071 the Steam Engine. 333 



to that which the steam to be used in the engine should have. The 

 second plate fuses at 10°, (18° Fah.) above the lirst. 



" Although many cases may be cited in which fusible plates have 

 probably prevented explosions, they are employed unwilhngly by 

 riiost, preference being given to the common valves, with which, in 

 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 vt^ithin 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 J it is therefore feared that it may give way under a tension 

 much less than that which would produce its fusion. At the outset, 

 this did actually take place, but the difficulty has been obviated by 

 covering the plate with a wire 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 tlie steam. 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- 

 tain cases, this sudden absence of the moving power, might occasion 

 serious 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 

 true, 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 ; 

 as 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 

 the highest advantages of these valves, the physical impossibility of 

 changing their limit of acdon, thuS placing, them beyond the reach of 

 imprudent workmen. It is true that with these plates, all overcharge. 



Vol. XX.— No. 2. 43 



