OF STEAM ENGINES'. 167 



wooden casing or jacket, as it is technically called, that is ap- LECT. 

 plied round the exterior of the steam cylinder to retain the heat. ^^^ A^ 

 It was made in separate staves, and fixed in its place by iron hoops 

 like a barrel. G the eduction pipe leading from the bottom of 

 the steam cylinder, to the upper part H of the condenser, which 

 is a hollow metal vessel of any convenient form placed below 

 the steam cylinder in a large cistern of water 1 1, called the cold 

 water cistern : b a cock called the eduction valve, for opening 

 and shutting off the connection between the steam cylinder and 

 the condenser. K the air pump also fixed in the cold water 

 cistern, but without any internal communication with the water 

 it contains. This pump is of the common lift construction, ex- 

 cept that its valves are of metal, on account of the heat of the 

 water : its lower valve c is called the foot valve. L is its piston 

 rod, the upper end of which is connected to, and works by the 

 engine beam. 37 is its suction pipe communicating with the 

 bottom of the condenser H, in order that it may draw off all the 

 air and water it contains ; and this water, being in a hot state is 

 delivered into the smaller cistern X, therefore called the hot water 

 cistern. O is a common pump, called the cold water pump, 

 because it supplies cold water to the cistern II, being worked 

 by the beam, in the same manner as the air pump. The surplus 

 cold water runs off into a drain by the spout P, and the hot 

 water by another spout at Q. 



After what has been said.uponthe steam engine, but little expla- 

 nation can be necessary in order to understand the operation of this 

 machine : for if thepiston C is at the bottom of the cylinder, and the 

 steam valve a be opened, steam will rush into B, and permit the 

 piston to ascend in obedience to the counterpoise weights at the 

 opposite end of the beam, until it is sufficiently raised, and at the 

 same time the piston of the air pump will rise and produce a par- 

 tial rarefaction in the condenser. So soon as the piston has got 

 to its proper elevation, the cock a must be shut, and b opened, 

 when the steam in the cylinder will rush into the condenser and 

 be condensed, thereby producing such a vacuum as will cause 

 the steam piston to descend ; when b must be shut, and a opened, 

 to produce a second rising of the piston C, during which the air 

 pump K will draw off any condensed water that was deposited 

 in H, and deliver it into A^, thus preparing the condenser for 

 making a second vacuum, which it is enabled to do by the cold 

 water pump O, keeping the cistern II constantly replenished. 

 To increase the power of condensation, Mr. Watt found it neces- 

 sary to place the cock as at g, for admitting a small stream of 



