IN METEOROLOGICAL REGISTRATION. 91 
as to produce a palpable daily change in its weight and 
pressure, we may apply the knowledge thus obtained to 
enquiries into the larger alterations that occur irregularly. 
Little doubt can be entertained that the same cause which 
makes the barometer decline daily, reduces atmospheric 
pressure during storms to a larger extent. The daily 
reduction of pressure, when no rain falls, is not often more 
than equal to half an inch of mercury ; but if condensa- 
tion has begun, at any time when there is a sufficient sup- 
ply of vapour, it may go on until the atmosphere becomes 
much heated in one part to a great height, and its weight 
in that part may be so much reduced, that the barometer 
may fall one or two inches — or even more, — the vapour 
to effect this large decline being brought from distant 
parts by winds. 
