surface and to the pressure of steam. The lower the pressure the larger must the safety-valve be. When steam flows through an orifice with a square edge such as a safety-valve, its flow is considerably reduced, and the weight. in lbs. of steam discharged per minute, per square inch of opening, corresponds nearly with three-fourths of the absolute pressure in the boiler, when that pressure is not less than 25 lbs., or 10 lbs. above the atmosphere. The area of opening requisite for the discharge of any given constant weight of steam, is in inverse ratio of the pressure; that is to say, it requires an orifice of three times larger area, to discharge steam of 30 lbs. pressure, than is required to discharge the same weight of steam per minute at 90 lbs. pressure.

The opening for the escape of steam, through a conical valve with cone of 45°, is about one-third less than the lift.

To find the proper area of a Safety-Valve, multiply the area in square feet of fire-grate surface, by one of the following multipliers, corresponding with the pressure at which the safety-valve is to blow off, and the product will give the area in square inches of that safety-valve; to which must be added the area of the wings of the valve, when the valve is constructed with wings.

Pressure as shown by the steam gauge 10 lbs., constant multiplier 1'4

Direct Load upon the Valve.-When the valve is loaded by a weight or spring, placed direct upon the valve, without the intervention of a lever. To find the necessary weight in lbs. to attach, or the amount of tension to put upon the spring, to prevent the valve blowing off before the blowingoff pressure is reached, multiply the area of the valve in square inches by the pressure of steam in lbs. per square inch, and to the product add the weight of the valve.

To find the pressure in lbs. per square inch, divide the load in lbs. upon the valve, by the area of the valve in square inches.

Safety Valve with Lever.-The centre of gravity of the lever, is the point at which it will balance, when placed upon a knife-edge. In Fig. 46,

F is the fulcrum, or joint where the lever is fixed, V is the centre of the valve, W is the weight.

The best angle for the seat of the valve is 45°; the width of mitre should not exceed inch; the lift of the valve should not exceedinch; the distance between the fulcrum and the centre of the valve, should equal the diameter of the valve; the pivot should bear upon the valve considerably below the level of the valve-seat. When a weight is used the total length of lever should equal one-third the diameter of the boiler; when the lever is held down by a spring-balance, the distance between the fulcrum and the

centre of the valve should equal the diameter of the valve, and the distance between the fulcrum and the spring-balance, should equal as many times the diameter of the valve, as there are square inches in its area.

Safety-Valve Loaded by a Lever and Weight.-When a lever and weight are employed to load a valve, it is necessary to find the resistance due to the weight of the lever and the valve. This may be ascertained by securing the valve to the lever with a piece of wire, and attaching a spring balance directly over the centre of the valve, which will give the load due to the weight of the valve and the action of the lever. This result divided by the area of the valve in square inches, will give the pressure in lbs. per square inch, at which the steam will raise that valve.

To calculate the action of the lever when the above method cannot be employed, Approximate Rule: Multiply the weight in lbs. of the lever, by the distance between the fulcrum and the centre of gravity, and divide the product, by the distance between the fulcrum and the centre of the valve; which will give the approximate resistance in lbs. due to the action of the lever, to which result add the weight of the valve and pivot.

To find the pressure in lbs. per square inch, at which the valve will begin to blow off :—

1. Multiply the weight in lbs. of the ball, by the distance in inches it is placed from the fulcrum.

2. Multiply the weight in lbs. of the lever, by the distance in inches between the centre of gravity and the fulcrum.

3. Multiply the weight in lbs. of the valve, by the distance in inches between the centre of the valve and the fulcrum.

4. Multiply the area of the valve in square inches, by the distance in inches between the centre of the valve and the fulcrum, then add together the first 3 products, and divide the sum by the 4th product.

To find the position of the weight on the lever, so that the safetyvalve will blow off at a given pressure :—

1. Multiply the weight in lbs. of the lever, by the distance in inches between the centre of gravity and the fulcrum.

2. Multiply the weight in lbs. of the valve, by the distance in inches between the centre of the valve and the fulcrum.

3. Multiply the area of the valve in square inches, by the pressure of the steam in lbs. per square inch, and multiply the product by the distance in inches between the centre of the valve and the fulcrum; then add together the first two products, and subtract the sum from the 3rd product, and divide the remainder by the weight of the ball in lbs.

To find the weight to place on the lever, so that the valve will blow off at a given pressure:-Multiply the area of the valve in square inches, by the required pressure of steam in lbs. per square inch, from which result deduct the weight of the valve and action of the lever in lbs. ; then multiply by the distance from the fulcrum to the centre of the valve in inches, and divide the product by the distance in inches, between the fulcrum and the point of the lever at which the weight is placed.

PROPORTIONS OF STEEL SPRINGS.

