TRANSACTIONS OF SECTION 6. 



'77 



end of its stroke. The air-valve, operated by the cam as already mentioned, has 

 communicating passages with the air-pump, the furnace or combustion-chamber, 

 and the annular air or packing space in the main cylinder. Consequently, the 

 compressed air is forced partly through the fire and combustion-chamber, and 

 partly into the annular air-space, the flow of air continuing during the time the 

 air-piston performs the second half of the stroke. Meantime, the main piston 

 receives its charge from the combustion-chamber, and cold compressed air passes 

 into the annular space, and practically acts as a packing, eSectually preventing 

 grit and dust rising from the fire to the working faces of the cylinder. AVhen the 

 air-pump has finished its stroke, the air-valve is closed, and the air in the working 

 cylinder is allowed to expand for tbe remainder of the stroke. 



The cylinder is kept cool by means of a circulating-water jacket. 



The bottom of the combnstion-cbamber is hinged, and tbe fuel is coke. As tbe 

 combustion takes place under pressure, an air-valve, working automatically, is 

 employed for feeding tbe fire. 



The consumption of coke is about 3 lbs. (one kilogramme and a half) per brake 

 horse power and per hour. 



3. Oji the Internal and External Work of Evaporation. 

 By W. WoKBt Beaumont, M.Inst. C.E. 



Several of tbe most interesting problems in connection with the st'^am-engine 

 turn upon the view that is taken of the mode of employment of the heat equivalent 

 of the external work of evaporation. 



When steam is generated under constant pressure external work is performed 

 = PV, P being the pressure and V the volume generated. It may therefore, in 

 accordance with the thermodynamic conceptions, be assumed that more heat is 



