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 ata 
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 
asteam engine, and having such a degree of fusibility as to melt at a 
— so low as to let off the steam before it could acquire 4 
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 presen! 
i. certainty of security, it was proposed to replace them by an entirely 
>. ifferent 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 sh ‘ 
have a very high temperature, and but little elasticity, but not poss 
sas ble that a great degree of elasticity should not be accompanied by @ 
high temperature. Experiments have determined the lowest temper 
ratures necessary for steam to acquire a tension of one, two, threes 
ten, &c. atmospheres. By using these results, we can know W as 
temperature the.steam must not surpass, after we have fixed upor. 
the pressure. If then we cover an opening in the: boiler with @ 
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 1; 
the plate melts and gives vent to the steam. In France, a roy we 
tee 
dinance requires that every boiler shall be “provided with two ft nor ae 
plates of unequal sizes. The fusing point of the smaller 5 ™ 
_. (18° Fah.) above the temperature of steam having an elasticity 4 
we = : -~ i 
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