Page:Elementary Text-book of Physics (Anthony, 1897).djvu/160

146, discussed in § 120, that a stream of liquid running down a cylinder diminishes the pressure upon its walls. In the Sprengel pump the liquid used is mercury. It runs from a large vessel down a glass tube, into the wall of which, at a distance from the bottom of the tube of more than 760 millimetres, enters the tube which connects with the receiver. The lower end of the vertical tube dips into mercury, which prevents air from passing up along the walls of the tube. When the stream of mercury first begins to flow, the air enters the column from the receiver, in consequence of the diminished pressure, passes down with the mercury in large bubbles, and emerges at the bottom of the tube. As the exhaustion proceeds, the bubbles become smaller and less frequent, and the mercury falls in the tube with a sharp, metallic sound. It is evident that, as in the case of the ordinary air-pump, a perfect vacuum cannot be secured. There is no leakage, however, in this form of the air-pump, and a very high degree of exhaustion can be reached.

The Morren or Alvergniat mercury-pump is in principle merely a common air-pump, in which combinations of stop-cocks are used instead of valves, and a column of mercury in place of the piston. Its particular excellence is that there is scarcely any leakage.

The compressing-pump is used, as its name implies, to increase the density of air or any other gas within the receiver. The receiver in this case is generally a strong metallic vessel. The working parts of the pump are precisely those of the air-pump, with the exception that the valves open downwards. As the piston is raised, air enters the cylinder, and is forced into the receiver at the downstroke.

124. Manometers.—The manometer is an instrument used for measuring pressures. One variety depends for its operation upon the regularity of change of volume of a gas with change of pressure. This, in its typical form, consists of a heavy glass tube of uniform bore, closed at one end, with the open end immersed in a basin of mercury. The pressure to be measured is applied to the surface of the mercury in the basin. As this pressure increases, the air contained in the tube is compressed, and a column of