Some types of large generating plants have a power-driven feed, the power
usually being from some kind of motor other than one operated by a weight,
such as a water motor, for instance. This motor is started and stopped by
the operator when, in his judgment, more gas is wanted or enough has been
generated. This type of machine, often called a "non-automatic generator,"
is suitable for large installations and is attached to a gas holder of
sufficient size to hold a day's supply of acetylene. The generator can then
be operated until a quantity of gas has been made that will fill the large
holder, or gasometer, and then allowed to remain idle for some time.

_Gas Holders._--The commonest type of gas container is that known as a
gasometer. This consists of a circular tank partly filled with water, into
which is lowered another circular tank, inverted, which is made enough
smaller in diameter than the first one so that three-quarters of an inch is
left between them. This upper and inverted portion, called the bell,
receives the gas from the generator and rises or falls in the bath of water
provided in the lower tank as a greater or less amount of gas is contained
in it.

These holders are made large enough so that they will provide a means of
caring for any after generation and so that they maintain a steady and even
flow. The generator, however, must be of a capacity great enough so that
the gas holder will not be drawn on for part of the supply with all torches
in operation. That is, the holder must not be depended on for a reserve
supply.

The bell of the holder is made so that when full of gas its lower edge is
still under a depth of at least nine inches of water in the lower tank. Any
further rise beyond this point should always release the gas, or at least
part of it, to the escape pipe so that the gas will under no circumstances