STEAM NAVIGATION. 



imparts a considerable portion of its heat to the cold feed which 

 comes in ; and it is found that by this expedient the brine discharged 

 into the sea may be reduced to a temperature of about 100. 



This expedient is so effectual, that when the apparatus is pro- 

 perly constructed, and kept in a state of efficiency, it may be 

 regarded as nearly a perfect preventive against the incrustation, 

 and the deposition of salt in the boiler^, and is not attended with 

 any considerable waste of fuel. 



40. It is maintained by some practical men, that the economy 

 of heat effected by brine-pumps, such as have been just described, 

 is more than counterbalanced by the risk which attends them, if 

 not accompanied by proper precautions. The pipes through which 

 the salted water is discharged are, it is said, apt to get choked, in 

 which case the pumps will necessarily cease to act, though they 

 appear to the engineer to do so ; and thus the water in the boiler 

 may become salted to any extent without the knowledge of the 

 engineer. "When the process of blowing out is executed in the 

 ordinary way, without brine-pumps, the engineer looks at his. 

 water-gauge and keeps the blow-off cock open, until the water 

 level has fallen to the required point. Under these circumstances 

 there is a certainty of having discharged from the boiler a certain 

 quantity of salted water, a certainty which does not exist in the 

 case of a continuous discharge of water by brine-pumps. 



Such expedients, therefore, it is contended, should always be 

 accompanied by some indicator, which shall show the degree of 

 saltness of the water in the boiler, such as we shall presently 

 explain. 



41. In practice, if a marine boiler be regularly attended to, 

 and the salted water be discharged either by the common 

 method of bio wing-off cocks or by brine-pumps, or any other expe- 

 dient which shall impose the necessary limit on the degree of con- 

 centration of water in the boiler, the evil arising from incrustation. 

 will be quite inconsiderable. 



A scale will in all cases be formed on the inner surface of the 

 boilers, which must be removed from time to time when the vessel 

 is in port. The best method of effecting this is by lighting some 

 shavings, or other light and flaming combustible, in the furnaces 

 when the boilers are empty and the safety-valves open. The 

 expansion of the metal by the heat thus produced being greater 

 than that of the matter composing the scale, the latter will be 

 detached and will fall in pieces to the bottom of the boiler, from 

 which it can be withdrawn with the water at the man-holes. 



In some cases, however, it will be preferable to detach the scale 

 by the hammer or chisel. 



42. It is a great error to suppose that incrustation is either the 

 142 



