230. What is said of steam? In what consists the power of the steam-engine? How is the expansive force of the steam in the boiler estimated? Describe the working of the engine by Fig. 210.
231. What is the difference between high and low pressure engines? What is said of the communication of heat? How many modes of communication are there, and what are they? What is the mode called convection?
232. Give examples of convection. What is the conduction of heat?
233. State the experiment represented in Fig. 211. State the experiment represented in Fig. 212. What is said of non-conductors of heat? Give the examples cited.
234. Explain Davy's safety-lamp.
235. Give what is stated in the note about Stephenson and Davy. What is said of the influence of density on the conduction of heat? Give the illustration about melting snow.
236. State the experiments which show that liquids are poor conductors of heat. What is said of air as a non-conductor of heat? What is said of double windows?
237. What is said of arrangements of the walls of buildings? What of an arrangements for preventing the spreading of fires in blocks?
238. How are animals in very cold regions protected from the cold? What is it in their coverings that affords the protection? What is said of the coverings of quadrupeds that are natives of warm climates? What of the elephants whose remains are found in Siberia?