it we cannot tell. Fortunately, a beautiful method has been discovered by which we can determine whether this is so—whether the star is coming towards us, and how fast it is approaching. Halley showed us how to watch the stars moving across our line of sight, and another great Englishman, Sir William Huggins, showed us how to find out with the spectroscope whether they are coming end-on to us, or moving away from us: and I want now to say something of how he did it.
Let us return to the train for a moment and suppose it whistles as it approaches the station. It will whistle a high note; let us say high D on the piano. Now if the train is coming at us the note will not sound like D, but like a note higher in the scale, say D sharp; if on the contrary it is running away from us, the whistle will sound lower in the scale, say D flat. Sometimes a train rushes right through a station whistling all the time, and then you can notice the change from D sharp to D flat quite easily. Many people have thought that the train changed its whistle as it went through, but that is not the case, it keeps on whistling the same note all the time; but while it is still approaching you hear a higher note; immediately it has passed you and begins to run away you hear the lower note.
You can hear much the same kind of thing with a passing motor-car if it happens to sound its horn. Or without the horn at all, if you listen to the hum of the machinery you will hear it change its note as it passes. If the motor-car is going slow, the change will not be much; but if it is going very fast, the change will be great. It would be quite
