242 • .METKOROI.OGV. 



the leKistance is legist. Now if the air, whicli coniPf; into this region, 

 would continue of the same density as before it leaves the surrounding 

 pans, then an equilibrium would be established and the barometer con- 

 sequently would have to rise. But the atmosphere, like all other gas- 

 eous matter, expands when it comes under less pressure, and the result 

 is, that so soon as it passes within the region v>'here the barometer is 

 lowest, it must expand. This concentring body of air, now expanding, 

 must become specifically lighter than it was before, and of similar den- 

 sity to the air previously within tliis region ; and now being lighter it 

 must ascend and give place to the pressure of the more dense atmos- 

 phere around it. There must then he an ascending column of air over 

 that space., because the air within is always more expanded and lighter 

 than the air without the storm. This phenomenon is illustrated in min- 

 iature by what takes place in a flue of a chimney communicating with 

 the fire. The fire, heating the column of air above it, makes it specifi- 

 cally lighter and causes it to ascend, and so soon as this upward motion 

 is given to it an inward motion is communicated to the surrounding air 

 below, which ruslies in from all sides towards the base of the flue, im- 

 niodiatoly expands, and also ascends. This is proven by the light bo- 

 dies which are frequently carried in to and up the flue by what is usu- 

 ally called the draught of the chimney, or, in other and more philoso- 

 phical language, by atmospheric pressure. 



Allhougli the air expands so soon as it comes within the region of 

 diminished pressure, yet it continues to expand more and more as it as- 

 cends in consequence of coming successively under less pressure. Here 

 another difliculty appears to arise. If the air flows in from all sides and 

 ascends, it must, unless afterwards disposed of, create an immense col- 

 umn, whose accumulated weight, though specifically lighter than the air 

 around, ought to raise the barometer in the centre of a storm. But 

 there is a method by which this great influx of air is rapidly disposed 

 of, and even had we no means of explaining its efflux, the indications 

 of the barometer would prove the fact beyond all doubt. As it is my 

 desire to unfold this theory regularly, step by step, it will be premature, 

 at this stage of the inquiry, to enter into the explanation of this part of 

 the phenomenon. It will be necessary, for the present, only to an- 

 nounce the fact that the air which comes in below passes outward above, 

 and thus prevents the accumulation within the column of all the air 

 which enters it. I may again refer to the flue in illustration : the smoke 

 and light bodies, which are carried up, soon spread out above and glide 

 away on tlie surrounding atmosphere, thus oflering no impediment to 

 the continuance of the upward current. 



