88 APPLIED SCIENCE 



the absolute zero, and therefore is equal to the observed 

 temperature plus 273 or 459.4 as the case may be. 



The lowest temperature which has thus far been attained 

 is -252 C. Dewar produced it by the evaporation of liquid 

 hydrogen. 



87. Principle of the Barometer. Gases, though gener- 

 ally lighter than air, all have a definite weight. This weight 

 depends upon the volume of the gas and the pres- 

 sure exerted, as may be proved by means of an 

 instrument called a barometer (Fig. 53). The 

 principle on which the barometer is based may 

 be explained in the following manner. 



If you put one end of a tube into a bowl of 

 water and the other end into your mouth, you can 

 draw the water up through the tube into your 

 mouth by sucking. You may think that you 

 suck the water up, but you do not; you merely 

 suck the air out of the tube by means of the 



FIG 53 muscles of your mouth. The weight of the outer 



Simple Ba- a i r then presses down on the water in the bowl 

 romctGr 



and forces it up into the tube. As soon as you 



let the air into the tube again the water runs back into the 

 bowl. If you had a tube 40 ft. in length and could suck all 

 the air out of it, the water would rise up in the tube nearly 

 34 ft. It would stop at that height, because the weight of 

 the column would just balance the weight of the air which 

 presses down on the surface of the bowl. As the tube is 

 more than 34 ft. long, in the space above the water, there 

 would be nothing, not even air. Such a space is called a 

 vacuum, from the Latin word meaning space without air. 

 If you put the tube into a fluid lighter than water, such a^ 



