MARINE ENGINES.

467

lay in using valves of too small dimensions; and that with valvesproportioned as proposed, properly constructed and loaded bysprings, anything approaching a dangerous increase of pressure isentirely avoided.

“ On Loading Safety Valves by Direct Springs. — Ithas been shown that valves having half an inch of area persquare foot of grate surface require to lift 2 x diameter of val ve

order perfectly to relieve the boiler; and if proportioned as isrecommended in this report, then the lift would be in all cases

diameter of valve.

36

Having determined the requisite lift, it remains to fix any reason-able or desired per centage of the load, which is not to be exceededby the additional load due to the compression or extension of thespring, caused by the lift of the valve. Let this, for example, berestricted to 2^ per cent. of the original load.

Then the spring loading the valve should be so proportioned thatthe compression or extension, to produce the initial load, shall be40 times the lift of the valve.

So that with valves having half an inch area per foot of gratesurface, the initial compression or extension of spring would be =

80 x diameter of valve T , r . . . .......

-p-. With valves as recommended, the initial com-

pression or extension would be ri 1 x diameter of valve. Thefollowing formula refers to spiral springs, made of steel in the usualway:—

E= Compression or extension of one coil in inches.d — Diameter from centre to centre of Steel composing spring ininches.

w — Weight applied in pounds.

D = Diameter or side of square of Steel of which the spring ismade in löths of an inch.

C = A constant which, from experiments made, may be takenas 22 for round Steel and 30 for square steel.

P_ d z x w

^ ~~ d*1Tc'

The total compression or extension of such a spring is equal tothat of one coil into the number of effective coils, which maybe taken as two less than the apparent number, the end coils