side the pipes, the water rises in the delivery pipe.

As the lifting force is that due to the difference of the head of water outside the pipe, and that due to the broken water within, the depth of immersion must be proportioned to the height to which the water has to be lifted. The depth to which the immersion must be made is, however, found to vary rather widely. Mr W. H. Maxwell gained the most economical results at Tunbridge Wells when the. ratio was 3 of immersion to 1 of lift at the start, and 2.2 to 1 at the finish, the difference being due to the sinking of the water from rest level to pumping level. Tests of some Continental air lifts go down lower.

where X cubic feet of free air per minute. A = gallons of water required per minute. B= lift of water in feet.

It is not economical to instal air-lifting systems in all situations, or under all conditions. It is essential that the difference between the level of the water at rest and the level as reduced by pumping should not be excessive. The ratio of the immersion of the air pipe to the lift must be adjusted to these, and not vary much.

The great advantage which this system has, is that it does away with movable pumps, foot

Air Meter, or Anemometer.-An instrument for measuring the velocity of the air in blast mains, and pipes, and mines. It comprises a small fan, the rotation of which imparts movement to counters and dials on which the rate of velocity is indicated.

Air Motor.-See Air Engine.
Air Port.-See Gas Furnaces.

Air Pump. If it were not that water contains a considerable volume of air and other gases in solution, and that air leaks into a vacuous space through bad places in castings and imperfectly packed glands, it would be possible to work a condensing engine with a closed condenser supplied with a jet of cold water and with a discharge pipe carried down 34 feet to a pond below the water surface of which the pipe would extend to seal out the entrance of air. But it is not practically possible to achieve the exclusion of air, and it was for this reason that when Watt invented the separate condenser he had to provide an air pump to remove such inleaking air. From that time a steam-engine condensing plant has included an air pump of some form, the duty of which is to remove the air which would ultimately collect in the condenser and entirely neutralise its effect.

In the absence of air, the pressure in a condenser is that of water vapour at the temperature at which the condenser stands at the moment. Thus if the condenser temperature is 100 degrees Fahr. the pressure of water vapour at this temperature is 0.944 of 1 lb. or 28 inches below atmospheric pressure. The normal atmospheric pressure being 14.700 lb., the "vacuum corresponding with a condenser temperature of 100 degrees Fahr. will be 14-700 -0.94413·756 lb. or 28 inches.

If, however, for each 147 cubic feet of condenser space there be admitted 1 cubic foot of air, the pressure of this air at that degree of 1 of 14.7 lb. = 1 lb. per expansion will be square inch.

14.7

also a law that if water at any given temperature occupy a given vessel, the space not occupied by the water must be occupied by its vapour, and each cubic foot of the space will contain a fixed weight of water vapour proper to the temperature of the water. This is necessary by the law of molecular equilibrium (Dalton), and it holds good irrespective of what other gas be present with the water vapour. Obviously, therefore, if water vapour separately exerts a pressure of 0.944 lb. at a given temperature, the presence of air, by itself capable of creating a pressure of 1 lb., will combine to produce a joint pressure of 1·0 + 0·944 = 1.944 lb. Thus the vacuum gauge on a condenser always reads less " vacuum or higher pressure than that proper to the temperature, and the cause of the discrepancy is air. When this air is withdrawn by a pump, the pump takes into itself the mixture of air and water vapour. As the bucket moves and contracts the occupied space, some of the water vapour condenses, and the rise of pressure in the enclosed space is simply that due to the air as it is compressed; so far as regards the water vapour, its pressure does not vary, but remains at the pressure proper to the temperature.

In air pumps with solid buckets the bucket descends away from the discharge valve and creates above itself a vacuum free from air, and of a pressure which corresponds with that due to the water sealing on the bucket. When communication is then made with the condenser the mixed vapour in this at a higher pressure rushes into the air pump, and in this way the air pump at each stroke gets hold of a certain volume of mixed vapour, part of which is air, and is enabled to discharge the air on the next up stroke into the atmosphere. In air pumps of the type which have a valve in the bucket, the descending bucket also would leave an empty space above itself, but that its valves open, and the space above the bucket fills with mixed vapour from the condenser at the condenser pressure. In this case, therefore, the bucket at each stroke draws out the maximum amount of

air that can be drawn out, and this type of pump is still preferred by some engineers, while others prefer those with solid buckets generally known as the Edwards' type.

