ECONOMY OF HEAT. 



The cylinders are often cased in wood. The boilers of loco- 

 motive engines are always covered with a coating of boards. 



By these and many other expedients for the economy of heat, 

 and more especially by the extensive application of the expan- 

 sive force of steam, the mechanical power evolved from the 

 combustion of coals k has been increased to an almost incredible 

 extent. 



22. A system of public inspection, of the performance of 

 the engines worked in the mining districts of Cornwall, was 

 established about forty years ago, which has been continued 

 to the present time with the greatest advantage to the mining 

 interests in particular, and to the engineering and commercial 

 world in general. An exact account is kept, and periodical 

 reports published of the quantity of fuel consumed by each 

 engine, and the quantity of work done, the latter being expressed 

 always by an equivalent weight, raised one foot high. The ratio 

 of the fuel consumed to the weight thus raised is called the DUTY 

 of the engine. 



23. The improved efficiency of steam machinery is illustrated 

 in a striking manner by these reports. It appears by them, that, 

 in 1813, the average mechanical effect of a bushel of coals, 

 applied in the best of the Cornish engines, was 11785 tons raised 

 one foot. In 1837, this duty was 38935 tons raised one foot. 

 The duty was therefore augmented in the ratio of 1 to 3. 



The increase of the mechanical efficiency of fuel has still gone 

 on from year to year, and it may now be considered that a bushel 

 of coals, of average quality, applied under good conditions of 

 economy to the most efficient engines, is capable of producing a 

 mechanical effect equivalent to 50000 tons raised one foot. 



24. It follows, therefore, that a pound of coal has a mecha- 

 nical virtue expressed by six hundred tons weight raised one foot 

 high. 



25. It is only by comparison with other physical agents that 

 we can duly appreciate this prodigious mechanical power of coals. 



It is calculated that the materials composing the great pyramid 

 of Egypt might have been elevated from the level of its base to 

 their actual places by the combustion of 700 tons of coal. 



26. Those of the Menai Bridge might have been raised from the 

 level of the water by 400 Ib. of coal. 



27. A train of coaches weighing 80 tons, and conveying 240 

 passengers, is drawn from Liverpool to Birmingham, and back 

 from Birmingham to Liverpool by the combustion of 4 tons of 

 coke, the cost of which is 51. To carry the same number of 

 passengers daily on a common road would require an establish- 

 ment of 20 stage coaches and 3800 horses. 



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