.546 MULL. METEOROLOGY. BAROMETER. 



where the changes depend on causes of a very extensive 

 nature. 



In a practical view we are thus enabled to explain a 

 phenomenon of frequent occurrence, particularly in sum- 

 mer; namely, the inutility of the barometer on certain 

 occasions in pointing out either future or actual changes 

 from calm to storm or from fair to foul weather. Nothing 

 is more usual than for the mercury to maintain a steady 

 elevation for considerable periods during our summer, if the 

 wind undergoes no change of direction ; although rain is 

 predominant or alternates with fair weather. These changes 

 of weather are thus shown to be partial; and of this we have 

 generally sufficient evidence in the current reports from dif- 

 ferent districts. The inutility of this instrument as an uni- 

 versal indicator of weather is thus proved, if proof of a fact 

 so notorious were necessary ; but the cause is also pointed 

 out, since it is here seen that no changes of weather will 

 affect it unless they are of a general nature. It is for 

 this reason that in autumn, as it approaches towards 

 winter, and throughout this season as far as the vernal 

 equinox and beyond, the barometer becomes a more 

 accurate, often a very perfect register of these changes ; 

 which would thus be proved, even if that were not con- 

 firmed by collateral evidence, to be of a wide extent. 

 The next practical result to be drawn from this fact is 

 of still greater importance, and would be even more so, 

 had the marine barometer become an universal appendage 

 to the instruments used in navigation. In the instances 

 above related, not one gale of wind out of ten would have 

 been indicated by it ; and the mariner who, depending on 

 its warnings, should therefore neglect, in sea language, 

 to keep a good look-out, would endanger the loss of his 

 vessel by trusting to a fallacious guide. The use of this 

 instrument has hitherto been so limited, and I may add 

 the observations, even if carefully made, so inaccessible, 

 that I cannot, in the writings of those navigators who 

 have used it, refer to any proofs of this uncertainty. 