Spiral Springs.-The proportions of spiral springs for safety valves loaded with direct springs, may be determined by the following rules :The internal diameter of the coil, should equal 4 times the thickness of the steel of which the spring is composed.

The lift of safety valves for all sizes, may be taken at one-tenth part of an inch.

The compression or extension of the spring, to produce the initial load, should be forty times the lift of the valve, or 4 inches for all sizes of valves with the above lift.

To find the diameter of round steel, or side of square of square steel, for springs:-*

The Author is indebted for the above rules for safety-valve springs, and for some of the information on safety-valves to a report on safety-valves in the Transactions of the Institution of Engineers and Shipbuilders of Scotland.

Find the load, by multiplying the area of the safety-valve in square inches, by the pressure of the steam in lbs. per square inch; then multiply the load by the diameter of the coil, from centre to centre of the steel; divide the quotient by the constant number 3 for round steel, or by the constant number 4'29 for square steel, and the square root of the quotient will give the size of steel in sixteenths of an inch, that is, the diameter when round, and the side of the square when square.

To find the compression or extension of one coil in inches:— Cube the diameter in inches of the coil (from centre to centre of the steel), then multiply by the load in lbs., and divide the product by the product of the fourth power of the diameter (or side of square if square) of the steel in sixteenths of an inch, multiplied by the constant number 22 for round steel, and 30 for square steel.

To find the pitch of a spiral spring:-The distance between neighbouring coils should be equal to twice the compression (or extension as the case may be), found by the last rule, and the pitch will be twice the compression added to the diameter of the steel when round, or the side of the square when square.

To find the number of coils :-Divide the initial compression of spring (or 4 inches for all sizes) by the amount of compression, or extension of one coil (found by the above rule), which will give the effective number of coils.

To find the length of spring, multiply the number of coils found by last rule by the pitch of spiral, and add two more coils, to allow for the two end coils serving as bases for the spring.

The above rules are for valves loaded with direct springs, but the same rules apply to springs acting at the end of levers, in which case the lift of the end of the lever where the spring is attached, must be taken instead of the lift of the valve.

Laminated Springs for Locomotive Engines, railway carriages and waggons, and conveyances.-The thickness of steel plate for springs under 3 to 4 feet span, should not exceed inch in the smaller, and from to inch in the larger sizes; for larger spans the thickness is generally inch, with the two top plates inch thick. The deflection per ton of load, is about inch for railway waggons, to 1 inch for locomotive engines,

1 inch for horse boxes, and from 1 to 2 inches for railway carriages. The following are Mr. D. K. Clark's rules for laminated or plate springs.

Let D

the deflection in sixteenths of an inch per ton load.

S = the span of the spring in inches when loaded.

the breadth of the spring plate in inches, considered uniform.

t = the thickness of plates in sixteenths of an inch.

n = the number of plates.

W the working strength of spring in tons, or safe load.

and n necessary to a given elastic flexure, span, and size of plates =

CHIMNEYS FOR FACTORY STEAM BOILERS.

The source of power for the draught of a chimney, is the difference in weight of a vertical column of cool air outside the chimney, compared with that of a vertical column of the heated gases inside the chimney. These two columns of air being of unequal weight, motion ensues. The best draught takes place, when the temperature of the gases inside the chimney is at 552°, which weighs only one-half the weight of the air outside the chimney when at 62°. A quantity of heat is absorbed in producing draught, but only about one-fourth the quantity of the heat is required to raise 1 lb. of air one degree, which is required to raise 1 lb. of water one degree, and the heat carried off by the gases may be found thus Multiply the weight of air per lb. of coal, by the difference in temperature between the gases in the chimney and the external air, and multiply the product by 238. The quantity of air required is 24 lbs. for each lb. of fuel. The usual rate of combustion is 12 lbs. of coal per square foot of grate-area per hour in Cornish and Lancashire boilers.

Proportions of Brick Chimneys-For an ordinary factory chimney, say, one for a good-sized cotton factory, the thickness of brickwork is 9 inches at the top; 14 inches at a distance of one-fourth the height from the top; 18 inches at one-half the height; 23 inches at a distance of threefourths the height from the top; and 28 inches at the base.

To find the area in square feet at the top of a chimney for a given boiler: Rule, multiply the area of the fire-grate surface in square feet by 80, and divide the product by the square root of the height of the chimney in feet.

To find the maximum horse-power of a chimney, when the inside area at the top, and the height, are given, divide the area in square inches by 70, and multiply the result by the square root of the height in feet. This will give the maximum horse-power, but a chimney should always be made about one-third larger than necessary, to allow for contingencies.

Flues. The horse-power of a chimney reduces with the length of flue. The power with longer flues than 50 feet, may be found by