13 



niometer be plunged into a mixture of pounded ice and 

 common salt, from the intense cold produced by the 

 conversion of the ice into water, the mercury will sink 

 to zero, or 32 below the freezing-point of Fahrenheit ; 

 if the tube of the thermometer should not be long enough 

 to admit of so low a graduation, the mercury will shrink 

 into the bulb. The expansion of mercury is ^g o of its 

 bulk for each degree of Fahrenheit between 32 and 

 212. For convenience tables have been computed, 

 from which may be taken out, at sight, the amount to 

 be subtracted from the height of the mercurial column, 

 on account of the expansion of the mercury from tem- 

 perature. 



The words Change, Fair, and Rain, engraved on the 

 plate of the barometer, were placed there by the first 

 observers of its variations : no great importance should 

 be attached to them ; for from the observations of two 

 centuries we find, that heavy rains, and of long conti- 

 nuance, take place with the mercury at 29*5 inches or 

 Change ; that rain frequently falls when it stands as high 

 as 30-00 inches, or Fair ; and, more particularly in winter, 

 a fine bright day will succeed a stormy night, the mer- 

 cury ranging as low as 29'00 inches, or opposite to Rain. 

 It is not so much the absolute height as the actual rising 

 and falling of the mercury which determines the kind 

 of weather likely to follow. The late great elevation of 

 30*90 inches in February of the present year 1849, was 

 succeeded by a minimum of 29'25 inches, which produced 

 a storm of wind so violent that the horizontal pressure of 

 many of the gusts amounted to 201bs. upon the square 

 foot; a pressure which is rarely exceeded, even when 



