MANOMETE R 495 A -5 -4 vapours, or liquids against the sides of the closed vessels in which they are confined, as, for instance, the pressure of steam in a steam-boiler. The simplest and at the same time most accurate form of manometer is that known as the " mercury manometer," sometimes also called the "free-air mano meter," and represented in fig. 1. It consists essentially of two vertical communicating tubes. One of these, AB, open at both ends, and made of thick glass, with a narrow uni form bore, is fixed hermetically in the neck of a large wrought-iron cylinder C, its lower end dipping below the surface of mercury contained in the cylinder. The other tube, EF, is attached at its lower end to the cylinder by the cross pipe D, and at its upper end can be put in communication with the vessel the pressure in which is to be ascertained. Usually the tube EF, the cross pipe D, and the space above the mercury in C are filled with water. At first the tube EF is left open to the atmosphere, and the height of the mercury in AB noted. When EF is then put in communication with the vessel in which the pressure (above atmo spheric) is to be determined, the mercury in AB rises, and from the height to which it rises the pressure is deduced. For accurate "work corrections must be made for the fall of the mercury in C as it rises in AB, and for the temperature and the height of the barometer at the time of the experiment. The great drawback to the employment of the simple mercury manometer for measuring very great pressures is the mechanical difficulty of obtain ing a sufficiently long column of mercury. E. H. M. Amagat, however, has lately (1880) worked with a column one-fifth of a mile high. His experiments were undertaken to find out how the various gases, nitrogen, oxygen, air, hydrogen, &c., departed from Mario tte s law when subjected to enormous pres sures. At the bottom of a coal-mine at Verpilleux, near St ^tienne, which had a depth of 327 metres, was placed the glass manometer tube containing the compressed gas, while the mercury tube (made of steel) extended up the shaft, being gradually built up in sections. See Nature, vol. xxii. pp. 62, 63. By means of Amagat s tables of the volume and corresponding pressure of the several gases, and with special forms of manometer to suit par- FIG. 1. Mercury Manometer. ticular circumstances, accurate and delicate measures of enormous pressures can now be obtained. Professor Tait, for instance, has recently applied these tables along with a manometer of his own devising for testing the behaviour of the thermometers supplied to the "Challenger" expedition under a pressure of as much as 10 tons to the square inch. " Regnault s manometer " is shown in fig. 2. AB is a strong metal tube, closed at the lower end, and carrying at the upper a bent pipe for admitting the compressed gas and a stop-cock R pierced with holes in a T form. DE and FG are two graduated glass tubes communicating at their, lower ends by a narrow passage in the metal block to which they are hermetically fixed. DE and AB also com municate at the upper ends by a passage in a metal piece attached hermetically to them. By the stop-cock R , having radial holes at right angles to each other, DE can communicate either with AB or with the atmosphere at O ; and by the stop-cock R" it can communicate either with FG or with the open air. The three tubes are surrounded by a cylinder MM containing water to keep the tempera ture constant. The tube AB is filled with the compressed gas whose pressure is to be ascertained. The stop-cock R being then placed as in fig. 2, mercury is poured into FG till it fills DE and runs out at O. The stop- si cocks R and R are then turned as in fig. 2a, so that AB and DE communicate with each other. Part of the com pressed gas flows over into DE, and the mercury in FG rises. By manipu- Fi,Lj. 2a lating the stop-cock R" as shown in fig. 2&, part of the mercury is allowed to run out of DE till a conveniently measurable differ ence of level be tween the mercury surfaces in DE and FG is attained. Let "k be this differ ence. Also let x be the pressure of the gas originally filling the volume V of AB, V the additional volume occupied by the ex panded gas, and H be the height of the baro meter at the time ; then we have by Mari- otte s law FIG. 2. Regnault s Manometer. from which (1). V is determined by weighing the mercury required to fill the space it occupies, and V can be calculated from (1) when AB is filled with dry air at. pressure H. In Regnault s apparatus the length of AB and DE was 1 metre, the diameter of AB 5 mm. and of DE 20 mm. The section of DE was thus sixteen times that of AB, and in this way a very great pressure could be measured by a comparatively small difference of level between the mercury surfaces in FG and DE. The instru ment, however, is subject to errors, arising chiefly from the diffi culty of measuring accurately the volumes V and V . The " compressed air manometer " (fig. 3) consists of a strong graduated glass tube of uniform narrow bore, closed at the top and fixed hermetically into the neck of a wide iron cylinder. The tube contains dry air, and its lower end dips below the surface of mercury contained in the cylinder. Attached to the side of the cylinder is a tube A, with a stop-cock, to afford communication with the vessel the pressure in which is to be measured. When the manometer is attached to the vessel containing compressed gas the mer cury rises in the glass tube till the pres sure of the air confined in the tube (reck oned in millimetres of mercury) plus the height of the mercury column above the level of the mercury in the cylinder is equal to the pressure on the surface of mercury in the cylinder. " Desgoffe s manometer" depends upon the same principle as the hydraulic press, and can be employed to measure the enormous pressure reached in the cylinder of that instrument. It is represented in perspective in fig. 4 and in _ , section in fig 5. V is a strong circular iron ;; ; vessel, in which moves up and down for a short v, 1 . distance a flat piston D attached to a cylindrical plug T. The lower part of V contains mercury which has free communication with a graduated vertical glass tube AB fixed her metically into the side of V. Above the mercury in V is placed a thin layer of water, and above that is stretched a thin memorane of india-rubber bolted down water-tight by an iron ring. The chamber C contains a cavity in which the plug T moves water-tight. By means of the tube t the instrument can be put in communication
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