STEAM NAVIGATION. 



process of blowing out should be regulated by tlie indications of 

 that instrument. To blow out more frequently than is necessary- 

 is attended with a waste of fuel ; for hot water is thus discharged 

 into the sea while cold water is introduced in its place, and con- 

 sequently all the heat necessary to produce the difference of the 

 temperatures of the water blown out, and the feed introduced, is 

 lost. If, on the other hand, the process of blowing out be observed 

 less frequently than is necessary, then more or less incrustation 

 and deposit may beproduced, and the injurious effects already 

 described ensue. 



36. As the specific gravity of water holding salt in solution is 

 increased with every increase of the strength of the solution, any 

 form of hydrometer capable of exhibiting a visible indication of 

 -the specific gravity of the water contained in the boiler, would 

 rserve the purpose of an indicator, to show when the process of 

 blowing out is necessary, and when it has been carried to a 

 sufficient extent. The application of such instruments, however, 

 would be attended with some practical difficulties in the case of 

 sea boilers. 



37. The temperature at which a solution of salt boils under a 

 given pressure varies considerably with the strength of the 

 solution ; the more concentrated the solution is, the higher will 

 be its boiling temperature under the same pressure. A comparison, 

 therefore, of a steam-gauge attached to the boiler, and a ther- 

 mometer immersed in it, showing the pressure and the temperature, 

 would always indicate the saltness of the water ; and it would 

 not be difficult so to graduate these instruments as to make them 

 at once show the degree of saltness. 



If the application of the thermometer be considered to be 

 .attended with practical difficulty, the difference of pressures under 

 which the salt water of the boiler and fresh water of the same 

 temperature boil, might be taken as an indication of the saltness 

 of the water in the boiler, and it would not be difficult to construct 

 upon this principle a self-registering instrument, which would 

 not only indicate but record from hour to hour the degree of 

 saltness of the water. A small vessel of distilled water being 

 immersed in the water of the boiler would always have the tem- 

 perature of that water, and the steam produced from it com- 

 municating with a steam-gauge, the pressure of such steam would 

 be indicated by that gauge, while the pressure of the steam in 

 the boiler under which pressure the salted water boils might be 

 indicated by another gauge. The difference of the pressures 

 indicated by the two gauges would thus become a test, by which 

 the saltness of the water in the boiler would be measured. The 

 two pressures might be made to act on opposite ends of the same 

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