THE EARTH'S VELOCITY. 



507 



already noticed when considering the sun, it is to that ruler of the day 

 that we are indebted for everything. 



What would happen, then, if the earth were suddenly to increase her 

 velocity, or the sun to contract his mass ? We should be flung into infinite 

 space, and in a short time would be frozen up completely. Our present 

 diurnal course would probably proceed, but all life and existence would 

 cease as we whirled with distant planets through infinity. 



Suppose, on the contrary, we were to stop suddenly. We have some 

 of us read in a foregoing part of this volume that heat is the motion of 

 molecules in ether, and that when a body strikes another heat is developed 

 by contact and friction. If the earth were to be stopped suddenly, " an 



Fig. 556. Transit of earth across Sun, seen from Mars.*? 



amount of heat would be developed sufficient to raise the temperature of a 

 globe of lead of the same size as the earth 384,000 of the Centigrade 

 thermometer. The greater part, if not the whole of our planet, would be 

 reduced to vapour, as Professor Tyndall says. 



In the diagram (fig. 546, on page 497) we shall at once find the explana- 

 tion of the constantly-recurring seasons, and the amount of our globe which is 

 illuminated by the sun at various times. It will be easily understood that the 

 poles have six months day and six months night. When the earth is at an 

 equinox, one half of the surface is illuminated and the other half in shade, 

 therefore the days and nights are equal. But when the north pole turns 

 more and more towards the sun, the south pole is turning away from it in 

 the same ratio, the days and nights respectively are getting longer and 



