boiling point is elevated to 216.7°; hence, to find the degree of temperature at which steam of a given density would be obtained from salt water equal to that from fresh water, the excess of temperature due to the saltness of the water must be added; thus

Suppose the temperature to produce steam at 12 pounds per square inch from fresh water be 245.1° Fah., the temperature to produce the same from water holding 3ds its weight of salt, will be 245.14.7 249.8° Fah.,

4.7 being the difference between 212° and 216.7.

18. How is approximated saturation of the water in the boiler, to an injurious extent, prevented?

By constant extraction of a portion of the water by means of brine pumps, or by brine cocks, from or near the bottom of the boiler; also by frequently blowing out an indicated accumulation of the feed water which has been intentionally forced into the boiler during a given period of time, say, at the greatest extent, the end of every two hours.

19. Observing, relative to the steam-engine at

sea, the high degree of temperature necessary for the production of steam from sea-water, and also the loss of heat that must be occasioned by the frequent changing of the water in the boiler, through which to prevent saturation,-is no advantage attempted whereby to effect a saving of fuel, and compensate in some measure for the extra quantity of heat required?

Yes, by means of a refrigerator, that is, a vessel of any convenient form containing a number of tubes, into which vessel, and surrounding the tubes, the saturated water or brine from the boiler is conducted in its course outward. And through the interior of the tubes the water to the boiler is compelled to pass by the action of the feed pumps; hence, by such means, a portion of the hreat from the brine is abstracted by the feed water previous to its entering the boiler, thus effecting a saving in proportion to the quantity of heat it may by such means imbibe.

20. By such application, how is the theoretical amount of advantage estimated?

Rule:-From 1212° subtract the heat of the brine after leaving the refrigerator; multiply the

remainder by the grade of saturation, and from the temperature of the water in the boiler subtract also the heat of the brine after leaving the refrigerator; add together the product and the remainder obtained by each process: then, as the sum of the two is to the remainder, so is the quantity of fuel required in a given time to the quantity saved, in equal terms of unity.

21. Suppose, in a marine boiler, the water be maintained at three times the saltness of sea-water, and at a temperature of 244° in the boiler, the water from the refrigerator at 100°, and the consumption of coals per day, without the refrigerator, thirty-six tons,—what, with the refrigerator, might be the anticipated amount of saving during the same period of time?

3 × (1212° — 100°) = 3336° and

244 -100 = 144

+

= 3480

Then, as 3480:144:: 36: 1.49 tons of fuel saved.

22. In regard to boilers for the generation of steam, what are the best proportions of capacity, heating surface, and extent of furnace, so as to

supply efficiently with steam any dimension of steam-engine given?

When practicability will admit, the capacity for water and steam ought not to be less than twenty-five cubic feet, extent of heating surface not less than nine square feet, and one square foot of furnace bars to each nominal horse-power.

23. What is meant by the term nominal horsepower of an engine?

By nominal horse-power is to be understood that of an engine having a cylinder of a given diameter and a given length of stroke, with a constant, uniform, effective pressure of seven pounds per square inch on the piston.

The rule for estimation being as follows:Multiply the square of the cylinder's diameter in inches by the cube root of the stroke in feet, and by .0213; the product is the nominal horse-power. Suppose a cylinder of 30 inches diameter with a stroke of six feet.

302900 and

61.817, hence 900 x 1.817

x .0213 34.8 horse-power..

=

24. What is the expected evaporation of water,

or volume of steam generated by a boiler of the preceding proportions to each horse-power?

About thirty-three cubic feet of steam generated per minute, or one cubic foot of water evaporated per hour by fuel of an average quality, or one pound of coal to about 8.5 pounds of water evaporated.

25. How is the pressure or elastic force of steam determined relative to the steam-engine practically?

By means of the safety-valve, that is, properly, a circular disk covering a circular opening in the top or upper part of the boiler, and either loaded directly or indirectly by the action of a lever and weight, or spring balance, to any degree of pressure required.

26. What is meant by the action of the lever? The action of the lever is the accumulated effect of its weight upon the valve, more than would be effected by actual gravity. Thus, if the distance between the fulcrum and the valve be equal to half the lever's length, the same being of parallel dimensions, the action upon the valve equals the weight of the lever; but in proportion as the dis