392 Introduction to the Study of Science 



aurora has been found by exact measurement to reach the height 

 of one hundred miles. 



The temperature of the upper atmosphere shows a similar change. 

 It gradually falls until at the height of seven miles the ther- 

 mometer registers about 60 below zero Centigrade. At greater 

 altitudes the fall is such that it appears to be altogether certain 

 that the space beyond the earth's atmosphere has a temperature 

 very nearly as low as what is known as " absolute zero," or 

 -273 C. 



180. Why balloons float. In studying the conditions that 

 cause objects to float in water, it was found that a floating body 

 displaces its own weight of water. A gigantic log raft, contain- 

 ing millions of feet of lumber, floats because it has apparently 

 less density than water. But a ship may be built of steel, which 

 has a greater density than water. The reason is this : the 

 steel structure incloses a vast body of air from which water is 

 excluded, so that the total volume of steel and air has an 

 average density that is very much less than that of water. 

 Consequently a steel ship heavily loaded floats (page 342 ff.). 



Average density. The principle involved in floating a ship 

 on water is exactly the same as that in floating a balloon in air. 

 The average density of the balloon and its load must be less than 

 that of the air. It is impossible to construct a steel shell for a 

 balloon and exhaust from it the air in order to reduce its average 

 density enough to allow it to float. A steel shell light enough to 

 float would be crushed before the air it contains could be ex- 

 hausted. What would be the pressure per square foot? If 

 a shell inclosing a vacuum could resist the pressure, its average 

 density would be far greater than that of the air. Hence, a 

 light strong material such as silk or rubber is selected for the 

 construction of the bag or envelope. This is filled with hydrogen 

 or other suitable gas, which by its pressure against the inner 

 surface of the envelope displaces the air, and the balloon rises. 



Example. Suppose a balloon, including envelope, basket, ropes, 

 and other equipment, weighs 150 kilograms, and the envelope holds 



