24 THE ATMOSPHERE. 



CHAPTER IY. 



CHANGES IN THE ATMOSPHERE. INSTRUMENTS TO MEASURE 

 THEM. CLIMATE. 



77. The state of the atmosphere is continually changing, 

 and several instruments have been contrived for the pur 

 pose of measuring its changes, and of showing what its 

 state is. The three most important are, 



(1.) The thermometer, (heat-measurer,) which shows 

 the changes in the heat of the air ; 



(2.) The barometer, (weight-measurer,) which shows 

 the changes in the weight or pressure of the air ; and, 



(3.) The hygrometer, (moisture-measurer,) which 

 shows the changes in the amount of moisture in the air. 



78. The Thermometer is constructed on the principle 

 that almost every substance known is swelled or expanded 

 by being heated, and contracted by becoming cooler ; and 

 that the expansion is in proportion to the degree of heat. 



79. This may be proved by various experiments. If a 

 hole in a plate of iron is just large enough to admit a rod 

 of iron when cold, it will be found that, when the rod is 

 heated, it will no longer enter. If the rod be left to cool 

 down to its former temperature, it will enter as at 

 first. This shows that the rod has been expanded by 

 heat, so as to take up more room than it had previously 

 taken up. 



When a wheelwright makes an iron tire for a wagon 

 wheel, he makes it just long enough to bring the fellies 

 closely together. In order to do this most effectually, he 

 makes it a little too short to go on while cold. He there 

 fore expands it by placing it on a circular fire, and when 



