Steam-Engine. 423 



linder, was entirely lost so far as the steam was concerned, since 

 it was raised simply by the excess of weight on the other arm of 

 the large lever. Here was a saving both of time and expense ; 

 for the piston was accelerated each way by steam, and the 

 quantity of fuel employed to keep it hot, during its ascent, was 

 not wasted. Watt took care, moreover, to surround the cylin- 

 der with a case of w r ood, or some other substance which is a non- 

 conductor of heat; into the interior of which, he also occa- 

 sionally introduced the steam as a means of keeping it warm. 

 He likewise used so much economy in the construction of the 

 different parts of his engine, that he succeeded in saving two 

 thirds of the steam employed by Newcomen. The steam 

 engine, thus improved, is represented in figure 253, the ex- 

 planation of which will now be easily understood. FD is 

 the boiler, in which the water is converted into steam by the 

 heat of the furnace below. This boiler is sometimes made 

 of copper, but more frequently of iron. Its bottom is con- 

 cave and the flame curls around it. Towards the top, it 

 has a safety valve S" fitted to resist a greater or less effort before 

 opening, according to the degree of elastic force to be em- 

 ployed. That the conversion of water into steam may be con- 

 stant, it is necessary that the water in the boiler should be kept 

 always at the same level, and consequently that it should be 

 supplied as fast as it is vaporized. This is effected by a tube 

 -o r, which supplies tht> boiler from a small reservoir r, filled 

 with water, already heated, which the pump 1 1 takes from the 

 condenser and forces into the lateral pipe t' i f . But in order 

 to introduce this into the boiler only when it becomes necessary, 

 the upper orifice of the tube v v is closed by a stopper, which is 

 raised or lowered by means of a small lever a b ; and at the 

 other arm b of the lever hangs a wire b ra, drawn downwards by 

 a weight ra, which is so adjusted in the boiler as to keep on a 

 level with the upper surface of the water. Then if the water 

 falls below this level, the weight m, which it supports in part, will 

 descend with it ; the lever turning will raise the stopper, and 

 suffer the water to pass into the boiler ; but as soon as the level 

 is re-established, the lever a b will again become horizontal, and 

 return the stopper to its place. From the top of the boiler pro- 

 ceeds the steam tube VV, which conveys the steam to the top of 

 the cylinder TT : by the valve S, and to the bottom, by the 



