on the Steam Engine. 333 
- 
second plate fuses at 10°, (18° Fah.) above the first. 
_“ Although many cases may be cited in which fusible plates have 
probably prevented explosions, they are employed unwillingly by 
_ Most, preference being given to the common valves, with which, in 
r addition to the plates, the boilers must be provided. Let us then 
examine the objections to these plates: . It was said at first that since 
these plates were affected by temperature, and not by pressure, they 
might melt when the steam within was very hot, but not elastic in 
Proportion, but this can happen only when the vapor is not saturated 
with moisture: that is, only when there is not a sufficient supply of 
| water within the boiler ; then a portion of the boiler must become 
heated, perhaps even to redness, and then there is eminent danger 
of explosion. This first objection, therefore, seems to be refuted. 
| The plate does not approach the point of fusion, without being soft- 
ened ; it is therefore feared that it may give way under a tension 
Much less than that which would. produce its fusion. At the outset 
to that which the steam to be used in the engine should have. The 
rst 
- Covering the plate with awire gauze, of small meshes, before it is 
fixed by bolts to the aperture which it is to close. Even now parts 
of the plate yield partially, swelling out in different places as the 
fusing point approaches; but experience has shown that it is only 
very near to this point that the metal yields entirely, opening a free 
Passage to the steani. When the fusible plate has been melted, all 
the steam escapes through the opening which it closed. It may take 
some time to replace it, to fill anew the boiler, and to heat the water, 
and during this time the engine stands still. In a steam boat, in cer- 
+ 
Setious-accidents. This is a real and a great difficulty, and perhaps 
is the reason why our neighbors have not adopted the fusible metal 
valve, but give preference to the ordinary safety valve. These it is 
tue, never suffer all the steam to escape. If they open, it is only 
When the elasticity of the steam within has passed a certain limit ; 
8 soon as this elasticity has returned within the limits fixed by the 
engineer before hand, they fall, closing the aperture ; and thus the 
moving power can never fail entirely. . 
“The advocates of the fusible metal. plates, considered as one of 
¢ highest advantages of these valves, the physical impossibility of 
anging their limit of action, thus placing them beyond the reach of 
tprudent workmen. It is true that with these plates, all overcharge, 
Vou. XX.—N 0B ; 
this did actually take. place, but the difficulty has been obviated by 
tain cases, this sudden absence of the moving power, might occasion _ 
ie 
