THE STEAM-ENGINE. 435 



Fig. 10. 



forms the passage which it is the office of the valve to open and close. When 

 the valve falls into its seat, it fits the aperture like a plug, so as entirely to 

 stop it. The spindle plays in sockets or holes, one above and the other below 

 the aperture which the valve stops ; these holes keep the valve in its proper 

 position, so as to cause it to drop exactly into its place. 



Fig. 11. 



In the experimental engine made by Mr. Watt at Kinneal, he used cocks, 

 and sometimes sliding covers, like the regulator described in the old engines ; 

 but these he found very soon to become leaky. He was, therefore, obliged to 

 change them for the spindle-valves just described, which, being truly ground, 

 and accurately fitted in the first instance, were not so liable to go out of order. 

 These valves are also called puppet-clacks, or button-valves. 



In the earlier engines constructed by Watt, the condensation was produced 

 by the contact of cold surfaces, without injection. The reason of rejecting 

 the method of condensing by injection was, doubtless, to avoid the injurious 

 effects of the air, which would always enter the condenser, in combination 

 with the water of condensation, and vitiate the vacuum. It was soon found, 

 however, that a condenser acting by cold surfaces without injection, being 

 necessarily composed of narrow pipes or passages, was liable to incrustation 

 from bad water, by which the conducting power of the material of the condenser 

 was diminished ; so that, while its outer surface was kept cold by the water 

 of the cold cistern, the inner surface might, nevertheless, be so warm that a 

 very imperfect condensation would be produced. 



At the time that Watt, in conjunction with Dr. Roebuck, obtained the patent 

 for his improved engine, the idea occurred to him, that the steam which had 

 impelled the piston in its descent rushed from the cylinder with a mechanical 

 force much more than sufficient to overcome any resistance which it had to 

 encounter in its passage to the condenser ; and that such force might be ren- 

 dered available as a moving power, in addition to that already obtained from 

 the steam during the stroke of the piston. This motion involved the whole 



