457 



MANOMETER. 



MANOMETER. 



468 



word, ready, in a low voice, when the second rank man fires and loads, 

 care being taken that the muakets of both ranks are not unloaded at 

 the same time. 



On the appearance of cavalry the nearest supporters and the reserve 

 move towards the threatened part, and form squares ; the skirmishers 

 at the same time run to any cover from whence they may aid the 

 supports by a cross fire; or run in behind the supports, and form 

 square under the protection of their fire ; or if too suddenly attacked 

 for this, form small knots or rallying squares, by running together at 

 convenient points round their officers. 



When light troops have to advance across a bridge, or through a 

 short defile, on arriving at the bank of the river, or at the entrance of 

 the defile, the skirmishers lie down in line and fire ; the supports, 

 strengthened by the reserve, charge the enemy on the bridge, or in the 

 defile, drive him back, and then form an extended line as skirmishers, 

 while the fonner skirmishers pass the bridge or defile, and now con- 

 stitute the supports and reserve. In retreating the supports pass first 

 over the bridge or through the defile, covered by the skirmishers and 

 immediately deploy, in order to act as skirmishers themselves; the 

 former skirmishers then rapidly pass, followed by the supports, and 

 the whole form in column in rear of the present skirmishers, who 

 then, by then* fire, protect the retreat if it is to be continued. 



MAXO'METER (from two Greek words puu/fa, thin or rare, and 

 litTfov, a measure) is the name given to instruments which measure 

 the rarity of the atmosphere or other gas. As however the rarity of a 

 gas is proportional to its elastic force, so long as its temperature and 

 chemical composition remain unchanged, such instruments as measure 

 the elastic force of gases are also, with this restriction, properly termed 

 manometers, and accordingly it is to the latter instruments that the 

 term is most frequently applied, both in this country and upon the 

 Continent. 



The " statical barometer " of Robert Boyle was a manometer of the 

 simplest kind, consisting of an exhausted glass globe suspended from 

 one extremity of a dtlicate balance, and counterpoised by a metallic 

 weight at the other extremity, the adjustment being made when the 

 atmosphere was in its mean state of density. Any subsequent variation 

 in the specific gravity of the air would, by a known law of hydrostatics, 

 destroy the equilibrium, and the motion of the globe would indicate 

 whether the variation had inclined towards an increase or diminution 

 of density, as in the former case it would ascend, in the latter it would 

 descend. 



Captain Phipps, in his north-polar voyage, and Colonel Roy, in order 

 to correct his barometric observations, employed manometers, which 

 gave the elastic tension of the atmosphere. They consisted of glass 

 imilar in form to thermometer-tubes, and of various sizes. Those 

 of Colonel Roy were from 4 to 8 feet in length, with bores from J,th to 

 '.tu of an inch in diameter. The bulb and part of the tube being 

 tilled with air of known tension, and the remainder of the tube being 

 partially occupied by a small column of mercury sufficient to cut off 

 the communication between the internal and external air ; any variation 

 in the elastic tension of the latter, arising from change of weight, 

 would be accurately measured by the ascent or descent of the mercurial 

 column ; for whenever the tension of the atmosphere exceeded that of 

 the contained air, the column would move towards the bulb, and the 

 contrary. But if the change in the tension of the atmosphere were 

 partly attributable to a change of temperature, then the motion of the 

 column would merely measure the difference of the variations in the 

 tension of the internal and external air, because the tension of both 

 would be equally affected by the change of temperature. The bulb 

 was pear-shaped, so that the " point being occasionally opened, dry or 

 moist air could be readily admitted, and the bulb sealed again without 

 any sensible alteration in its capacity." (' Phil. Trans.,' vol. Ixvii., 

 p. 689.) The manometers of Varignou and Wolf were similar to the 

 preceding. 



A more convenient instrument, and one of more general use, con- 

 gists of a siphon-barometer, the basin of which is enclosed air-tight in 

 a globular or other conveniently shaped vessel, furnished with a number 

 of cocks, by means of which and the pneumatic pump the contained 

 gag may be removed, and other gases successively substituted in its 

 place. If equal parts by weight of different gases be thus successively 

 introduced, they will not be affected by any change which may take 

 place in the surrounding atmosphere, except in so far as such change 

 may affect their temperature; so that, providing the temperature 

 remain constant, the relative tensions of these gases will be accurately 

 measured by the weight of the mercurial column suspended in the 

 : arm of the barometer, above the level of the mercury in the 

 basin ; care being had to allow for any variation in the capacity of the 

 receiver, arising from alteration in the level of the mercury in the basin, 

 and likewise for the small tension always indicated by the birometer 

 immediately previous to the introduction of a fresh gas, arising from 

 the impossibility of forming a perfect vacuum, 



If an approximate vacuum be formed in the receiver enveloping the 

 basin of the barometer, and a small quantity of any liquid be then 

 introduced, it will bo immediately converted into vapour, anil the 

 elastic teusion of this vapour will be measured in precisely the same 

 way as that of permanent gases. The receiver is sometimes of suffi- 

 cient size to contain animals and plants, the eiicct of which in increasing 

 or diminishing the tension of the enclosed gas is then measured by the 



rise or fall of the mercury. If this manometer be transported from 

 one place to another, where the temperature is the same, but the force 

 of gravity different, this variation in the force of gravity will be mani- 

 fested by a corresponding variation in the length of the mercurial 

 column ; that is, if the gravity increase, the tension of the enclosed 

 gas will be counterbalanced by a shorter column of mercury, and the 

 contrary ; but as this method of measuring the variations in the force 

 of gravity is not susceptible of that accuracy which is attained by the 

 employment of the pendulum, it is rarely if ever resorted to. 



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, 1'7 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 ; 

 but 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 manumSlre <l air libre, le manumetre a air com- 

 j)rim(, and the metallic manometer. The open air manometer consists 

 of a glass tube of about five metres 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 Mariotte, 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 [BAROMETER], namely, that when a 

 tube with flexible sides and slightly flattened is rolled into a spiral 

 form in the direction of its smallest diameter, 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, 

 0'7 metre in length, of long flattened oval section, the major axis being 

 11 millimetres and the minor 4 millimetres iu 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 scala representing the tension of the 

 vapour in atmospheres. Vapour beiug admitted, the pressure causes 

 the spiral to unroll, and iu doing so the needle is carried over the scale. 

 This instrument has the advantage of being portable, and not fragile, 

 and heuce is used in connection with locomotive boilers. 



