126 COMBUSTION OF MAGNESIUM. 



of so much variety in the knocks of different persons, with- 

 out the danger of any of them being confounded with the 

 ti-adesrnen's or postman's knock, that almost every indi- 

 vidual comes to have his own peculiar rap, and thus the 

 knocker has a language, as it were, of its own, notifying 

 those who are in the house of the rank and position of the 

 person at the door, and, in case it is any inmate of the fam- 

 ily, making it known at once who it is. 



And this is how it happened that Lawrence, on hearing 

 the knocker sounded at the door while he was reading his 

 newspaper, said at once, " Here comes John." 



All this explanation, however, of the language of the 

 boatswain's whistle at sea, and of the knocker on the doors 

 of English houses, is a digression, and it would be some- 

 what irrelevant in a scientific treatise on light were it not 

 that a language belonging to this same class, and of substan- 

 tially the same character in respect to its principles, and of, 

 perhaps, about the same scope as to copiousness and ex- 

 tent, has gradually grown up among the light-houses on 

 sea-coasts, by means of which some simple but very im- 

 portant information can be communicated to ships at sea 

 through variations in the light, as will hereafter be more 

 particularly explained. 



John very soon came into the room, and as he entered 

 the door he held up a small object between his thumb and 

 finger. 



" See," said he. 



Lawrence looked up. John advanced toward him, hold- 

 ing out what had much the appearance of a watch-spring 

 coiled up, except that the color was of a bluish-gray. 



"What is it?" asked Lawrence. 



" Magnesium," replied John. " It is a yard long when it 

 is uncoiled. I bought it for sixpence." 



" That's cheap," said Lawrence. 



