separate combustion-chamber, or where the air is heated through an intervening metallic diaphragm. The drawing up of the grit and ashes is completely prevented in the present motor, this latter feature forming an important part of the invention. As will be seen by the drawings, the engine is constructed on the beam principle, and the combustion-chamber is really a prolongation of the working cylinder. The piston (or plunger) is of considerable length, the upper part only being made to fit the cylinder. The lower part of the piston is of slightly less diameter, consequently an annular space is formed between it and the cylinder. This space is connected with the main air-supply, which is controlled by a valve operated by

a connecting-rod and cam-lever worked from a cam on the crank-shaft of the engine. The air-pump is placed in the centre of the machine, immediately beneath the beam-standard, and is operated by a rod attached to the rocking-beam, and this is connected by a rod to the crank-shaft. Owing to the position of the beam, pump, and connecting-rods, the piston of the air-pump is at the outer end of its stroke when the working piston, on its return stroke, has reached a middle position. During the last half of the return stroke of the working piston the air-piston is pushed inwards, and compresses the charge of air previously drawn in until it has reached the middle of the stroke, at which moment the working piston is at the

end of its stroke. The air-valve, operated by the cam as already mentioned, has communicating passages with the air-pump, the furnace or combustion-chamber, and the annular air or packing space in the main cylinder. Consequently, the compressed air is forced partly through the fire and combustion-chamber, and partly into the annular air-space, the flow of air continuing during the time the air-piston performs the second half of the stroke. Meantime, the main piston receives its charge from the combustion-chamber, and cold compressed air passes into the annular space, and practically acts as a packing, effectually preventing grit and dust rising from the fire to the working faces of the cylinder. When the air-pump has finished its stroke, the air-valve is closed, and the air in the working cylinder is allowed to expand for the remainder of the stroke.

The cylinder is kept cool by means of a circulating-water jacket.

The bottom of the combustion-chamber is hinged, and the fuel is coke. As the combustion takes place under pressure, an air-valve, working automatically, is employed for feeding the fire.

The consumption of coke is about 3 lbs. (one kilogramme and a half) per brake horse power and per hour.

3. On the Internal and External Work of Evaporation.

By W. WORBY BEAUMONT, M. Inst.C.E.

Several of the most interesting problems in connection with the steam-engine turn upon the view that is taken of the mode of employment of the heat equivalent of the external work of evaporation.

When steam is generated under constant pressure external work is performed = PV, P being the pressure and V the volume generated. It may therefore, in accordance with the thermodynamic conceptions, be assumed that more heat is

used in the generation of steam under constant pressure than under constant

PV volume, the extra quantity of heat being U = J being Joule's equivalent, and J

U the heat units.

The author suggests the following explanation of the way in which the heat equivalent of the external work of evaporation is used. If heat flows out of steam when mechanical work is done by it during its formation, it must be supposed that steam is cooled by the outflow. If heat flows out and if cooling follows, the corresponding condensation or liquefaction takes place, and a further supply of heat is demanded to re-evaporate steam so liquefied; or, what is the same thing, the further supply of heat is used in continuously preventing the liquefaction from reaching more than the incipient stage. The action here sketched is readily conceived if for the purpose of explanation the evaporation and the external work be supposed to take place per saltum. Suppose a piston, immediately over the water in a simple evaporating vessel, to have been moved by the steam through a small distance A. Then heat corresponding to the work done in moving the piston through A will have flowed out of the steam, and this quantity of heat Qbeing Q gone, condensationmust have taken place in order that the temperature T and pressure P of the remaining steam may be unaffected (L being latent heat of evaporation). Now before the piston can be again moved through a further similar distance A1, that quantity Q must be restored by a further demand on the source of heat, and if Q' be the quantity of heat required to produce the volume of steam V, then the total quantity of heat Q required to move the piston through distance A1 will be Q2=Q1+Q, in order that volume V1 may be produced and the condensed steam Q be re-evaporated.

L

Now if A be taken as less than any assignable distance or the process assumed continuous, then evaporation and incipient liquefaction may be supposed to be contemporaneous, and Q and Q' will be supplied contemporaneously.

In this way it appears to the author that an explanation can be found of the mode of conversion of heat into the external work of evaporation under constant pressure, or of conversion of heat into the work performed by a steam-engine during the admission part of the stroke, or, more correctly speaking, the work done by the steam on the piston during admission. If this be a true statement of the actual mode of employment of the heat converted into the mechanical work of a steam-engine during the admission part of the stroke, then it follows that liquefaction takes place during admission, which must be sufficient to represent the mechanical work done. This being so, the question arises, To what extent will this liquefaction result in water or suspended moisture in the cylinder of a steamengine? The outflow of heat and corresponding liquefaction may be supposed to take place at the moving wall or piston, and in the hypothetic case supposed the liquefied steam may be assumed to be re-evaporated by the steam or water immediately below, which in its turn demands and receives more heat for its resuscitation from the source. In the case of the steam-engine cylinder, however, it is open to question whether the killed molecules in the cylinder or next the piston are resuscitated by the incoming steam, which follows up the movement of the piston. If they are not, then liquefaction will take place in the steam-engine cylinder during admission as a result of the performance of work, although the work is the external work of the evaporation which is performed in the boiler. The heat required for evaporation is that of Regnault's tables, but under the assumption here explained (when the liquefaction takes place in the cylinder and the resulting water does not return without loss of heat to the boiler), the heat required to raise the temperature of the quantity of feed water to the temperature of evaporation must be

