Hi 



BARO'METER. 



BARO'METER. 



83 1 



consequently to vary the height of the mercurial column so as to be able 

 to deduce from it the atmospheric pressure. In M. Blondeau's instrument, 

 the dimensions of which are very much less than in the ordinary 

 form, a volume of air of the pressure intended to be measured is 

 taken and dilated so as to occupy double its former volume. Under 

 such circumstances, the elasticity of the air producing equilibrium 

 at the pressure of only half an atmosphere, causes such a difference 

 in the height of the mercury in the tube communicating with 

 it, as to show at once the value of the pressure of this half-atmo- 

 sphere, whence may be deduced the pressure of the air at the time of 

 observation. 



Water-Barometer. Attempts have been made at various times to 

 construct a water-barometer, under the idea that its greater oscillations 

 as compared with the mercurial-barometer might throw some light on 

 several points of physical science. Mariotte formed a water-barometer, 

 and noticed that when the mercurial-barometer stood at 28 poucts, the 

 water indicated 31 J pieds. " Ce qui donne," he remarks, " le rapport 

 du inereure a 1'eau de 134 a 1." Otto Guericke had ft water-barometer 

 rising along the wall with a toy of the shape of a man floating on the 

 top of the water ; the lower part of the tube was concealed under the 

 wainscoting, so much so, that the little image only made its appearance 

 in fine weather. This contrivance was named an Anemoscope or Si-m/icr 

 Virum. It does not appear that in these instruments any attempts 

 were made to purge the water of air by boiling, and to prevent its 

 re-absorption, nor did the tube consist of one whole length. These 

 objections Professor Daniell determined to obviate in the water- 

 barometer, which it was proposed to erect in the hall of the Royal 

 Society at Somerset House, in the year 1831. The tube was drawn 

 without any difficulty by Messrs. Pellatt and Co., it was 40 feet in length, 

 1 inch in diameter at its lower extremity, and O'S inch at its upper. 

 The tube was conveyed in a packing-case to its position in the 

 Society's Hall, and its lower end was passed through a stuffing-box 

 into a steam boiler, so arranged that the water could be first thoroughly 

 boiled, and then, by closing a stop-cock, raised in the tube by the 

 elastic force of the vapour acting upon its surface until it issued in a 

 jet from a small stop-cock at the top of the tube. This operation was 

 at length, after some failures, conducted successfully. As the appa- 

 ratus cooled the column of water subsided until it was balanced by 

 the pressure of the atmosphere, and the water in the boiler, which 

 now constituted the cistern of the barometer, was covered with half-an- 

 inch of castor oil to prevent the atmosphere from communicating with 

 the water, and the air from being absorbed. The adjustment of the 

 scale with the necessary correction was next attended to. Of course 

 one of the most important corrections would be for the elasticity of 

 the aqueous vapour at the observed temperature. An extensive series 

 of observations during two years showed that the instrument was 

 undergoing deterioration, and on opening the cistern it was found that 

 some of the liquid had escaped, and that the oil had undergone decom- 

 position. On taking some of the water out of the boiler and placing it 

 under the receiver of an air-pump, air-Bubbles were extricated in 

 abundance, showing that the attempts to prevent the absorption of the 

 air had failed. The indications of this instrument, however, clearly 

 established the fact that the water-barometer precedes by one hour the 

 barometer of half-an-inch bore, and the latter, the mountain-barometer 

 of 0'15 inch bore by the same interval in their indications in the 

 horary oscillations. It was also curious to watch the water-barometer 

 in windy weather ; the column of water appeared to be in a perpetual 

 motion resembling the slow action of respiration. At the end of 1844, 

 the barometer was examined, the gas collected in the vacuum proved 

 pure nitrogen, the oxygen having been absorbed in passing 

 through the oil, making it rancid and producing other changes. In 

 January, 1845, the instrument was emptied, cleaned, and refilled as 

 before, when instead of covering the water in the cistern with oil, a 

 solution of caoutchouc in naphtha was substituted to the depth of 

 between two and three inches. This, however, did not prevent the 

 entrance of air, so that the attempt to establish a water-barometer 

 must be regarded :is a failure. 



In the theatre of the Natural Philosophy class in the University 

 of Edinburgh, is a water-barometer constructed by Adic, not for 

 meteorological purposes, but as a class illustration for exhibiting 

 the Torricellian experiment in a striking manner. It consists of 

 A fine drawn tube of tin of 4-inch bore, rising 30 feet from a copper 

 login, enclosed under the benches of .1 class below, and cemented 

 at the top to a glass cylinder, 2 inches wide ami about b feet high, 

 exposed t< view but ^terminating in a small basin containing water. 

 Both ends of this compound column are fitted with stop-cocks, 

 which may be opened or shut at pleasure by means of concealed 

 wires. 



