PRINCIPLE OF THE BAROMETER. 



181 



The principle of the barometer is very sim- 

 ple. It is an instrument, which, by virtue of 

 its peculiar construction, weighs or balances 'a 

 column of air, forty or fifty miles high, and of 

 the diameter of the barometer tube. 

 Suppose the diameter of this tube 

 to be equal to that of a goose quill. 

 Now, suppose that we could take a 

 piece out of the air of the size of 

 that quill, which extended from its 

 summit to the level of the sea, and 

 weigh it ; and that we then filled the 

 tube of the barometer with mercury, 

 up to the height of 30 inches, and 

 weighed it ; we should find the 

 weight of this short column of mer- 

 cury, and the forty or fifty mile-long 

 column of air, precisely equal ! This 

 may be easily put to the test. We 

 are able actually to weigh the one 

 against the other, in the following 

 manner. Let us, by means of the 

 blow-pipe, melt one end of a tube of 

 glass about 33 inches long until it is 

 quite closed at that end, and then fill 

 it with mercury, and putting the open end into 

 a saucer with a little mercury in it, let us invert 

 it. Yet the mercury does not run out. The 

 reason is, the tall, thin column of air, of which 



THE 

 BAROMETER. 



