost impossibility of caulking the seams of
thinner plates.
It is a rule well known to all practical boiler makers that the thinner
the metal (compatible with due strength) the longer the life of the
boiler under its varying stresses and the better the caulking will
stand.