EFFECTS OF SEA-WATER. 



be imparted to the lower plates which are incrusted, compared 

 with the higher parts which may not be so, an unequal expansion 

 is produced, by which the joints and seams of the boiler, are- 

 loosened and opened, and leaks produced. 



These injurious effects can only be prevented by either of two- 

 methods ; first, by so regulating the feed of the boiler that the 

 water it contains shall not be suffered to reach the point of 

 saturation, but shall be so limited in its degree of saltness that no 

 injurious incrustation or deposit shall be formed; secondly, by 

 the adoption of some method by which the boiler may be worked 

 with fresh water. This end can only be attained by condensing 

 the steam by a jet of fresh water, and working the boiler con- 

 tinually by the same water, since the supply of fresh water 

 sufficient for a boiler worked in the ordinary way, could never be 

 commanded at sea. 



The method by which the saltness of the water in the boiler i& 

 most commonly prevented from exceeding a certain limit, has- 

 been to discharge from the boiler into the sea a certain quantity 

 of over-salted water, and to supply its place by sea water intro- 

 duced into the condenser through the injection-cock, for the 

 purpose of condensing the steam, this water being mixed with the 

 steam so condensed, and being, therefore, a weaker solution of 

 salt than common sea water. To effect this, cocks called blow- 

 off cocks are usually placed in the lower parts of the boiler, where 

 the over-salted, and therefore heavier, parts of the water collect. 

 The pressure of the steam and incumbent weight of the water in 

 the boiler force the lower strata of water out through these cocks ; 

 and this process, called blowing out, is, or ought to be practised at 

 such intervals as will prevent the water from becoming over- 

 salted. When the salted water has been blown out in this, 

 manner, the level of the water in the boiler is restored by a feed 

 of corresponding quantity. 



This process of blowing out, on the due and regular observance 

 of which the preservation and efficiency of the boiler mainly 

 depend, is too often left at the discretion of the engineer, who is, 

 in most cases, not even supplied with the proper means of ascer- 

 taining the extent to which the process should be carried. It is 

 commonly required that the engineer should blow out a certain 

 portion of the water in the boiler every two hours, restoring the 

 level by a feed of equivalent amount ; but it is evident that the 

 sufficiency of the process, founded on such a rule, must mainly 

 depend on the supposition, that the evaporation proceeds always at 

 the same rate, which is far from being the case with marine boilers. 



35. An indicator, by which the saltness of the water in the 

 boiler would always be exhibited, ought to be provided, and the 



137 



