B.t.u. transmitted per square foot per hour per degree (Fahrenheit) difference in temperature between inside and outside air. _Brick work_ 4 in. thick = 0.68 { concrete } 8 in. thick = 0.46 { cement } 50 per cent. more than brick. 12 in. thick = 0.33 { stone 33-1/3 per cent. more than brick. Window = 1.090 { Wood as wall = 0.220 { concrete } 20 per cent. more than Double window = 0.560 { reinforced } brick. Important Topics 1. Heat a manifestation of energy. 2. Steam-engine and its action. 3. Mechanical equivalent of heat and heat equivalent of fuels and efficiency of engines. Exercises 1. Construct a working model of the cylinder and steam chest of a steam engine and be prepared to explain its action. 2. At $5.00 per ton how many B.T.U.'s should be produced from 1 cent's worth of bituminous coal? 3. Try the following experiment: Place a quart of water in a teakettle and place it over the fire for 5 minutes, and note the rise in temperature and compute the number of B.T.U.'s entering the water. Place another quart of water at the same temperature in an aluminum or tin dish and heat for 5 minutes, note the rise in temperature and compute the heat used before. Which of the dishes shows the greater efficiency? How do the efficiencies of the two dishes compare? How do you account for any differences in the efficiencies found? 4. How high would 8 cu. ft. of water be lifted if all of the energy produced by burning 1 lb. of coal were used in raising it? 5. What is the mechanical equivalent of a pound of coal expressed in horse-power hours? 6. If a furnace burns 100 lbs. of coal a day and its efficiency is 50 per cent. how many B.T.U.'s are used in warming the house? 7. How many B.T.U.'s can be obtained by burning 1/2 ton of bituminous coal? 8. when a pound of water is heated from 40°F. to 212°F., how many foot-pounds of energy are absorbed by the water? 9. How many loads of coal each weighing 2 tons, could be lifted 12 ft. by the energy put into the water in problem 8? [Illustration: FIG. 163.--An eccentric.] 10. When 3 cu. ft. of water are used for a hot bath and the water has been heated from 50°F. to 112°F., how many B.T.U.'s have been absorbed by the water? 11. If the average temperature of water at the surface of Lake Michigan is 50°F., how many B.T.U.'s would be given off by each cubic foot of water at the surface, if the temperature of the water should drop 5°F.? 12. In a cold storage plant carbon dioxide gas is used. The pipe leading from the compression pump to the expansion valve passes through a condensing tank of cold water. Why? 13. When the gas is compressed in a cold storage plant, what becomes of the energy used by the compression pump?