ISO IN STARRY REALMS. 



while the several planets can each be discriminated from 

 the others. 



There is, however, a crucial test which will detect every 

 object of a planetary nature. The very word planet means 

 a wanderer ; and the term is appropriately employed to 

 designate the wandering stars which move about over the 

 surface of the heavens. These bodies pass from one con- 

 stellation to another in the course of their circuits around 

 the sky, and they are thus widely distinct from the so- 

 called fixed stars by which the constellations are them- 

 selves formed. In the course of a few weeks, or even less, 

 it will be easy to observe, even without telescopic aid, the 

 actual changes in the positions of the planets ; while, with 

 the assistance of a good equatorial telescope, a single hour 

 is usually sufficient to disclose enough movement in a 

 planet to show that the object is something quite different 

 from a star. 



It is my object in this chapter to convey a sketch of 

 what is known, or can be reasonably conjectured, with 

 regard to Mars as a world. This globe is of particular 

 interest to us ; for it is natural to feel curious with regard 

 to the neighbouring globe, which is in many respects 

 placed in much the same conditions as is our earth. It 

 would seem that our globe occupies an intermediate posi- 

 tion, so to speak, in the general system of the planets. 

 I do not now refer only to the fact that there are some 

 planets which are nearer the sun than we are, and that 

 there are others more distant. This is undoubtedly true ; 

 but there are other circumstances of a still more signi- 

 ficant character. This world is a good deal larger than 

 some of the planets, while it is a good deal smaller than 



