INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE, 



BAEOMETEES. 



1. Principle of the Barometer. The first instrument which gave the exact 

 measure of the pressure of the atmosphere was invented by Torricelli, a Florentine 

 pupil of Galileo, in 1643. It is constructed 

 as follows : A glass tube, C D (fig. 1), about 

 34 inches long, and from two to four-tenths 

 of an inch in diameter of bore, having one 

 end dosed, is filled with mercury. In a 

 cup, B, a quantity of mercury is also poured. 

 Then, placing a finger securely over the 

 open end, C, invert the tube vertically over 

 the cup, and remove the finger when the 

 end of the tube dips into the mercury. The 

 mercury in the tube then partly falls out, 

 bat a column, A B, about 30 inches in 

 height, remains supported. This column is 

 a weight of mercury, the pressure of which 

 upon the surface of that in the cup is pre- 

 cisely equivalent to the corresponding pres- 

 sure of the atmosphere. As the atmospheric 

 pressure varies, the length of this mercurial 

 column also changes. It is by no means 

 constant in its height ; in fact, it is very 

 seldom stationary, but is constantly rising or falling in the tube. It is, there- 

 fore, an instrument by which the fluctuations taking place in the pressure of 

 the atmosphere, arising from changes in its weight and elasticity, can be shown 

 and measured. It has obtained the name Barometer, or measurer of heaviness, 

 a word certainly not happily expressive of the utility of the invention. If 

 the bore of the barometer tube be uniform throughout its length, and have its 

 sectional area equal to a square inch, it is evident that the length of the column, 

 which is supported by the pressure of the air, expresses the number of cubic 

 inches of mercury which compose it. The weight of this mercury, therefore, 

 represents the statical pressure of the atmosphere upon a square inch of 

 surface. 



In England the annual mean height of the barometric column, reduced to 

 the sea-level, and to the temperature of 32 Fahrenheit, is about 29'95 inches. 

 A cubic inch of mercury at this temperature has been ascertained to weigh 

 0-43967 Ibs. avoirdupois. Hence 29'95 xO'48967^14'67 Ibs., is the mean 



