Density of Brine in Boilers of Marine Steam-Engines. 283 



The rule of working them is nearly this : — Continue the 

 operation of blowing off until, if possible, the diiference of the 

 columns is less than an inch, it Avill be unnecessary to blow off 

 again until the difference is at least 6 inches. 



As a practical rule, I find that it is necessary to blow oflF 

 when the brine at the bottom has about three degrees of salt- 

 ness. But this will A'ary exceedingly, according as the con- 

 struction of the boilers is more or less judicious. When the 

 heat is greatest in the lowest portion of the boiler, and the 

 flues return above, they will be most liable to salt, and require 

 the most frequent cleansing. 



The following is Mr Laurie's description of the instrument. 

 The drawings give the details of the apparatus. — J. S. R. 



The fact that the specific gravity of salt-water is greater 

 than that of fresh, and that it increases with the degree of 

 saturation, is what the operation of this instrument depends 

 on ; by its means two columns of water, the one feed and the 

 other brine, are poised against each other, so as that any dif- 

 ference of weight betwixt these columns immediately becomes 

 apparent by the lighter of the two requiring an accession in 

 quantity to resist the upward pressure to which both columns 

 are subjected. This is accomplished by having two common 

 glass gauge-tubes close together, each of which is connected 

 with a separate tube ; that inside the boiler descends to the 

 level of the water, the specific gravity of which is to be mea- 

 sured, and having either or both of these tubes so connected 

 with the feed-pipe of the boiler, that by opening a cock one of 

 the pipes will be filled with feed-water, while the other re- 

 mains filled with brine, which cock being shut, the tubes re- 

 main so filled ; but inasmuch as feed-water is of less specific 

 gravity than brine, it will be forced up and stand in the glass 

 tube at a higher level than the brine, which difference of levels 

 increases with the saturation — and hence the index to judge 

 of the saltness. 



In Plates VI. and VII., A, B, are the two glass gauge-tubes ; 

 C, one of the tubes forming the connection betwixt one of these 

 gla.ss gauge-tubes and its tube D, that descends inside of the 

 boiler ; E, the tube forming the connection betwixt the upper 

 ends of these tubes and the inside of the boiler ; F, G, two cocks 



