VI.] THE ATMOSPHERE. 91 



of a column of air standing on a similar base and extending 

 upwards to the extreme limit of the atmosphere. Now a 

 column of mercury 30 inches long, in a tube of one 

 square inch in sectional area, weighs about 15 lbs.; hence 

 it is found, as before stated, that the weight or pressure of 

 the atmosphere is about 15 lbs. on every square inch. 



If instead of using a dense liquid, like mercury, the ex- 

 perimentalist took a lighter one, such as water, he would 

 naturally expect that the column required to balance the 

 weight of the external atmosphere would be proportionally 

 longer. As a matter of fact it is found that, when water is 

 used, the suspended column is something like 33 feet in 

 length : in other words, as water is about 13 J times lighter 

 than mercury, bulk for bulk, the column of water will be 

 about 13 J times longer than the mercurial column. It was 

 indeed, by observing a body of water raised in the suction, 

 pipe of a pump at Florence, that Torricelli was led to his 

 experiment with quicksilver. In working a common pump, 

 air is sucked out of the tube communicating with the source 

 of water below, and the pressure of the atmosphere then 

 forces the water up the pipe in order to supply the place 

 of the air which has been removed. But, when the pipe 

 reaches a length of more than about 30 feet, the column of 

 water which it contains balances the atmospheric pressure, 

 and consequently if the tube be longer than this no more 

 water rises, and the pump ceases to act. In seeking to 

 find out why water cannot rise higher, Torricelli was led to 

 make the experiment to which reference has been made, 

 and to construct the instrument which is represented on the 

 left-hand side of Fig. 21. It is called a Barometer?- 



1 Barometer, from fidpos, baros, weight, and fierpov, metron, measure ; 

 an instrument for measuring the weight of the atmosphere. Thermo- 

 meter, from Q^pixos, ihei-mos, hot ; an instrument for measuring tem- 

 perature. 



