as "high pressure."

Another important distinction is formed by the method of bringing the
carbide and water together. The majority of those now in use operate by
dropping small quantities of carbide into a large volume of water, allowing
the generated gas to bubble up through the water before being collected
above the surface. This type is known as the "carbide to water" generator.

A less used type brings a measured and small quantity of water to a
comparatively large body of the carbide, the gas being formed and collected
from the chamber in which the action takes place. This is called the "water
to carbide" type. Another way of expressing the difference in feed is that
of designating the two types as "carbide feed" for the former and "water
feed" for the latter.

A further division of the carbide to water machines is made by mentioning
the exact method of feeding the carbide. One type, called "gravity feed"
operates by allowing the carbide to escape and fall by the action of its
own weight, or gravity; the other type, called "forced feed," includes a
separate mechanism driven by power. This mechanism feeds definite amounts
of the carbide to the water as required by the demands on the generator.
The action of either feed is controlled by the withdrawal of gas from the
generator, the aim being to supply sufficient carbide to maintain a nearly
constant supply.

_Generator Requirements._--The qualities of a good generator are
outlined as follows: [Footnote: See Pond's "Calcium Carbide and
Acetylene."]

It must allow no possibility of the existence of an explosive mixture in