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APPENDIX 
I.—0 N MARINE BOILERS. 
BY MR. J. DINNEN, 
ASSISTANT ENGINEER, HER MAJESTY’S DOCKYARD, WOOLWICH. 
1. The paucity of information connected with steam navigation,—a subject, it must be 
allowed, of great national importance, and which has assumed a prominent feature in her 
Majesty’s naval service,—suggested the investigations from which the following practical 
essay on marine boilers has resulted. The incrusted state of the boilers of all steam vessels 
returning from the Mediterranean with mails, in the early stage of their employment, was 
calculated to impress the observer with the impracticability of making such voyages without 
submitting to like effects; no experiment having been made, or correct data furnished, at that 
period, to counteract the notion, so universally entertained, that the water of the Mediterra 
nean holds more marine deposit in solution than that of the Channel. It is probable that 
Dr. Halley’s experiments on the evaporation of water from the surface of the sea assisted 
materially in the formation of this general opinion. 
It is calculated that the Mediterranean evaporates no less than 5,280,000,000 tons daily: 
and this quantity far exceeds that evaporated from any other surface of equal extent, 
being entirely encircled by land ; but it does not follow from this that the water is salter. 
Whoever has been in the Mediterranean is aware that the dews there are exceedingly heavy, 
and that they begin to descend some time before the sun has set: in still evenings indeed 
the dew descends where it has arisen ; and no doubt much is precipitated on the land ; but it 
must be borne in mind, that land gives forth exhalations at all temperatures, as well as the 
sea. Again, the visible current always runs into the Mediterranean, in addition to the great 
rivers Nile, Danube, Rhone, &c., to supply that portion of water which the land may carry 
off. Experiments do not show the specific gravity to exceed that of other waters ; and hence 
the general opinion alluded to must have been conjectural, and not founded on the result 
of investigation. 
2. It cannot be too generally known that the thermometer is a good practical test of the 
concentration of a boiling solution of salt; of which the following is offered in illustration.