2. The condition of rarefaction, or reduction of pressure below that of the atmosphere, in a vessel, as the condenser of a steam engine, which is nearly exhausted of air or steam, etc.; as, a vacuum of 26 inches of mercury, or 13 pounds per square inch.

Vacuum brake, a kind of continuous brake operated by exhausting the air from some appliance under each car, and so causing the pressure of the atmosphere to apply the brakes.

Vacuum pan (Technol.), a kind of large closed metallic retort used in sugar making for boiling down sirup. It is so connected with an exhausting apparatus that a partial vacuum is formed within. This allows the evaporation and concentration to take place at a lower atmospheric pressure and hence also at a lower temperature, which largely obviates the danger of burning the sugar, and shortens the process.

Vacuum pump. Same as Pulsometer, 1.

Vacuum tube (Phys.), a glass tube provided with platinum electrodes and exhausted, for the passage of the electrical discharge; a Geissler tube.

Source: Webster's Revised Unabridged Dictionary (1913)