A third form of pump is that known as the ejector condenser. In this variety steam is condensed in a moving jet of water, and the velocity of the steam combines with that of the water to produce a velocity of the combined jet Vxm+ox M which is where V and v are the m+ M velocities of the steam and of the water, and m and M their mass respectively. The action of an injector condenser is exactly that of the feed injector, and a vacuum as high as 12 lb. can regularly be obtained by their means. Another air pump is known as the dry air pump, and it is attached to the head of the barometric condenser first named, and serves to withdraw the air which alone prevents a 34 feet drop pipe from giving the highest vacuum that it is possible to attain from that device. If the velocity of the exhaust steam be taken at 888 feet per second, and the water approaches at a velocity of 30 feet per second, and is of 29 times the weight of the steam, then by the above formula we have the velocity of the combined jet as follows:888 × 1 + (30 × 29) __ 1758 1+29 30 A velocity V of 60 feet per second is that due to a head of h where V = 8 √h, whence √√h=7}, and h=56 feet or 24 lb. Thus if of perfect efficiency an ejector condenser should give the highest possible vacuum consistent with the temperature, and actually vacuo of over 13 lb. have been obtained with it.

=

= nearly 60 ft. per sec.

This ex

ample serves to show the great importance of some velocity of approach of the water jet. In the present case it nearly doubles the value of the numerator. Had the mean jet velocity been 32 feet per second only, representing a head of 16 feet of water, the vacuum could not have reached even 7 lb., though with cold water and a ratio of water to steam of 21 only, the velocity of the final jet would have been 40, and the equivalent vacuum nearly 11 lb., but even this shows the advantage of initial water jet velocity.

Though vertical pumps are more usually employed, those of horizontal type are sufficiently common to be well known. In the manufacturing districts of Lancashire and Yorkshire horizontal air pumps have been often employed in order that they might be worked directly off the rear extension of the engine piston rod, and thereby avoid the complication of links, crosshead, and angle lever necessary where a vertical pump must be driven by a horizontal engine.

Some forms of horizontal air pumps have a horizontally moving bucket or displacement plunger, and these move in solid water, the surface of which, in end chambers of some height extended beyond and above the level of the pump barrel, rises and falls vertically in obedience to the movement of the bucket, and becomes itself the real acting surface of the pump, pushing above it any air through the delivery valve, and itself passing through the valve to the extent necessitated by the influx of condensed steam, or in case of jet condensers of the water It is of condensation from the condenser. obvious that in this form of pump a difficulty may arise in consequence of the high speed of the bucket. The water should be able to follow the bucket at the maximum velocity of this without cavitation. There is no air pressure above the water surface to push down the water and compel this to follow the bucket. Pressure of the water itself can alone be relied upon, and such pumps, when properly constructed, must have end chambers sufficiently large to reduce the rise and fall of the water surface to reasonable velocities and extent by giving the end chambers a cross-sectional area sufficiently greater than the barrel area to effect such reduction. Then the height of the water level above the pump centre must be such that the pressure due to the height shall be sufficient to give the necessary velocity of flow to the water to enable it to follow up the bucket closely. Thus a bucket moves 10 feet per second average velocity, or say 16 feet as maximum velocity at or near mid stroke. In getting up this velocity in a course of say 2 feet, it has occupied at, say 90 revolutions per minute, th second.

In th of a second, gravity gives a velocity

=

acceleration of 5 feet. Obviously the effect of gravity must be increased by pressure from above. The velocity of outflow of water under pressure of a given head is V = √2gh where V = velocity in feet per second, g = 32.2= gravity, hhead in feet. Now we have V 16 feet as one item given. Hence 16=8/h, and h will be therefore 2, and h=4 feet, and this would require to be the height of the water surface in the end chambers in order to force the water to follow close to the bucket, and so enable the pump to run without shock or water impact.

=3

== foot. In this case the bucket would fall freely 3 inches by gravity where it is driven down 4 inches, and any water upon it would lag behind the bucket 1 inch. In such pumps it is therefore the case that above a certain speed the uprising bucket simply meets a quantity of broken water which is spread evenly on the bucket. Some is discharged with the air, the rest is left behind by the more rapidly descending bucket, and churned up by the next incoming spray dashed in by the descending bucket round the curved passages at the base, as seen in Fig. 31, which represents an Edwards' type air pump

No such calculations are necessary with vertical pumps which simply deliver all the surplus which comes into them, but it is instructive to calculate from the velocity of high-speed air pumps of the solid type just what happens to the sealing water imprisoned within them. It will be found that the bucket velocity is often greater than gravity. Thus let a pump at 240 revolutions per minute be taken with a stroke of 4 inches. The mean duration of one pump stroke is second. In this time by the law of falling bodies, which gives the distance fallen through=8=1612, we have t=1 · · 22 = √

and 8

made by Messrs W. H. Allen, Son, & Co., Ltd., of Bedford. Obviously speeds and the influence and effects of gravity must be carefully considered in air-pump design, for it will not be safe for a high-speed bucket to meet solid water, though no harm can come from collision with a shattered mass of air-cushioned spray.

As with steam engines and air compressors, so also air pumps may be worked in two stages, if thought necessary. Two examples only need be cited, the one that of the Parsons steam jet vacuum augmenter, which induces a flow of air from a condenser at say 28 inches " vacuum,"