THE STEAM-ENGINE. 



sisting the pressure ; but if it be so, its point of fusion would be one at which 

 the steam would have a pressure of at least two atmospheres above its work- 

 ing pressure. The plug would therefore be capable of being fused only as 

 soon as the steam would acquire a pressure of 30 Ibs. per inch above its regu- 

 lar working pressure. 



When a boiler ceases to be worked, and the furnace has been extinguished, 

 the space within it appropriated to steam will be left a vacuum by the conden- 

 sation of the steam with which it was previously rilled. The external pressure 

 of the atmosphere acting on the boiler would, under such circumstances, have 

 a tendency to crush it inward. To prevent this, a safety-valve is provided, 

 opening inward, and balanced by a weight sufficient to keep it closed until it 

 be relieved from the pressure of the steam below. 



A large aperture closed by a flange secured with screws, represented at 

 in fig. 52, called the man-hole, is provided to admit persons into the boiler for 

 the purpose of cleaning or repairing its interior. 



The manner in which the governor regulates the supply of steam from the 

 boiler to the cylinder, proportioning the quantity to the work to be done, and 

 thereby sustaining a uniform motion, has been already explained. Since, 

 then, the consumption of steam in the engine is subject to variation, owing to 

 the various quantities of work it may have to perform, it is evident that the 

 production of steam in the boiler should be subject to a proportional variation. 

 For otherwise, one of two effects would ensue : the boiler would either fail 

 to supply the engine with steam, or steam would accumulate in the boiler from 

 being produced in too great abundance, and would escape at the safety-valve, 

 and thus be wasted. 



In order to vary the production of steam in proportion to the demands of the 

 engine, it is necessary to stimulate or mitigate the furnace, as the evaporation 

 is to be augmented or diminished. 



The activity of the furnace must depend on the current of air which is drawn 

 through the grate-bars, and this will depend on the magnitude of the space af- 

 forded for the passage of that current through the flues. A plate called a damper 

 is accordingly placed with its plane at right angles to the flue, so that by rais- 

 ing and lowering it in the same' manner as the sash of a window is raised or 

 lowered, the space allowed for the passage of air through the flue may be reg- 

 ulated. This plate might be regulated by the hand, so that by raising or low- 

 ering it the draught might be increased or diminished, and a corresponding 

 effect produced on the evaporation in the boiler : but the force of the fire is 

 rendered uniformly proportional to the rale of evaporation by the following ar- 

 rangement, without the intervention of the engineer. The column of water 

 sustained in the feed-pipe (figs. 52, 53) represents by its weight the difference 

 between the pressure of steam within the boiler and that of the atmosphere. 

 If the engine consumes steam faster than the boiler produces it, the steam con- 

 tained in the boiler acquires a diminished pressure, and consequently the col- 

 umn of water in the feed-pipe will fall. If, on the other hand, the boiler pro- 

 duce steam faster than the engine consumes it, the accumulation of steam in 

 the boiler will cause an increased pressure on the water it contains, and there- 

 by increase the height of the column of water sustained in the feed-pipe. 

 This column therefore necessarily rises and falls with every variation in the 

 rate of evaporation in the boiler. A hollow float P is placed upon the surface 

 of the water of this column ; a chain connected with this float is carried up- 

 ward, and passed over two pulleys, after which it is carried downward through 

 an aperture leading to the flue which passes beside the boiler : to this chain is 

 attached the damper. By such an arrangement it is evident that the damper 

 will rise when the float P falls, and will fall when the float P rises, since the 



