MECHANICS. 87 



The cost of a steam engine varies according to its 

 power. The smaller cost nearly ,100 for each horse 

 power, the largest not quite .40. The consumption 

 of coal is rated at one bushel, or 84 pounds, per hour 

 for an engine of ten horse power ; the quantity is 

 somewhat less in proportion in engines of great 

 power. 



In a steam boat of 100 horse power, common 

 pressure, 44 feet are required for the engine and fuel. 

 The high pressure engine would only require half 

 that space. 



Wolfe's Steam Engines. 



These are condensing engines, which work with a 

 power intermediate between high and low pressure. 

 One engine of this description at Whealvor, raises 

 50,000,000 of pounds one foot high, with one bushel 

 of coals. 



The expense of one of the large mines in Corn- 

 wall, in 1816, for coals to work their engines, was 

 about 25,000 per annum. 



Perkins's Steam Engine. 



In the ordinary condensing apparatus, where 1170 

 degrees of heat are absorbed in generating the steam, 

 1070 degrees are lost by entering into the condensing 

 water ; but Mr. Perkins has contrived a method by 

 which nearly all the heat is absorbed from the steam, 

 and returned to the generator. 



The highest that Mr. Perkins has ever used the 

 steam for his engines, is 800 pounds to the inch, or 

 about 57 atmospheres; but in some of the steam gun 

 experiments, a pressure of 1GOO pounds to the inch 

 was used with perfect safety. 



The great power of Mr. Perkins's steam engines 

 has been established, by the fact of its having lifted 

 a column of water forty feet high and forty inches in 

 diameter with a 9J inch piston. 



