276 IN STARRY REALMS. 



as far as the outward bound ship is concerned. When the 

 observer started on his voyage there were already seven 

 steamers at different situations in their homeward passage ; 

 all of these the outward-going steamer must pass, but 

 during the week that he is on his voyage seven more 

 steamers must have left New York, and these, too, must 

 also be passed by the outward bound ship before she 

 reaches harbour on the other side of the Atlantic. Thus 

 you see that our passenger outward bound will, in the 

 course of a week, pass fourteen vessels coming the other 

 way. In fact, he will pass two vessels per day, whereas, 

 if he remained at Liverpool, he would only find one per 

 day arriving, on the supposition which we have made. 

 Suppose that the sensations caused by the moving of the 

 vessel were overlooked, so that, in fact, the observer was 

 unconscious of the fact that he was in rapid motion ; he 

 would not unnaturally seek for an explanation of the 

 phenomena he had observed in the movements of the 

 other vessels. In other words, he would conclude that the 

 vessels were leaving New York twice every day, the fact 

 that he was himself in motion having had the effect of 

 making the other vessels seem to arrive twice as often as 

 they would otherwise have done. 



Instead of ships crossing the Atlantic, let us now return 

 to the conception of waves of light radiating from a star 

 in the depth of space. If the star is voyaging towards 

 the earth, then the number of waves that arrive every 

 second will depend not alone on the light itself, but also 

 on the relative velocity with which the bodies are ap- 

 proaching or receding. If the two bodies are approach- 

 ing, then more waves will reach the eye in the same 



