PNEUMATICS, 



11 



are not so much indicated by the actual 

 height of the mercury as by its varia- 

 tions in height. The following rules 

 may, in a certain degree, be relied on, as 

 corresponding generally to the concomi- 

 tant changes in the barometer and the 

 weather : 



] . Generally, the rising of the mer- 

 cury indicates the approach of fair wea- 

 ther ; the falling of it that of foul wea- 

 ther. 



2. In hot weather the fall indicates 

 thunder. 



3. In winter the rise indicates frost, 

 and in frost the fall indicates thaw, and 

 the rise snow. 



4. If fair or foul weather immediately 

 follows the rise or fall, little of it is to 

 be expected. 



5. If fair or foul weather continue for 

 some days, while the mercury is falling 

 or rising, a continuance of the contrary 

 weather will probably ensue. 



6. An unsettled state of the mercury 

 indicates changeable weather. 



By these rules it will be seen that the 

 words engraved on the plate are fre- 

 quently calculated to mislead the ob- 

 server. Thus, if the mercury be a.i.much 

 rain, and rise to changeable, fair wea- 

 ther is to be looked for. Again, if it be 

 at set fair, and fall to changeable, foul 

 weather may be expected. 



CHAPTER IV. 

 Of the Elasticity of Air. 



(25.) We have already mentioned the 

 properly which fluids in' general possess 

 of transmitting; pressure (13.) Air and 

 other elastic fluids possess this quality 

 as perfectly as liquids, but there are 

 some circumstances attending the man- 

 ner in which they exert it which must 

 be attended to. If we suppose the closed 

 vessel A B C D E F G H (fig. 9.) de- 

 scribed in (13,) to be filled with air in- 

 stead of water, while the air is of the 

 same density as the free external air, 

 let the piston P be laid upon the aper- 

 ture a b so as to confine the air, but not 

 compress it or reduce its bulk. 



From the aperture a b let a cylinder, 

 open at both ends, enter the vessel to 

 any distance c d. Upon urs^ng the pis- 

 ton with any pressure, as one pound, it 

 will enter the cylinder to a certain dis- 

 tance a' b' (Jig. 10.) at which its further 

 progress will be arrested by the increas- 

 ed resistance of the confined air becom- 

 ing equal to the urging pressure. In 

 this state of the apparatus, every part of 



the inner surface of the vessel wiH sus- 

 tain a bursting pressure amounting to 

 one pound on every square inch, sup- 

 posing that to be the magnitude of the 

 aperture a b. Thus the air or other 

 elastic fluid has the same extraordinary 

 facility in transmitting pressure as we 

 already ascribed to liquids or inelastic 

 fluids. The only difference between the 

 cases is, that the elastic fluid yields to 

 the incumbent pressure, and suffers 

 itself to be driven into a diminished 

 space, while it transmits the incumbent 

 pressure to the inner surface of the ves- 

 sel in which it is confined. 



(26.) The next property of air which 

 calls our attention is its elasticity, a 

 property intimately connected with the 

 last-mentioned, though not dependent 



fiff.9. 



on it. This, as we have already stated, 

 is the line of distinction between gase- 

 ous fluids and liquids. 



In the apparatus which has been just 

 described, the air confined in the vessel 

 yielded to the pressure which urged the 

 piston P, so as to allow the piston to 

 enter the cylinder ; but the pressure on 

 the piston continuing the same, the pro - 



