56 Bird - Lore 



see that part of the weather. The air full of heat or cold we feel, and we call 

 it hot or cold weather, as the season may be. 



When the air is full of dust-particles or smoke, then we smell not the air, 

 but the foul matter in it. 



It is when the air is full of pure, health-giving gases and vapors, sometimes 

 moist, sometimes dry, but always fresh and invigorating, that we actually 

 smell the weather — the very best kind of weather in all the world. 



Not only man but all other animals, as well as the trees and plants, are so 

 made that changes in the weather have much to do with their growth and 

 habits. 



Plants and animals in hot, rainless regions are quite unlike plants and 

 animals which live where it is always moist or cold. Even the same kind of 

 plants and animals become changed somewhat in habits and growth, when put 

 into very different kinds of weather for a long time (climate). 



Most plants and animals, especially those living in cold climates, change 

 also in habit, and more or less in growth, with the seasons of the year. 



It is for this reason that a study of the changes in weather which take place 

 during the four seasons of the year helps us to understand and look for the 

 growth of vegetation in spring, the coming of migrating birds and the awaken- 

 ing of animals which sleep during winter (hibernation). 



In order to know when spring may be expected, we must watch the weather 

 from day to day. We must not only watch it, but must learn to measure it, 

 to weigh it and to make pictures of it. 



Although it sounds strange to speak of measuring, weighing and making 

 pictures of the weather, this is really what is done in weather-bureaus all over 

 the world. 



The air is measured in different ways, for example, by its hotness or cold- 

 ness (temperature) ; by the rate at which it travels when in motion (velocity) ; 

 or by the amount of moisture in it (humidity). 



It is weighed by finding out how much it presses down upon the earth. 



A thermometer, or heat-measure, is an instrument which tells just how hot 

 or cold the air is. A barometer, or weight-measure, is an instrument which 

 tells the weight (pressure) of the air. 



A weather-vane is a very simple instrument, telling in which direction the 

 air is moving, while an anemometer, or wind-measure, tells how fast the air 

 moves. 



Other instruments are made to show how wet or how dry the air is, how 

 much rain or snow it sheds upon the earth, and other things important for 

 man to know. 



In order to make pictures of the weather, it is necessary first to weigh and 

 measure it in different places, and at different heights. An isotherm is a picture 

 of the temperature, while an isobar is a picture of the pressure of the air, or 

 atmosphere. 



