The canister
was made of two curved pieces of sheet tin with soldered edges, cloth
or paper being used at the ends. It was surrounded with sand or earth,
so that the effect of the blast was practically the same as though the
hole were drilled in the shape of the canister. In other words, the
old Portland system was to drill a large, round hole, put in a
canister, and then fill up a good part of the hole. Were it possible
to drill the hole in the shape of the canister, it would obviously
save a good deal of work which had to be undone. The Portland system
was, therefore, an extravagant one, but the results accomplished were
such as to fully warrant its use. Straight and true breaks were made,
following the line of the longer axis of the canister section, as in
Fig. 2.
[Illustration: Fig. 2.]
It was found that with the old Portland canister two breaks might be
made at right angles by a single blast, when using a canister shaped
like a square prism. In some of the larger blasts, where blocks
weighing in the neighborhood of 2,000 tons were sheared on the bed,
two holes as deep as 20 ft.
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