288 



BAROMETE R. 



Baromiter. 



Ceneral di- 

 rections for 

 the con- 

 structing of 

 barometers. 



Tubes. 



Mercury. 



in consequence of* variation* in the weights of the at- 

 mospherical columns. Some pretend that Otto de 

 Guericke first proposed to employ the barometer 

 as a weather-glass j but this idea, so obvious, was a 

 natural consequence of the observations of Perrier 

 and Descartes, and mu6t have occurred at a very ear- 

 ly period to these philosophers. Boyle made many 

 meteorological observations with it in 166(5 ; and he 

 was also at great pains to refute a hypothesis advan- 

 ced by Linus, concerning the cause of the elevation 

 of the mercury in the Torricellian tube. This hy- 

 pothesis, which prevailed for some time,' was called 

 the Funicular hypothesis: it assumed, that the mer- 

 cury was supported by an invisible rope of the same me- 

 tallic fluid ; an opinion so extremely absurd, that, in 

 the present state of science, we have some difficulty 

 in believing that it was ever maintained, or that it 

 should have been deemed worthy of a grave refuta- 

 tion. 



The experiment of Torrieelli was so simple, and yet 

 so easy to be exhibited under a variety of forms, that 

 a great number of barometers were soon proposed, 

 either with a view of rendering them more correct, 

 or enlarging the extent of the barometrical scale. 

 Before we proceed to give particular descriptions of 

 these instruments, and of the various attempts which 

 have been made to increase their accuracy and sensi- 

 bility, it may not be improper to make some previous 

 remarks applicable to barometers in general. 



The tubes intended for barometers ought to be 

 sealed hermetically at both ends, immediately after 

 they are made at the glass-house, and to be kept in 

 that state till they are to be fitted up. Without this 

 precaution, they are apt to be sullied with dust, 

 moisture, and other impurities, which it is almost 

 impossible afterwards to remove, on account of the 

 smallness of their diameters. When they are opened, 

 which may be done with a file, care should be taken 

 not to breathe into them, or to wash them with spi- 

 rit of wine, or any other fluid ; experience having 

 proved, that in tubes so treated, the mercury al- 

 ways stands a little below its proper level. This is, 

 no doubt, owing to the adhesion of a little of the 

 spirit of wine to the sides of the tube, which being 

 afterwards converted into vapour, renders the vacuum 

 above the mercury imperfect. If any cleaning is ne- 

 cessary, it may be done with a fine linen rag, that 

 has previously been well dried. 



The tubes ought to be as perfectly cylindrical as 

 possible, though, in some cases, this is not absolute- 

 ly necessary. They should be about 33 inches in 

 length, and the diameter of their bore should be at 

 least 2 or 2\ lines, otherwise the friction, and capil- 

 lary action, will be apt to affect the free motion of 

 the mercury. The glass should not be very thick, 

 as it is apt, in that case, to break, when the mercury 

 is boiled in the tube : half a line is sufficient. 



The mercury ought to be perfectly pure, and free 

 from all foreign metals. The best is what has been 

 recently revived from cinnabar ; the common mer- 

 cury of the shops being often adulterated intention- 

 ally with tin, lead, and bismuth, stands at various 

 heights in the tube, according to the nature and 

 quantity of the foreign substances with which it is 

 amalgamated. 



Method of 

 filling the 

 tubes. 



The different mechanical methods which have been Barometpr 

 proposed for purifying mercury, are, for the most 

 part, ineffectual; we would, therefore, recommend 

 the revivification of the metal from cinnabar, for nice 

 barometers, as being least liable to uncertainty. For 

 this purpose, take a pound of cinnabar, and reduce 

 it to powder ; mix it well with live or six ounces of 

 iron, or steel filings ; and having put the mixture in- 

 to an iron retort, expose the whole to the heat of a 

 revcrberatory furnace ; the mercurv will soon pass 

 over in a state of great purity, and may be obtained, 

 by adapting to the retort an earthen receiver which 

 has been previously half filled with water. 



Before being introduced into the tube, the mercu- 

 ry ought to be well heated, or even boiled in a gla- 

 zed earthen pipkin, in order to drive off any mois- 

 ture which may adhere to it ; but this will be unne- 

 cessary, if the mercury has been recently revived. 



The mercury ought likewise to be boiled in the 

 tube, to expel any air or moisture which may still 

 remain attached to it, or to the inside of the tube. 

 This is done in the following manner: Pour as much 

 mercury into the tube as will make it stand to the 

 height of three or four inchq sj. and introduce a long 

 wire of iron to stir it during trie act of boiljng. Ex- 

 pose the mercury in the tube gradually to the heat 

 of a chafing dish of burning charcoal ; and when it 

 begins to boil, stir it gently with the iron wire to 

 facilitate the disengagement of the bubbles of the 

 air. When the first portion of the mercury has been 

 sufficiently boiled, and all the air extricated, remove 

 the tube from the chafing dish, and allow the whole 

 to cool, taking care not to bring it into contact with 

 any cold substance. Introduce an equal quantity of 

 mercury, and treat it in the same manner, withdraw- 

 ing the wire a little, so that it may not reach below 

 the upper part of the mercury already freed from air. 

 The chafing dish must also be placed immediately 

 under the mercury which has been last poured in. 

 Repeat the same process with each successive por- 

 tion of mercury till the tube is filled, always apply- 

 ing the heat very cautiously ; and be equally careful 

 in allowing it to cool, before a fresh portion of mer- 

 cury is poured in. 



It sometimes happens, when the tube is carefully 

 inverted, as in the Torricellian experiment, that the 

 mercury, after being completely freed from air, in 

 the way we have described, remains suspended in the 

 upper part of the tube, and does not assume its pro- 

 per level, with respect to the pressure of the atmo- 

 sphere, till the tube has been gently shaken. This 

 fact, which seems to have given great difficulty to 

 Huygens, is certainly owing to the capillary attrac- 

 tion of the tube, and the mutual attraction of the 

 particles of mercury, as it takes place only in tubes 

 of a small bore. To say, that it is owing to the in- 

 fluence of an invisible ethereal fluid more subtile than 

 air, is no less unphilosophical than the semicircular 

 hypothesis of Linus, or the principle of the horror of 

 a vacuum, particularly when we can assign a cause 

 for it, of whose operation we have many simple and 

 obvious proofs. 



The Torricellian experiment exhibits the barome- 

 ter under the simplest, though not the most conve- 

 nientform. The tube AB,{ Fig. 2.PlateLIII.) which 

 4 



Causeofthc 

 occasional 

 suspension 

 of the mer- 

 cury above 

 its proper 

 level. 



Plate 

 UII. Fig. D.. 