U
L

U

added, because in order that one pound of steam may be supplied to the cylinder as steam at cut off, the extra quantity of feed water must be supplied to the boiler. The quantity of water actually evaporated in the production of one pound

U
L

when the evaporation

of steam in the steam-engine cylinder will thus be 1 lb. + takes place under constant pressure, although it is only 1 when evaporation takes place under constant volume. The heat required for evaporation under the author's U assumption for the one pound of steam in the steam-engine will be L + - (T-to), L

T being temperature of evaporation and to the temperature of the feed water. (In the elementary case T = to.) This, it must be noted, is the heat that will be required for each pound of steam accounted for by the indicator.

4. On a new System of Screw Propulsion with non-reversible Engines.1 By W. WORBY BEAUMONT, M. Inst.C.E.

At the present time all screw propellers are driven by engines, which must be so designed that they may be fitted with all the paraphernalia necessary for reversing.

A considerable part of this reversing gear must be at work during the whole of the time the engines are running. Thus, although it may not be necessary to reverse the propeller or the direction of motion of a ship during a long run, the quickly moving parts of this gear must nevertheless be kept at work all the time. In order to avoid the practical objections to this, and the stresses which are brought to bear on the propeller and screw shaft by reversing the direction of their rotation, it is now proposed to effect the reversal of the direction of motion of the ship by means of the propeller, and the object of this paper is to bring before the Mechanical Science Section of the Association a description of the apparatus designed for this purpose by Mr. Robert McGlasson.

For several years the feathering screw propeller has been in use on a considerable number of vessels. By means of this, known as Bevis' propeller, the angle of the blades may be shifted by gear in the screw shaft tunnel, so that they may be placed fore and aft, and thus offer no impediment to the motion of the ship when it is desired to employ sails instead of engines.

By means of the same propeller the angle of the blades may be set so as to alter the pitch to that which may be found best for the ship, or to suit it for very low power when only slow steaming is wanted. As employed for these purposes this form of propeller has been long enough in use to show its practical sufficiency.

By an extension of the application of the principle of this propeller, it is now seen to be possible to achieve several ends which are considered to be of great importance. Some of these may be enumerated as follows:

1. The propulsion of ships by means of screws, which rotate always in the same direction, and may be actuated by non-reversible engines and screw-shafts.

2. The simplification of marine engines, by dispensing with all the parts at present used for making the engines reversible.

3. The complete and quick reversal of the direction of propulsion of the ship, without any of that heavy stress which often amounts to strain and rupture of the screw-shaft, or couplings, or crank-shaft.

4. The facile adjustment of the pitch of the screw blades while the engines are running, so that the pitch may at all times be set to suit the form, trim, and condition of the ship, the requirements of navigation, or any sudden emergency requiring prompt action.

The extension of the application of the principle of the feathering screw consists in the employment of apparatus by means of which the pitch or angle of the blades is always under control, and may be changed from moment to moment with the same facility as is the rudder by means of steam or hydraulic steering gear.

Either form of the apparatus thus employed operates by moving in one or other direction a sliding collar on the tail shaft. This collar is connected to the rod of levers which gives angular motion to the screw blades.

1

The discussion on this paper was given in Engineering, September 4, 1891, p. 269; and the paper with illustrations was published in Industries in September 1891; and in the Marine Engineer, October 1891.

Generally a hydraulic cylinder and piston will be employed for moving this collar, and the valve for admitting the water to either side of the piston will generally be operated in the engine-room, but may be operated from the bridge.

5. Action of Screw Propellers. By Major R. DE VILLAMIL, R.E. Resultant action of a screw propeller is similar to a piston with an infinite stroke and velocity v. The speed of screw is revolutions multiplied by effective pitch. Effective pitch diameter pitch ratio. Minimum circumferential velocity, which gives a thrust√2gd. Most advantageous circumferential velocity

=

=

√2gd ✓m

m

where m is pitch ratio. Centre portion of a screw is inert. Inert area = Centre of screw acts as a drag or resistance-hence the 'Thrust deduction factor.' No screw will convert more than 70 per cent. of the power into longitudinal thrust. Thrust of screw depends on revolutions x effective pitch. Methods of improving propellers:

1. Adopting a form which feeds itself from the centre.

2. Forcing water to the centre by 'feeding blades' on leading side of propeller.

Desgoffe propeller satisfies the first requirement, and shows economy of 25 to 30 per cent. in fuel. Feeding blades will reduce or quite eliminate the 'Thrust deduction factor.'

The differences between theory now proposed and generally accepted theory were considered.

6. On the Comparative Values of Various Substances used as Non-conducting Coverings for Steam Boilers and Pipes. By W. HEPWORTH COLLINS, F.C.S., F.G.S., F.R.M.S.

The author has recently accurately determined the respective non-conducting values of several of the well-known substances and mixtures used as non-conducting material for covering steam boilers and pipes. These results are of much importance, more particularly as there does not appear to be any accessible record of an investigation in this country of a recent or reliable character.