246 ELEMENTS OF GENERAL SCIENCE 



different directions when seen from different points on the 

 earth's surface. By means of exact measurements of the dis- 

 tance between two points of observation, and of the two angles 

 of observation, the distance to the point of the third angle may 

 be accurately calculated. 



After the distance of the moon has been found, its size can 

 be determined. The computations show that its diameter is 

 2163 miles, or approximately J of that of the earth. Its 

 volume is about -$ of that of the earth, while its mass is 

 nearly -^ of that of the earth. 



252. Atmosphere and water on the moon. The moon dif- 

 fers from the earth in the very important point that it has no 

 atmosphere surrounding it. This is shown by the fact that 

 the illuminated part of its surface is always seen undimmed 

 by clouds or even an atmosphere. The reason that the moon 

 has no atmosphere is that its attraction is not great enough to 

 hold one in its vicinity. An atmosphere such as that which 

 surrounds the earth is made up of an exceedingly great num- 

 ber of small masses called molecules, which we have already 

 considered. Their incessant pounding on every object in our 

 air produces what we call the atmospheric pressure, which at 

 sea level is about fifteen pounds per square inch. 



There is no water upon the moon's surface. When observed 

 through powerful telescopes all that side of the moon which 

 is toward the earth is seen to be simply a rocky and desolate 

 waste. It has sometimes been supposed that on the side of 

 the moon which is never turned toward the earth there might 

 be water and conditions like those which make the earth 

 habitable ; but this is impossible, for if there were an atmos- 

 phere on the other side it would quickly spread all over, and 

 part of it would appear on the side of the moon toward the 

 earth. If there were water on the other side and no atmos- 

 phere, at least a part of the water would be evaporated and 

 would produce an atmosphere of water vapor, part of which 

 would pass to the side toward the earth. 





