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This instrument should be placed higher than the auxiliary oil distributor itself to enable the oil to drain by gravity feed to the oiler.

While the pressure and gravity feed systems are generally used in airplanes, it may be well to describe the vacuum lift system which has been widely applied to motor cars and which may have some use in connection with airplanes as these machines are developed.

It is claimed that all the advantages of the pressure system are obtained with very little more complication than is found on the ordinary gravity feed.

When using gravity feed without air pressure be sure to vent the tank to allow circulation of air.

An important step was the substitution of what is called the "force feed" for the gravity feed.