128 Transactions of tee American Institute. 



We have here still larger driving wheels, and on a previous occa- 

 sion, when I used this experiment in the opera house at Philadelphia, 

 where there were conveniences for the purpose, I employed a ten 

 horse-power steam engine as the driving force. 



By thus rotating this thin disk at the rate of 6,000 times a minute, 

 its surface moving only about (3^-) three and a half times as fast as an 

 express train, or some 216 miles an hour, and holding a file against its 

 edge, a beautiful shower of sparks is projected upward, composed of 

 fragments of the file torn off by the impact of the projecting particles 

 of the wheel, and rendered luminous by the intensely rapid vibrations 

 into which they are thus thrown. 



We have thus seen, by many conclusive experiments, that sound is 

 a vibration, and that heat and light may be produced by exactly the 

 same means as sound, provided we exalt the rapidity of the action in 

 the appropriate degree. 



This may serve us as a legitimate ground for the assumption that 

 light is one of the vibratory forces. 



I would not have you suppose, however, that what I have already 

 said includes the entire body of evidence on this subject ; in fact, it 

 hardly touches it, and not one lecture, but several, would be needed, 

 even in the most general manner, to unfold the vast array of facts and 

 arguments by which this doctrine is established. 



Passing, then, from our brief consideration of the nature of light, 

 we will next turn our attention to some of its sources. 



Our late theoretical consideration of its nature teaches us that any- 

 thing which would cause a very rapid vibratory movement in the par- 

 ticles of matter, might be a source of light. Such was the case in our 

 last experiment of the rotating iron disk, where mechanical force 

 expressed in sensible motion was the cause of light. Similar to this 

 is the spark produced by the clash of flint and steel, the foot stroke of 

 the iron-shod horse on the pavement, the flash produced by the cannon 

 ball when it strikes the side of the iron-clad, and the flashing trail of 

 the shooting star, and possibly we may go further still and look to a 

 like conversion of mechanical force in the inrushing of condensing 

 matter towards great centers of attraction and the impact of in-falling 

 bodies for the origin of light in our glorious sun and in the infinite 

 hosts of stars. But as the sun will be the special subject of a lecture 



Note.— (See Fig. 8.) This experiment was first made by an American, Jacob Perkins, who achieved 

 great reputation in England in connection with his steam gun, and steel-plate transfers, and who is 

 now represented in London by the firm of A. M. Perkins & Son, who manufacture steam engines 

 and boilers, running at very high pressures, 250 pounds and upward, and which are guaranteed to 

 give a duty of one horse-power for each two pounds of coal consumed per hour. 



