Density of Brine in Boilers of Marine Steam- Engines. 279 



in 1812, the boiler was found to boil over, or prime, as it is 

 technically called by engineers, when part of the water is forced 

 up so violently, along with the steam, as to pass over into the 

 cylinder of the engine — a circumstance always detrimental, 

 and sometimes destructive to the engines. This arises from 

 the thickening of the water, its density being increased by the 

 retention of the solid substances, which compose sea-water, 

 and which remain and accumulate in the boiler, while the 

 fresh portion of the water is passing off in the shape of steam. 



This process of accumulation of solid matter in the marine 

 bpiler is by no means slow. The whole of the water which a 

 marine-boiler usually contains is evaporated in three or four 

 hours, leaving the solid substances in the cubic content of 

 boiler behind it, and being replaced by salt water, with an equal 

 quantity of depositary matter, accumulating as rapidly as be- 

 fore ; and since it is known the solid matter amounts to as 

 much as ^^ of the whole mass of water, it would follow, if the 

 process of ebullition could continue so long as 150 hours, there 

 would be deposited in the boiler a quantity of solid matter 

 equal to the number of tons of water in the whole content of 

 the boiler. 



Long, however, before this degree of solidification can take 

 place, evils of a different description intervene to impair and 

 put an end to the functions of the boiler. The solid consti- 

 tuents of salt water which are left behind do not diffuse them- 

 selves uniformly over the whole liquid mass, so as to constitute 

 a homogeneous brine ; on the contrary, the new supplies of 

 sea-water, as they enter the boiler, remain secluded from the 

 former more saturated brine, rise by their less specific gravity 

 into an upper stratum, while the denser brine forms a bed in the 

 lower part of the boiler, and surrounds the fire-box and heater- 

 flues occupying the water-spaces and legs, which are usually 

 at a high temperature, and which, in double-tiered boilers, are 

 generally the most intensely heated. The intense heat of the 

 metal expels the water from the brine in contact with it most 

 rapidly in the hottest places, and salt is deposited on the hot- 

 test parts of the furnaces and flues, extending rapidly to those 

 less heated, and so not only diminishing the evaporative power 

 of the boiler, but injuring its substance, and endangering its 

 existence. 



