MECHANICS AXD USEFUL ARTS. 



71 



This process was illustrated, by reference to the mode of producing the 

 intense heat and luminosity required for the oxy-hydrogen microscope. In. 

 the latter, the piece of lime or carbon on which the heat was projected, 

 was instantly raised to the temperature of extreme luminosity, neither the 

 lime or carbon, however, suffering rapid combustion. In the former, the 

 carbon of the gas was raised by the same means, (the combustion of the 

 hydrogen), to the high temperature, but could not suffer combustion until 

 it was brought into contact, in its turn, with its equivalent of the oxygen 

 of the air. If, however, that supply of air was not provided before the 

 carbon lost its high temperature, it returned to its previous and natural 

 state of a black substance, and gave the black character to the products 

 called smoke. 



In effecting the combustion of the gas generated from the coal in a fur- 

 nace, the first process was merely mechanical, and consisted in bringing 

 the atoms of the gas, and those of the air, into the most intimate state of 

 mixture ; such mixture being the sine qua non of subsequent chemical 

 union. The mode or means by which this chemical admixture could be 

 effected in the most rapid and intimate manner, involved all that art or 

 human ingenuity could do, towards producing perfect combustion. Refer- 

 ence was then made, to the area recommended by some authorities, as 

 being sufficient to allow the quantity of air to enter a furnace. It had 

 been considered, that even half a square inch of aperture, for each square 

 foot of furnace grate, was sufficient for the combustion of the fuel. This 

 was, however, stated to be insufficient for practical purposes, the proper 

 area for admission being from four to six square inches for each foot of 

 grate, according to the extent of draught and the nature of the coal. This 

 serious difference was supposed to have been caused, by an erroneous cal- 

 culation of the rate of the current of air entering. For if half a square 

 inch of area was all that was allowed, the air must have a velocity ten 

 times greater than could be shown to have been ever attained. Thus, 

 supposing a furnace to be four feet by two feet six inches, equal to ten 

 square feet of bar surface, this would effect a combustion of 2 cwt. of 

 coal per hour, and require, for the gas alone, a supply of 10,000 cubic 

 feet per hour, or for 20 cwt. of the coal, 100,000 cubic feet. The following 

 comparison of velocities, of the entering air for the supply of the gas, gave 

 some idea of the cause of underrating the required area of admission : 



Then, if the area were reduced to half a square inch, it would require a 

 velocity of 80 feet per second, to provide for the admission, within the 



