352 On Steam- Engines . [Nov. 



A second advantage of high pressure engines, and one that 

 is even greater than the former, is the economy of fuel which 

 results from the effects of a high temperature. This will be 

 readily granted, when it is stated that the repairs and expenses 

 of the steam-engines employed in draining a single large coal- 

 mine in England, amount annually to the sum of '25,5001. 



On this account several large proprietors of copper and tin 

 mines, in Cornwall, adapted machinery to their engines, in 

 1811, by which an account is regularly kept of the work, which 

 they perform ; and, from the results of these experiments, con- 

 ducted on the largest scale, the comparative effect of the dif- 

 ferent kinds of engines has been ascertained for more than ten 

 years. 



In the month of August, 1818, the Cornish steam-engines 

 raised 15,760,000 lbs. one foot high, for each bushel of coals 

 consumed. 



From December of the same year, the improvements were so 

 material, either in the management of the engines, or in some 

 of their parts, that the mean total product was increased to 

 17,075,000 lbs. 



By a series of similar improvements, and by the construction 

 of new and more perfect engines, the product was, 

 In December, 1812, 18,200,000 lbs. 



1814, 19,784,000 



1815, 20,766,000 



and since 1815, the product is even still larger, in conse- 

 quence of the improvements that have been made in the con- 

 struction of the fire places, and boilers, and in short in every 

 part of the machinery. 



At the present day, it is calculated that Watt's improved 

 steam-engines raise more than 30,000,000 lbs. of water one foot 

 high, by the consumption of one bushel of coals. 



By the side of this augmentation we must place that 

 which results from the employment of Woolfe's steam-engines, 

 which, as is well known, are condensing engines, and work 

 with a pressure intermediate between that of the high and the 

 low pressure engines. 



Such a machine, with a double cylinder, has been constructed 

 for the mine Whealvor, in Cornwall ; the diameter of the large 

 cylinder is 53 inches, and that of the small one 5*3 inches. 

 This engine has raised 49,980,822 lbs. one foot high, by one 

 bushel of coals, whilst the mean product of the other engines 

 was only 20,479,350 lbs. raised to the same height. 



In 1815 the mean product of two of Woolfe's engines was 

 46,255,250 lbs. 



One of the inconveniences attending engines of mean and 

 high pressure, is loss of power by the wear of the more delicate 

 parts of their structure, and the consequent loss of steam ; at 



