QUANTITATIVE INDUCTION. 109 



When we have the power of varying the quantity of a 

 cause entirely at our will it is easy to discover whether 

 a certain effect is due to that cause or not. We can then 

 make as many regular or irregular changes as we like, 

 and it is quite incredible that the supposed effect should 

 by chance go through exactly the corresponding series of 

 changes unless by dependence. Thus, if we have a bell 

 ringing in vacuo, the sound increases as we let in the air, 

 and it decreases again as we exhaust the air. Tyndall's 

 singing flames evidently obeyed the directions of his own 

 voice ; and Faraday when he discovered the relation of 

 magnetism and light found that, by making or breaking 

 or reversing the current of the electro-magnet, he had 

 complete command over a ray of light, proving beyond all 

 reasonable doubt the dependence of cause and effect. In 

 such cases it is the perfect coincidence in time between 

 the change in the effect and that in the cause which raises 

 a high improbability of casual coincidence. 



It is by a very simple case of variation that we infer 

 the existence of a material connexion between two bodies 

 moving with exactly equal velocity, such as the locomotive 

 engine and the train which follows it. Elaborate observa- 

 tions were requisite before astronomers could all be con- 

 vinced that the red hydrogen flames seen during solar 

 eclipses belonged to the sun, and not to the moon's atmo- 

 sphere as Flamsteed assumed. As early as 1706, Captain 

 Stannyan noticed a blood red streak in an eclipse which 

 he witnessed at Berne, and he asserted that it belonged 

 to the sun ; but his opinion was not finally established 

 until photographs of the eclipse in i86o a taken by Mr. 

 De la Eue, showed that the moon's dark body gradually 

 covered the red prominences on one side, and uncovered 

 those on the other, in short, that these prominences 

 moved precisely as the sun moved and not as the moon 

 moved. 



