330 MISCELLANEOUS. 



re-acts upon the rocket, and causes it to ascend, on 

 the same principle as a boat is put off by a man in 

 it who pushes against the shore with a boat-hook. 



Fire-works by means of irtflammable air are the 

 most elegant; and being free from smell or smoke, 

 may be exhibited in a room without any disagree- 

 able effect. 



By referring to what has been already said when 

 treating on hydrogen gas, the principle of these fire- 

 works will easily be understood. Small copper or 

 tin tubes must be provided, of about a quarter of 

 an inch in diameter ; these tubes must be formed 

 into the shapes required, and pierced with very 

 small holes where a flame is wanted to appear; to 

 these pieces must be attached large bladders, or 

 air-tight bags, filled with hydrogen gas. There 

 must also be a stop-cock between the tubes and 

 the bladders, to open or close the communication. 

 Having prepared every thing properly, open the 

 stop-cock, and press the bladders; the hydrogen 

 will be forced out through the holes in the tubes, 

 and by means of a taper may be inflamed. A con- 

 stant stream of fire may be kept up as long as 

 there is any air in the bladders. 



Upon this principle were constructed the beau- 

 tiful fire-works exhibited in London by Mr. Cart- 

 wright. 



The philosophical candle, formed by hydrogen, 

 has been already described under chemistry. 



Of Artificial Grottos and Shell-Work. 



The idea of artificial grottos is often wrongly 

 conceived. They are usually intended as imita- 

 tions of nature; yet natural grottos are never seen 