It may also be noticed, that in the Great Exhibition of 1851, Mr. 

 Brown exhibited a barometer 39 feet high, containing two immiscible 

 liquids of nearly equal specific gravity, and their point of meeting 

 which may be placed at any part of the scale, was indicated by one 

 liqtu'd being coloured. 



The Aneruid Barometer. The weight of a column of air, which in 

 the ordinary barometer act* on the mercury, in the aneroid presses on 

 a small round metal box, from which nearly all the air has been 

 extracted, and as its indications are obtained without the use of mer- 

 cury or any other fluid, we have the etymology of the word in the 



Greek, d without, and vripbs fluid.* It was invented some years ago 

 by M. Vidi of Paris. It is inferior in value to the mercurial barometer, 

 but has some advantages in sensibility and portability. It admits of 

 being carried in the pocket, and may be used on a journey in situations 

 where the mountain barometer would be inconvenient. Its variations 

 from temperature seldom exceed T ' r ,th inch in a range from 28 to 80", 

 a variation to which the mercurial barometer is liable, only in this case 

 its exact amount for every degree of temperature has been determined. t 

 The external appearance of the aneroid is that of a brass box with an 

 enamelled dial face covered with glass. In the centre of the dial is 

 a pivot for the reception of a needle, which moves over a series of 

 graduations marked from 28 to 31 to represent the inches of the 

 ordinary barometer, and over these graduations are the usual words 

 ' stormy,' ' much rain,' ' rain,' &c. Attached to the face are also two 

 thermometers, one graduated according to Fahrenheit, and the other 

 according to the centigrade scale. On removing the dial-face, but 

 retaining the needle, we have the appearance represented in the 

 accompanying figure, in which a is the flat circular box made of white 

 metal, previously exhausted of air through the short tube 4, and made 

 air-tight by soldering : the upper and lower surfaces of the box are cor- 

 rugated in concentric circles, which improves the elasticity of the metal . 

 This box is attached to the bottom of the brass external case. In the 

 centre of the upper surface of the elastic box is a solid cylindrical 

 socket a', about half-an-inch high, to the top of which the principal 

 lever e, d, e, is attached; this lever brings the box into a state of 

 tension by separating the surfaces, and rests partly on a spiral spring rf, 

 and partly on two fulcra, having knife-edges, which are free to move. 

 The end e of the principal lever is attached to a second lever /, from 

 which a fine watch-chain y, extends to li, where it works on a drum 

 attached to the arbour of the hand. A hair-spring at A, attached to 

 the metallic plate i, regulates the motion of the hand. Aa the weight 



of the atmosphere becomes increased or diminished the surface of the 

 box becomes depressed or elevated, while the spiral spring d, upon 

 which the principal lever rests, shares in the motion, which is thus 

 communicated through the levers to the arbour of the hand at h . At 

 the back of the instrument is a screw for the purpose of adjusting the 

 hand to the, height of the mercurial barometer. 



The neglect of this last point has led to considerable misconception on 

 the part of persons using the instrument, and also on the part of some 

 writers who liavc described it. They regard the aneroid as an inde- 

 pendent instrument, which it was never intended to be, for it requires 

 to be set originally by a Kirometer, and to be adjusted from time to 

 time by reference to the .same instrument. In the Meteorological 

 Papers published by the Board of Trade there is the following testi- 

 mony to its value : " Tin: aneroid is quick in showing the variation in 

 atmospheric pressure, and to the navigator who knows the difficulty of 

 using barometers at times, this instrument is a great boon, for it is 

 steady under all circumstances of weather, at the same time giving 

 indications of increased or diminished pressure. It is a good \vrMln r 

 glass, to be suspended on or near the upper deck for easy reference. 

 In ascending or descending elevated land, the hand of the aneroid may 

 be seen to move like the hand of a watch, showing' the height above the 

 level of the sea." Mr. Belville of the Royal Observatory, Greenwich, 

 in his " Manual nf the Barometer," in giving the meteorological results 

 of a tour in Wales obtained by means of the aneroid, says, " It was a 

 delightful companion and highly useful; its indications preventing 

 many an excursion which would have ended in disappointment." 



* It has been however said, that M, Viili intended this word to be derived 

 rather from a.f)ip, a man, because the pressure is similar to that produced by 

 the same cause on a man's body. Thence he intended to call it an aneroid 

 baroscope. [BAUOSCOM;.] The ' derivation given above, is probably more 

 correct. Others derive it from aWpo^uai, to inquire or observe. 



The inventor has intioduccd into the vacuum-box a small portion of gas, 

 which, changing its bulk with change of temperature, is intended to act as a 

 compensation for the effects of heat and cold. 



