CHAPTER X. THE BAROMETER. (73) Principle of the Barometer. THE fundamental principle underlying all barometers in which fluids are used is that the heights of two columns of two fluids are inversely as their specific gravities; air is 10,784 times lighter than mercury, therefore a column of thirty inches of mercury would support an atmosphere five miles high if it were all equally dense. A barometer is made by filling a tube of sufficient length closed at one end, with the fluid and inverting it into a cistern of the same fluid. The liquid sinks a certain distance until it is balanced by the atmospheric pressure. Its level then remains within certain limits stationary. If the atmosphere from any cause gets lighter the barometric fluid in the tube falls, in the cistern rises, if the atmospheric pressure increases the barometric fluid rises, that in the cistern falls. The longer the column, the more obvious will be the oscillations. With the knowledge experimentally acquired that mercury of 13 59 sp. gravity will have when used as a barometric fluid a column of about thirty inches, it is easy to calculate the length of the column of any other fluid provided the specific gravity of the fluid is known; for instance glycerin has a specific gravity of 1.26 required, the length of a barometric column of glycerin. As 1 26 the specific gravity of glycerin: 30 inches :: 13:59 the specific gravity of mercury = 323-5 inches or 27 feet. Water would have a still longer column, alcohol longer still, ether longer than alcohol, for the lighter the liquid the longer the column. Since a long column is far more sensitive than a short column, Physicists early turned their attention to constructing barometers with water. But water barometers are no longer made, the few which were constructed gave untrustworthy results, mainly from the effect of temperature on the aqueous vapour in the top of the tube and from other causes. (74) The Glycerin Barometer. The barometer of the future seems decidedly to be the glycerin barometer invented by Mr. Jordan, which has been now for several years in use at Kew. The body of the instrument is formed of an ordinary metal gas tube five-eighths of an inch internal diameter, and furnished at the top with a gun metal socket in which is cemented a glass tube four feet long, inside diameter one inch; the upper end is formed in the shape of an open cup, and it has fitted into its neck a stout india-rubber stopper. The height of the fluid is observed in this tube, and read by means of brass scales placed on either side of the tube and fitted with indices and verniers, which are moved by means of milled heads placed at the bottom of the scales. The cistern is a cylindrical vessel of copper tinned inside, five inches deep and ten inches diameter. It is fitted with a cover screwed on, and air has access through a pinhole in the cap attached to the cover; the cap has a recess packed with cotton wool, so that the air filters in dust-free. There is an arrangement by which the lower end of the tube can be for temporary purposes closed by a plug. To fill the tube with glycerin, the glycerin, warmed to 100° F., and tinted with aniline, is poured into the cistern, and air sucked out at the top by an air-pump to the full extent to which the glycerin will rise; the lower end is then stopped with the plug and the rest of the tube filled absolutely full with glycerin. The india-rubber stopper is now inserted and the lower plug withdrawn ; the glycerin now sinks to about 323 inches, and bubbles of air in the course of a day or two find their way up into the vacuum. These are most conveniently removed by applying an air pressure pump to the surface of the fluid in the cistern by making a connection with the cap, the upper india-rubber stopper being of course temporarily removed. This operation may have to be repeated; lastly, to prevent absorption of water, petroleum specially purified is poured in a layer of about an inch on the top of the glycerin in the cistern. Glycerin has a very small coefficient of expansion, according to Reinold 000303 for 1° F. between 32° and 212°; readings have to be corrected for temperature and reduced to sea-level. It may also be desirable to express the readings in terms of the mercurial barometer; if done by calculation it must be remembered that 323 571 inches equals 30-3 inches Kew standard; a convenient plan is to have mercury equivalents attached to the scale of the instrument. The following is one of the well-known diagrams which appear daily in the Times, the glycerin values on one side, the mercurial on the other. (75) Mercurial Barometers. The advantage of the mercurial over the glycerin barometer is its portability and moderate size. It is not every one who cares to have a tube over twenty-seven feet long carried through his rooms, while the length of the mercurial barometer is under three feet. The simplest of mercurial barometers is the syphon form. A glass tube is bent in the form of a syphon, the longer limb being |