820 EEPORT— 1888. 



4. The Efficiency of Steam at High Pressures and the Carnot Theorem.^ 

 By W. WoEBT Beaumont, M.Inst. G.E. 



The object of the author of this paper is to show that the Carnot theorem as 

 expressed by the equation E = — =- is inapplicable for the calculation of the effi- 

 ciency of steam as used in a steam-engine cylinder. It is shown that the work 

 done by expanding steam bears no relation to the fall in temperature due to fall in 

 pressure, and that with steam at pressures between 350 lbs. and 50 lbs. per square 

 inch the work done by expansion varies from 692 foot-lbs. to 1,050 foot-lbs, per 

 degree of fall in temperature. 



The argument leads to the conclusion that pressures higher than those now used 

 can be very advantageously emploj-ed. 



When the exchange of heat due to difference in latent heat, total heat, and 

 temperature at the different pressures is taken into account, the difference in 

 favour of the high-pressure steam becomes much greater. An explanation is 

 afforded of the great efficiency of triple stage expansion engines as compared with 

 the lower pressure simple engines. The steam in the steam-engine must be con- 

 sidered with reference to that part of its history which is comprised between its 

 admission to the cylinder and the instant of exhaust, and with reference to the 

 heat used, not to the fall in temperature. By increasing the pressures used, an 

 extra number of stages of expansion may be obtained at a small extra expenditure 

 of heat in the boiler. The difference A II between the total heat of steam at 300 

 lbs. and at 250 lbs. is under five units, but the work done by expansion from 300 lbs. 

 to 250 lbs. is 12,240 foot-lbs. or about 2,400 foot-lbs. per unit of heat employed in 

 raising the pressure from 250 lbs. to 300 lbs. The work done per unit of heat 

 disappearing on expansion for each stage of 50 lbs. is given in a table. 



5. Revolving Sails, or Air-pro]pellers.'^ By H. C. VoGT. 



If ships could be towed instead of propelled by means of the ordinary pro- 

 pellers, which disturb the stream lines round the ship, a saving on the average of 

 40 per cent, in coals and engine-power would be gained ; the screw especiallv 

 sucks the sustaining water from the ship, causing its resistance to be augmented 

 in the same proportion ; the thrust of the propeller must therefore exceed the net 

 resistance of the ship by this amount. This excess in the resistance can only be 

 reduced by destroying the shape of the ship. Formerly the length of ships was three 

 to four times the breadth, now it is ten times the breadth. Both these proportions 

 cannot be correct, and it must be remembered that the weight of the hull increases 

 with the second power of the length, tlie middle section remaining unchanged. 

 The screw-propeliei' increases, further, the change in the trim of the ship when the 

 speed increases, also causing an increased resistance ; and this ill-effect can only be 

 counteracted by giving the driving thrust considerably above instead of below th** 

 centre of gravity of the ship. 



Air-pressure on large canvas areas has been used from time immemorial : the 

 author only proposes to use comparatively small driving areas, radiating from an axis 

 parallel to the keel, and supported between two low pillars. The external appear- 

 ance of such an air-propeller is similar to tliat of a screw-propeller ; but the wings 

 of the air-propeller are broadest near the circumference, and they are made of thin 

 sheet-steel. Rotated quickly in the elastic air, a rarefaction on the front side 

 of the wings is created, and the consequent difference in pressure constitutes the 

 thrust, which, with a given engine-power, is quite as high as vnth the common screw 

 in inelastic water. To create such a rarefaction, speed rather than area is required. 

 Take, for instance, the wings of a wild swan, which possess 25-28 times the area 

 of the web feet, while the thrust yielded from the wings is many times that yielded 

 from the web feet. Suppose the horizontal water-resistance when the swan swims 

 fast to be four pounds ; when it flies the horizontal resistance may perhaps also be 



' Paper is published in extenso in Tndustrii'.t, vol. v. No. 119. 

 - Printed in extenso in Industries, vol. v. p. 335. 



