26 CHANGES IN THE ATMOSPHERE. 



The thermometer is an instrument of great utility to 

 the farmer, and indeed to every intelligent person. 



81. A Barometer is constructed upon the principle, now 

 a well-known fact, that air has weight. It can be weighed 

 by a delicate balance, by first filling a flask with air and 

 weighing it, and then drawing out the air by an instru 

 ment called an air pump, and weighing the flask without 

 the air. At the level of the sea, one hundred cubic 

 inches of air weigh 805 grains, while water weighs 816 

 times as much. 



82. The air seems to be pressed towards the earth by 

 its weight, just as water is kept in the ocean and in lakes 

 by its weight. Its pressure is greatest at the level of the 

 sea, because of all the air in the sky above. As we ascend 

 a hill or mountain, the pressure becomes less, because 

 there is less air above us, and because the attraction of 

 gravitation is diminished. The air is constantly in motion ; 

 and its pressure upon the surface of water, and upon all 

 other surfaces, is constantly varying. The purpose of a 

 barometer is to measure this varying pressure. 



83. A barometer is made of a large tube of glass, pre 

 cisely like that of the thermometer (A B fig. 1,) but much 

 longer, not less than 32 or 83 inches long, with a bag 

 or bulb at one end, filled with mercury, or quicksilver, 

 so contrived as to rise to a certain height in the tube, 

 while it has the air bearing upon it in the bag or bulb. 

 From the upper end of the tube the air is first completely 

 withdrawn or exhausted, by the tube s being held upside 

 down. The tube is then turned back and fastened to a 

 wooden frame, or enclosed in a case with a graduated 

 plate behind the upper end of the tube, on which plate 

 are marked the heights of the column of quicksilver. 



