THE BAROMETER. 17 



great bodies of water. A few moments only will serve to give a 

 proper idea of this most important instrument. 



A barometer is an instrument used for ascertaining and measur- 

 ing the weight of the air or more strictly speaking of our atmos- 

 phere. It was invented in 1643, when Torricelli was making his 

 examinations as to the reason why water ascended in pumps to the 

 height of thirty-two feet, and there remained. Taking a glass tube 

 some four feet in length and closed above, he filled it with mercury 

 and reversed the open end in a basin of that same substance. The 

 column sank to a level of 27.5 inches and there remained. Com- 

 paring the column of water with that of mercury he found them to 

 be to each other in height in an inverse ratio of the specific gravi- 

 ties of these two substances; that is, a column of mercury 27.5 

 inches high would balance a column of water 32 feet; and both 

 would remain the same, while the pressure of atmosphere from outside 

 upon the open basins of mercury and water in which the columns 

 were inverted remained the same. In 1646, at Rouen, one Pascal 

 repeated this experiment, and further proved that while one column 

 of mercury remaining at the bottom of a mountain underwent no 

 change, a similar column taken to the top of the same mountain 

 was reduced in height by several inches by the diminution of pres- 

 sure, while it regained that same height, and corresponded with 

 the one left at the bottom of the mountain when brought down. 

 By this means he proved the possibility of measuring heights by 

 the variations of the barometer, for thus the tube of mercury was 

 named. Boyle, in 1666, discovered that the atmosphere was elas- 

 tic and compressible, and Mariotte that the density of the atmos- 

 phere was in proportion to the weight with which it was com- 

 pressed. The layer of air nearest the earth was heaviest and 

 sustained the weight of all the rest of the atmosphere above it and 

 each succeeding stratum was lighter and lighter. Aqueous vapor is 

 the amount of water evaporated and held in suspension in the at- 

 mosphere. As evaporation is promoted by dry air, wind, a diminu- 

 tion of pressure, and heat, the quantity thus held in suspension 

 depends upon the temperature. Since " heat expands the gaseous 

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