6o IN STARRY REALMS. 



oiated, for if a proportionate amount of force be given to 

 all the details, it is still useful as expressing the general 

 character of the object, and valuable as exhibiting a fair 

 relation between its several parts." 



As soon as the prominent features on the planet came 

 to be recognised, it was found necessary to devise means 

 by which their positions on the globe could be conve- 

 niently indicated. The analogy of our own earth showed 

 that the proper method of accomplishing this was to 

 adapt to Mars a system of latitudes and longitudes. As 

 to the former, there was but little difficulty. Once the 

 rotation of the planet was sufficiently observed, its axis 

 became known, and also its equator. The latitude of an 

 object on Mars is, of course, its angular distance from the 

 equator. But the longitude presents a difficulty as to the 

 zero from which it should be measured. This difficulty 

 is not exclusively found on Mars. We have it on our own 

 globe. Englishmen naturally measure all longitudes from 

 the meridian of Greenwich ; but national susceptibilities 

 arise, which prevent us from actually asserting that all ter- 

 restrial longitudes are measured in the same manner that 

 we use. 



On Mars the difficulty is of a somewhat different nature. 

 The first essential for the zero of longitude is that it shall 

 be at all events a mark so clearly denned that there shall 

 be no ambiguity. This is the point about which there 

 has been some difference of opinion among astronomers. 

 But I think there can be little doubt that the proposition 

 made by Mr. Knobel, in 1884, is the correct one. He 

 points out that there is on the face of the planet one spot 

 so very clearly defined and so very easily observed with 



