140 OF THE GENERATION AND [SECT. in. 



The greatest power of steam should be a little more than is required to work 

 the engine ; suppose it be 5 Ibs. on the circular inch, and the diameter of the 

 lowest part of the seat of the safety valve should be 3 ; then 3 times 3 = 9, the 

 area of the valve in circular inches, and 9 times 5 = 45 Ibs., which is the required 

 weight for the load to be placed upon the valve. This valve will not open 

 till the steam presses it with greater force than 5 Ibs. on the circular inch. 

 The metal box for the valve being locked up, of course no one but the pos- 

 sessor of the key could alter the load on the valve ; but a handle passing through 

 the cover is necessary to move it, to prevent it rusting fast. 



For further security it has been proposed that another safety valve should be 

 placed upon the same boiler, but with rather less load upon it, in order that it 

 may open first, and give notice to the engine man when the steam is likely to 

 become too strong. This should have a stronger handle for moving it, either for 

 letting off the steam when not required, or other purpose ; but the handle to raise 

 the locked valve should be either connected by a chain or slight wire, so that it 

 could not be fixed so as to increase the load on the valve. It would be better 

 to rely on the common valve, than one locked up till it had become stuck fast 

 with rust. 



262. A conical seated valve does not appear to me to be the best ; for the 

 locked valve I would prefer a flat seat, and that the metallic surfaces in contact 

 should be narrow, and of metal not liable to corrosion, nor to fix by unequal 

 expansion. 



263. To prevent the danger of adhering in steam boat boilers, Mr. Nimmo 1 

 proposed that the valve should be a hemisphere with its convex surface down- 

 wards, to rest in a seat formed to fit it, and the weight he proposed to hang to 

 the lowest part of the valve. See U, Plate xvii. Fig. 1. By this means the 

 motion of the boat would be constantly changing the position of the valve, while 

 its form would render it steam-tight in all positions, without danger of adherence. 

 A chain might be also attached to the upper side of the valve, to lift it without 

 opening the case inclosing it. This method deserves attention : its defect would 

 most likely be want of stability in its seat. 



264. The most certain and safe method for low pressure boilers, is to balance 

 the pressure of the steam by a column of water, of a diameter adequate to allow 

 of the escape of the steam as rapidly as it would be possible for the fire to generate 

 it. A feed pipe is to a certain degree a safety tube of this kind, but neither of 

 the size nor construction which safety requires. The tube or pipe T W should 

 be made recurved at the lower end T, Plate n. Fig. 2. its mouth being not lower 



1 Report on Steam Boats ; or, Partington's Historical Account of the Steam Engine, p. 92. 



