270 



LECTURE XXIX. 



([space which the evolved gases, &c. have to travel before they quit the 

 neighbourhood of the water, enables them both to be thoroughly consumed, 

 and to part with all their heat to advantage. The communications between 

 the cylinder and boiler, and cylinder and condenser, are commonly made by 

 means of a sliding valve, which, from its shape, is known by the- name of 

 the D valve. It is seen in figures (1) and (2), 



Fig. 1. 



Fig. 2 



and consists of nothing more than a slide G of this shape, 



placed in the steam chest, the opening be being sufficiently 



wide to allow a free communication between the passage 



which leads to the eduction pipe T, and one of the passages 



to the cylinder, whilst it closes the latter from the steam chest. 



Thus, in figure 1, the communication is between the bottom 



of the cylinder and the condenser, whilst steam is entering to the top of the 



cylinder. In figure 2 it is the reverse. 



The apparatus by which the valve is slid up and down is seen at 

 figure (3), 



