A Practical Manual On Sea Water Distillation
With A Description Of The Necessary Machinery For The Process

36 SEA WATER DISTILLATION. raise the temperature of 1 lb. of pure water at 39'1° one degree (i.e., to 40'1°). This is about the temperature at which water is at its greatest density—i.e., when the 1 lb. of water has the smallest volume. But, although 39'1° is the specified temperature, practically any other tem- perature holds good for ordinary calculations ; sometimes the B.T.U. is based on a temperature of 60° as being ordinary summer temperature, instead of 39°, as above referred to. Note.—The thermal unit in France is called a “ Cal- orie/' and is further described as a grande calorie ’ or a “ petite calorie/’ the former being the heat required to raise the temperature of 1 kilogramme of water from 4 C. to 5° C., whilst the latter (“ petite calorie ”) is the heat required to raise the temperature of 1 gramme of water from 4° to 5° C. From this it will be seen that the British thermal unit (B.T.U.) equals 0’252 (or, more accurately, 0’251996) “ grande calorie/’ or 252 “ petites calories.” Metrical System. 6. In France and on the Continent, where the metrical system is used, the pressures are often indicated by the number of atmospheres or kilogrammes per square metre. The following short Table will, therefore, be useful when pressures are mentioned in foreign terms :— TABLE F (on page opposite). A. = Atmosphere (i.e., 29'9 inches of mercury) at ocean level, per barometer—Allow 0’1" reduction for every 260 feet of ascent. P. = Pounds per square inch. K.M. = Kilos, per square metre. H. = Head of water per square inch in feet high.