254 PROGRESS OF SCIENCE. 



by less waves than if it were moving towards them. By this 

 principle, which Doppler (180353) discovered in reference 

 to sound (1841), but which holds good for light, it is clear 

 that if a celestial body moves towards us, the succession 

 of light waves upon the eye will be more rapid than if it 

 recedes from us ; and therefore the spectral colour will 

 shift towards the red end of the spectrum if the body be 

 approaching us, and towards violet if the body be receding. 

 Now this shift of the spectral lines from their normal 

 position in the spectrum is detected with absolute certainty 

 by the spectroscope, and by that means we are able to 

 tell not only that a star moves, but also in what direction 

 it does move, and the rate at which it moves. Thus THE 

 SPECTROSCOPE which can reveal to us what bodies are 

 made of, whether these bodies be air, straw, paper, wood, 

 lava, stone, or heavenly bodies ; whether heavenly bodies 

 are gaseous, liquid, solid ; whether they shine by intrinsic 

 or reflected light, can also reveal NEW WORLDS, together 

 with the DIRECTION AND RATE of their MOTION. In these 

 researches Dr. Huggins, Dr. MILLER, and Mr. ALEXANDER 

 HERSCHEL contributed their share the last mentioned 

 rinding different kinds of spectra among meteorites Norman 

 Lockyer meanwhile working in the same direction, and 

 gathering data upon which to build his meteoritic hypothesis 

 as we have seen. 



b. 1824. Thomson (Sir William), now Lord Kelvin, invented 



the MARINE GALVANOMETER, which made MARINE CABLE 

 telegraphy a complete success; invented the electrometer for 

 measuring the amount of electrical charge; also an apparatus 

 for investigating atmospheric electricity ; in collaboration 

 with Prof. Haughton, determined the numerous varieties of 

 ocean tides (lunar diurnal, solar diurnal, lunar fortnightly, 

 monthly, yearly ; solar semi-annual and annual, etc.) ; applied, 

 with John Hopkinson, to lighthouses a SYSTEM OF SIGNALS, 

 suggested by Babbage, by which each lighthouse perpetually 

 spells its own number or name " by flashes or obscurations 

 of various duration and succession." Concurrently or con- 

 jointly with other physicists, and amongst others, Clausius, 

 CLERK MAXWELL, and HELMHOLTZ, Sir William Thomson 



