The exact determination of the elastic force of aqueous vapour at high temperatures being essential to the safe construction and manage ment of steam-engines, the French government requested the Royal Academy of Sciences to institute a course of experiments, with a view to the attainment of so important an object. The care of making these experiments was confided by the Academy to MM. de Prony, Arago, Girard, and Dulong, who made their report in 1830 (` Annales de Chimie,' t. xliii., p. 74.) The manometer constructed for this purpose consisted of a straight glass tube of uniform bore, metres (67 inches) in length, and 5 millimetres (1.25 inches) in diameter and thickness, closed at the upper and open at the lower extremity. The capacity having been accurately determined, it was filled with perfectly dry air of known density, and enveloped in a cistern of water, which was kept at a uniform temperature. Another tube of, equal bore and thick ness, but 26 metres (85 feet) in length, and open at both ends, was then erected, and the lower extremities of the two tubes were made to communicate with apertures in the opposite sides of a cylindrically shaped reservoir, capable of holding about 1 cwt. of mercury. By means of a forcing-pump adjusted to the top of this reservoir, the pressure upon the surface of the contained mercury could be increased at pleasure ; and this increased pressure, being transmitted to the lateral apertures already mentioned, would obviously cause the mercury to rise in both tubes, but to unequal heights ; for in the longer tube it would rise until the weight of the mercurial column, together with that of the superincumbent atmosphere, were 'equal to the pressure ; hut in the shorter tube, only until this pressure was counterbalanced by the rapidly augmenting expansive force of the confined air, added to the weight of the small column of mercury forced into it.. The expansive force of the compressed air would be measured by the dif ference of these two columns ; and by this means, the shorter tube having been carefully graduated corresponding to pressures varying from one to twenty-nine atmospheres, the construction of the mano meter was complete. The longer tube and the forcing-pump were then removed, as no longer necessary, and instead of the latter was substi tuted the actual pressure of steam at successively increased tempe ratures, the tension of which was indicated by the compression of the air in the manometer.
The French apply the term manometer to any apparatus which measures the pressure of aeriform bodies, such as the pressure-gauge of a steam-engine or of a gasometer. They distinguish three classes of manometers, namely, le manomatre a air fibre, le manometre a air cent prime, and the metallic manometer. The open air manometer consists
of a glass tube of about five mitres in length, one end of which is cemented into a cast-iron cistern containing mercury. Parallel with the glass tube is a tube of iron four metres in length, the lower extremity of which passes by a lateral opening into the mercury cistern. This iron tube is filled with water, which transmits the pressure of the steam, &c., to the mercury, the ascent of which in the glass tube marks the pressure in atmospheres and tenths. This form of instrument is not used for greater pressures than those of five or six atmospheres. In Cazalet's manometer the steam is made to act on the sectional area of the rod of a piston, the plunger of which, of much greater sectional area, presses on and so sustains in the other leg of the inverted siphon into which it plunges a mercurial column of sectional area equal to its own. The compressed air manometer is based on the law of Marlette, that the volumes of gases are inversely as their pressures. It consists of a glass tube closed at its upper extremity, filled with dry air, and fastened securely in an iron cistern containing mercury, from which proceeds a side tube which is connected with the boiler, or vessel containing the vapour whose elastic force is to be measured. Any pressure acting through this tube on the surface of the mercury in the cistern will force it up the tube, which is graduated according to the above law, and by its indications gives the pressures put in operation.
The metallic manometer of M. Bourdon is constructed on the same principle as his metallic barometer [Banosffirsa), namely, that when a tube with flexible sides and slightly flattened is rolled into a spiral form in the direction of its smallest dialneter, any internal pressure on the sides tends to unroll the tube, while any external pressure tends to roll it up more closely. In this manometer the curved tube is of brass, metre in length, of long flattened oval section, the major axis being 11 millimetres and the minor 4 millimetres in length. To one end is attached a tube and a stop-cock for, putting it in communication with a steam-boiler ; the other end is closed, and carries a needle, the arrow of which moves over a graduated scale representing the tension of the vapour in atmospheres. Vapour being admitted the pressure causes the spiral to unroll. and in doing so the needle is carried over the scale. This instrument has the advantage of being portable, and not fragile, and hence is used in connection with locomotive boilers.