4 HEAT OR CALORIC. 



by the operation of the machinery ; the atmosphere does not ope 

 rate, except on the horizontal section of the rod of the piston. In 

 this machine, the steam is constantly working, while in Newcomen's 

 it was inert half the time, and not only was the cylinder below the 

 piston, chilled at every stroke, by the cold water, but above the pis- 

 ton, by the cold air. Mr. Watt's great improvement consisted in shut- 

 ting out the atmosphere entirely, and in causing the condensation of 

 the steam, at a distance from the cylinder, which is in that way main- 

 tained at the boiling point. Thus both the upward and downward 

 movement of the piston, is caused by the elastic effort of the steam. 



Wolf's, Evans's or the high Pressure Engine. There is no con- 

 densation of the steam, which is driven out alternately, above and be- 

 low the piston, against the atmosphere. As these engines work simply 

 by dead lift of expansive steam, great strength is necessary in the 

 machinery. The principal advantage is in economy of machinery, 

 and room ; not of fuel. On account of the strength and smaller size 

 of the boilers, explosions are less frequent, than in the low press- 

 ure engines, but they are more destructive. Dr. Hare remarks, 

 that " the engines in our steam boats, generally combine the two prin- 

 ciples using steam that will support a weight, of from seven to fif- 

 teen pounds, per square inch, and that a true Bolton and Watt steam 

 engine, having an ample supply of water, cannot explode while the 

 safety valve is of a proper size, and not improperly loaded."* 



Perkin's Generator.-^ The pressure is far beyond any thing here- 

 tofore used ; eight hundred pounds, and even one thousand pounds, 

 on the square inch, is not an uncommon pressure and fifteen hun- 

 dred has been frequently used. The generator is very small ; it is 

 heated in a furnace ; there is no boiler, but water is injected by the 

 machinery, as it is wanted, about one gallon at a time. At Woolwich 

 of late,J the steam was so heated, as to set fire to wood, tow, &c. 

 and to ignite the iron generator, at the orifice made for the emission 

 of the steam. Mr. Perkins says, that 4000 atmospheres = 6 5, 000 

 Ibs. on the square inch, is the maximum pressure of steam.< 



(#.) Mr. Perkins states that his high steam will not issue from an 

 orifice, in his generator, one fourth of an inch in diameter, the pressure 



* If these conditions were observed, all steam engines would be much safer than 

 they are ; but in the high pressure engines, the metal is necessarily exposed both 

 to the weakening effect of heat, and to the mechanical strain arising from vast pres- 

 sure ; while in the low pressure engines, these causes are comparatively feeble in 

 their operation. The rule for loading the valve in Mr. Watt's original engines, 

 was two and a half pounds for each square inch. 



t See Am. Jour, especially Vol. XIII. 



t Jones' Journal, Nov. 1827. 



The elastic energy of common steam is derived from the latent heat X ^P- gr. -{- 

 the temperature or thermometric tension. Ure. 



