THE RADIOMETER. n 



mitted over those 92,000,000 miles between the earth and the sun, is 

 still one of the greatest mysteries of Nature. 



In the science of optics, as is well known, the phenomena of light 

 are explained by the assumption that the energy is transmitted in 

 waves through a medium which fills all space, called the luminiferous 

 ether, and there is no question that this theory of Nature, known in 

 science as the Undulatory Theory of Light, is, as a working hypothe- 

 sis, one of the most comprehensive and searching which the human 

 mind has ever framed. It has both correlated known facts and pointed 

 the way to remarkable discoveries. But, the moment we attempt to 

 apply it to the problem before us, it demands conditions which tax 

 even a philosopher's credulity. 



As sad experience on the ocean only too frequently teaches, energy 

 can be transmitted by waves as well as in any other way. But every 

 mechanic will tell you that the transmission of energy, whatever be the 

 means employed, implies certain well-known conditions. Let it be that 

 the energy is to be used to turn the spindles of a cotton-mill. The 

 engineer can tell you just how many horse-power he must supply for 

 every working-day, and it is equally true that a definite amount of 

 energy must come from the sun to do each day's work on the surface 

 of the globe. Further, the engineer will also tell you that, in order to 

 transmit the power from his turbine or his steam-engine, he must have 

 shafts and pulleys and belts of adequate strength, and he knows in 

 every case what is the lowest limit of safety. In like manner, the 

 medium through which the energy which runs the world is transmitted 

 must be strong enough to do the immense work put upon it ; and, if 

 the energy is transmitted by waves, this implies that the medium must 

 have an enormously great elasticity, an elasticity vastly greater than 

 that of the best-tempered steel. 



But turn now to the astronomers, and learn what they have to tell 

 us in regard to the assumed luminiferous ether through which all this 

 energy is supposed to be transmitted. Our planet is rushing in its 

 orbit around the sun at an average rate of over 1,000 miles a minute, 

 and makes its annual journey of some 550,000,000 miles in 365 days 

 6 hours 9 seconds and T B 7 of a second. Mark the tenths; for astro- 

 nomical observations are so accurate that, if the length of the year 

 varied permanently by the tenth of a second, we should know it ; and 

 you can readily understand that, if there were a medium in space which 

 offered as much resistance to the motion of the earth as would gossa- 

 mer threads to a race-horse, the planet could never come up to time, 

 year after year, to the tenth of a second. 



How, then, can we save our theory, by which we set so much, and 

 rightly, because it has helped us so effectively in studying Nature ? If 

 we may be allowed such an extravagant solecism, let us suppose that 

 the engineer of our previous illustration was the hero of a fairy-tale. 

 He has built a mill, set a steam-engine in the basement, arranged his 



