290 



s 



THE BAROMETER. 



and we accordingly find the words " rain," " fair," " changeable," " frost," &c., 

 engraved on the scale attached to common domestic barometers, as if, when 

 the mercury stands at the height marked by these words, the weather is always 

 subject to the vicissitudes expressed by them. These marks are, however, 

 entitled to no attention ; and it is only surprising to find their use continued in 

 the present times, when knowledge is so widely diffused. They are. in. fact, 

 to be ranked scarcely above the vox stellarum, or astrological almanac. 



It has been already explained, that in the same state of the atmosphere the 

 height of the mercury in the barometer will be different, according to the eleva- 

 tion of the place in which the barometer is situated. Thus two barometers, 

 one near the level of the Hudson and the other on the heights of West 

 Point, will differ by half an inch ; the latter being half an inch lower than the 

 former. If the words, therefore, engraved upon the plates, are to be relied 

 upon, similar changes of weather could never happen at these two situations. But 

 what is even more absurd, such a scale would inform us that the weather at the 

 top of a high building, such as Trinity church, New York, must always be 

 different from the weather in Wall street, at its foot. 



The variation in the altitude of the barometer in a given place, together with, 

 the corresponding vicissitudes of the weather, have been regularly recorded 

 for very long periods. It is only by the exact comparison of such results that 

 any general rule can be found. The rules best established by such observations 

 are far from being either general or certain. It is observed that the changes 

 of weather are indicated, not by the actual height of the mercury, but by its 

 change of height. One of the most general, though not absolutely invariable 

 rules is, that when the mercury is very low, and therefore the atmosphere very 

 light, high winds and storms may be expected. 



The following rules may generally be relied upon, at least to a certain ex- 

 tent : 



1 . Generally the rising of the mercury indicates the approach of fair weather : 

 the falling of it shows the approach of foul weather. 



2. In sultry weather the fall of the mercury indicates coming thunder. In 

 winter the rise of the mercury indicates frost. In frost its fall indicates thaw ; 

 and its rise indicates snow. 



3. Whatever change of weather suddenly follows a change in the barome- 

 ter may be expected to last but a short time. Thus, if fair weather follow im- 

 mediately the rise of the mercury, there will be very little of it ; and in the 

 same way, if foul weather follow the fall of mercury it will last but a short 

 time. 



4. If fair weather continue for several days, during which the mercury con- 

 tinually falls, a long succession of foul weather will probably ensue ; and 

 again, if foul weather continue for several days, while the mercury continually 

 rises, a long succession of fair weather will probably succeed. 



5. A fluctuating and unsettled state in the mercurial column indicates 

 changeable weather. 



The domestic barometer would become a much more useful instrument if 

 instead of the words usually engraved on the plate, a short list of the best es- 

 tablished rules, such as the above, accompanied it, which might be either en- 

 graved on the plate, or printed on a card. It would be right, however, to ex- 

 press the rules only with that degree of probability which observation of past 

 phenomena has justified. There is no rule respecting these effects which will 

 hold good with perfect certainty in every case. 



One of the most important scientific uses to which the barometer has been 

 applied, is the measuring of heights. If the atmosphere, like a liquid, were 

 incompressible, this problem would be very simple. The pressure on the met- 



