116 EFFECTS OF HEAT. 



with great power, and forces it out of the 

 spout. 



Q. How can the PRESSURE of STEAM on the 

 SURFACE of the water, FORCE the water through the 



KETTLE-SPOUT ? 



A. In the same manner as the pressure 

 of air on the mercury of a barometer, 

 forces the quicksilver up the glass tube. 



Q. What causes the RATTLING NOISE so often 

 made by the LID of a saucepan or boiler ? 



A. The steam (seeking to escape) 

 forces up the lid of the boiler, and the 

 weight of the lid causes it to fall back 

 again : this being done frequently, pro- 

 duces a rattling noise. 



Q. If the steam COULD NOT LIFT UP THE LID 

 of the boiler, how would it escape ? 



A. If the lid fitted so tightly, that 

 the steam could not raise it up, the 

 boiler would burst into fragments, and 

 the consequences might be fatal. 



Q. When steam pours out from the spout of a 

 kettle, the STREAM begins apparently HALF AN INCH 

 off the SPOUT ; why does it not begin CLOSE to the 

 spout ? 



A. Steam is really invisible ; and the 

 half-inch (between the spout and the 



