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A

A

e, which is acted upon by the water whose surface is intended to be preserved at the same constant level, and the supply of which is derived from the conduit a; if then the surface at e rise, the sluice is depressed, and the discharge by the conduit lessened, and vice versa. This arrangement is evidently only suited to an open. conduit, in which no great pressure can be brought upon the sluice; if applied to the mouth of a closed pipe with a great head of water pressing on it, the friction in the grooves of the sluice-frame would be so great as to require an enormous float e, and the action could not fail to be of an irregular character.

The double-beat valve, invented by Hornblower (Pole on the Cornish Engine, pp. 85-88), is represented fully opened in transverse section, at D, C, Fig.

43, and shut in Fig. 44. It is intended that the water or steam should pass from A to B when the valve is opened, and that the communication between them be intercepted when it is shut. The dark lines at

B

B

A

D, D represent the movable parts of the valve; those at C, C indicate the parts that are fixed. The value of its peculiar construction may be best appreciated by

considering the tests of a good valve, which should, in the first place, evidently afford a large passage to the steam or water, with a small displacement; and, secondly, should be capable of being opened with a small force. These conditions are fulfilled in the double-beat valve, which consists of a fixed part or seat C, formed by five partitions, which radiate from a central axis, and are joined below to a ring, a, Fig. 43, and closed on top by a circular disc, in one piece with the partitions, and covering the spaces between them, and also by a movable part, D, the valve proper, which is a sort of case surrounding the seat C, and having a vertical motion, sliding up and down the exterior edges of the partitions in C; this case is open on the top, and connected with its actuating rod n, by the arms r, r.

A

C

C

B

B

Fig. 44.

When it is at the lowest point of its stroke, and shut, it bears upon the bevilled or conical surfaces a and a', which have but a very small breadth; when, on the contrary it is raised, as in Fig. 43, it permits the passage of the water through the different openings shown by the bent arrows. It is evident that by this arrangement it is not necessary to raise the valve through any great height in order to afford a large passage to the water, thus satisfying the first test mentioned above; on the other hand, the valve D, being pierced on its upper part by a circular opening nearly as great as that on the lower part, the force required to raise it is the excess of the pressure of the water or steam per square inch in A over that in B, multiplied into the difference of the

circular areas above mentioned, this difference being evidently the annulus formed by the sum of the horizontal projections of the upper and lower conical surfaces, a and a', shown at E in Fig. 43, projected down from the transverse section.

If this valve or case D should have been a simple disc with bevilled edges, as in the lower part of Fig. 43, we should have required to lift or start it a force equal to the excess of pressure in A over that in B, multiplied into the whole circular area of the top of the disc v, v; and this would not only have to be provided by the prime mover, but a very greatly increased size and strength given to the rods, joints, &c., which actuate the valve. In a large disc-valve, as, suppose, 12 inches diameter, the area being 113.1 sq. inches, and with an excess of pressure in A above that in B of 15 lbs. per square inch, it would require a force of 1696.5 lbs. to lift it. If in an equal double-beat valve each annulus was inch broad in the horizontal projection, the sum of their areas would be (12292) × 0.7854 = 49.48 sq. inches; thus the force required for the starting of such a double-beat valve is less than half that necessary for an equal disc-valve, being 49.48 x 15 742 lbs., or 954 lbs. in favour of the double-beat valve, and so in proportion for pressures other than 15 lbs.

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In the particular case of the valves raised by the float n, Fig. 41, it may be, moreover, remarked that the force necessary to raise them has to be applied but for a very short time, the instant it is raised, the pressure on each side is brought to a state nearly that of equilibrium; the less, then, the resistance to the float at the moment of raising the valve is, the more sensitive it becomes to any alteration in the surface of the water in 7,, with an absence of any irregular or jerking motion. The flap-valve a is consequently retained more steadily at its proper adjust

ment.

The woodcut, Fig. 45, illustrates a somewhat simpler arrangement to effect the same object, as in Fig. 41, which has been adopted by Mr. Gale at the Kilmarnock Water Works. It represents a vertical section through the centre line of the valve house, showing half the roof and the end walls, the entrance door being at P, and, at the opposite end, the pipe A from the reservoir enters; N and

N being the section of the foot of the external slope of the embankment impounding the water, and B the culvert conveying it to the filter beds; the plane of the section is taken transversely to the embankment and perpendicular to its length. The lever L, its support, and the flap-valve are of the same construction as those described above. The weight H is sufficient by its leverage to close the flap-valve and prevent any discharge taking place: all the supply, therefore, must be given by a reduction of the pressure so produced.

A chain attached to the outer end of the lever L

passes over the pulley E suspended at F from a transverse beam shown in section. The other end of the chain is fixed to a float D, working in a cast-iron circular well C; on the cover of it is bolted a tube K, which rises above the highest water level in the valve house. The bottom of the well is in communication with the distributing reservoir by the horizontal pipe G, so that the water stands at the same level in both: if, therefore, the consumption in the town were such as to cause that surface to descend, the float D, becoming less supported, pulls upon the chain, and, lifting the end of the lever L, increases the discharge through A: this increased volume passing down the culvert B to the filter beds soon arrives at the distributing reservoir, and tends to restore its level; on the other hand, if the surface were to rise, the float D, becoming more immersed, loses a portion of its weight, and, therefore, the valve at the end of the pipe A is proportionably closed, and the discharge lessened in correspondence with the lessened consumption. The depth of water in the reservoirs at these works was about 18 feet; at the Gorbals reservoir, about 50 feet, it would not have been possible with this greater pressure to have adopted the simpler arrangement just described; the dimensions of the float D and and the weight H would have been inconveniently increased.

The moderateur lamp affords a most ingenious example, though on a very small scale, of a constant flow of the oil, though the "head," or pressure, varies widely. The annular wick, or Argand burner, is placed on the upper part of the lamp, and is fed with oil from a cylinder which is placed at the lower part of it, and closed at the bottom. The oil is raised from this by the descent of a piston, forced down by the uncoiling of a spiral spring, which is compressed in winding up the piston from the bottom of its course after the former time of use;