332 Review of Renwick 



In order to obviate the dangers which result from a deficiency of 

 water in the boiler, and a consequent heating of the upper surface, 

 and of the flues when these are employed', several different methods 

 are used. Guage cocks are attached to the boiler, which the en- 

 gineer turns at short intervals, for the purpose of ascertaining the 

 level of the water ; and a thermometer ought to be connected with 

 the boiler in such a manner that its indications may be seen from 

 without. A still more effectual way of guarding against the conse- 

 quences of an accumulation of heat in the upper part of the boiler, is 

 by means of plates of fusible metal. It is well known that an alloy 

 composed of lead, tin, and bismuth, has the property of melting at a 

 very low temperature, sometimes even, when the best proportions 

 are observed, at a temperature below that of boiling water. By va- 

 rying the proportions of the ingredients, various degrees of fusibility 

 may be attained. A plate of this alloy connected with the boiler of 

 a steam engine, and having such a degree of fusibility as to melt at a 

 temperature so low as to let off the steam before it could acquire a 

 dangerous degree of heat, promises to afford the most effectual se- 

 curity hitherto devised against the dangers which arise from over- 

 heating the boiler, or any part of it. Upon this subject M. Arago 

 has the following remarks. " As soon as it was found that the com- 

 mon safety valves sometimes got out of order, and did not present a 

 certainty of security, it was proposed to replace them by an entirely 

 different contrivance, the action of which should never be uncertain. 

 This was the fusible metal valve. To understand rightly the nature 

 of these valves, we should know that it is possible that steam should 

 have a very high temperature, and but little elasticity, but not possi- 

 ble that a great degree of elasticity should not be accompanied by a 

 high temperature. Experiments have determined the lowest tempe- 

 ratures necessary for steam to acquire a tension of one, two, three, 

 ten, &:c. atmospheres. By using these results, we can know what 

 temperature the steam must not surpass, after we have fixed upon 

 the pressure. If then we cover an opening in the boiler with a 

 plate made of an alloy of lead, tin, and bismuth, in proportions such 

 that the alloy will melt at the limit of temperature fixed upon before 

 hand, this temperature can never be exceeded, for on reaching it, 

 the plate melts and gives vent to the steam. In France, a royal or- 

 dinance requires that every boiler shall be provided with two fusible 

 plates of unequal sizes. The fusing point of the smaller is 10°, 

 (18° Fah.) above the temperature of steam having an elasticity equal 



