THE NON-LIVING WORLD 173 



along a line equidistant from both, it will be horizontal. 

 The " dip " of the needle is subject, like its variation, to 

 daily, monthly, and yearly changes, as well as to sudden 

 storms. 



The external sources of the world's heat and light are, 

 of course, the " heavenly bodies " the sun, moon, and 

 stars, the study of which constitutes the science of 

 astronomy. 



The earth, as every one now knows, is not only a 

 sphere, revolving on its axis daily, but also accomplishes, 

 together with its satellite, the moon, an annual revolu- 

 tion round the sun. It is therefore a planet, i.e., one of 

 those various other spheres which also revolve round the 

 sun, together with their satellites ; and which planets, 

 with certain comets, and clouds of more or less relatively 

 minute bodies, called meteoroids, constitute a planetary, 

 or solar, system. This system again is but one of many 

 systems of suns (it may be with or without attendant 

 planets) which make up, together with some dark globes 

 and masses of gases or vapours termed nebulce the 

 visible sidereal universe. 



The various bodies of this universe, which vary 

 immensely as to size, are continually changing their 

 relative positions according to the mechanical laws of 

 dynamics, already noted, and the force of gravity. 

 These bodies are all material bodies, and we have seen * 

 that all such are (by gravity acting between them) drawn 

 together directly as their masses and inversely as the 

 squares of their distances. It is the study of the 

 relative movements of the heavenly bodies which has 

 revealed to us the universality, so far as we have been 

 able to test it, of the law of gravitation. 



* See ante, p. 66, 



