40 PRACTICAL LESSONS IN SCIENCE. 



about 33 feet high, varying somewhat with the condition of the 

 air, as to temperature, as to the presence of water vapor, etc. 

 At higher elevations some of the air is below the experimenter, 

 so that the pressure is less and the barometer column is shorter. 

 In this way elevations may be measured with considerable ac- 

 curacy. The mercurial barometer consists of a tube filled with 

 mercury inverted in an open cup of mercury, fitted with a scale, 

 an arrangement for making corrections, etc., the whole provided 

 with a case of wood or metal for protection. It is used for meas- 

 uring elevations, but its most important use is in connection 

 with navigation as a weather glass. It shows the varying press- 

 ure of the atmosphere, from which one may gain some idea of 

 what weather to expect. In general, a rising column, or increas- 

 ing pressure, indicates fair weather, while a falling column, or 

 diminishing pressure, indicates foul weather. An instrument 

 called an aneroid barometer is much used because of its porta- 

 bilitya good one need not be much larger than a watch. 

 Atmospheric pressure is indicated by its effect on an elastic box 

 containing slightly rarefied air. The aneroid is not as reliable 

 as the mercurial, and should be adjusted frequently by a stand- 

 ard mercurial, if one expects to get reliable indications. The 

 upward pressure or buoyancy of the air interferes somewhat 

 with the accuracy of weighing operations. 



The molecules of gases are so far apart that one gas does not 

 hinder the expansibility of another. If a number of gases are 

 mixed in a limited space, each will be diffused throughout the 

 whole space, gases acting as vacuums to each other. In this way 

 poisonous gases, the products of respiration, combustion and 

 decay, and of various manufacturing operations, instead of 

 accumulating in one place are continually dissolved away and 

 dispersed in the great atmospheric ocean. 



The siphon is an instrument by which liquids may be trans- 

 ferred from one vessel to another by atmospheric pressure. It 

 consists of a bent tube, one arm of which is longer than the 

 other. Take a piece of glass tubing about twelve inches long 

 and bend it into a U shape at five inches from one end. Fill the 



