BOTARY ENGINES.] 



APPLIED MECHANICS. 



pose that the piston D E measures 10 inches each way, 

 and therefore presents a surface of 100 square inches, 

 and that it is acted on by steam having a pressure of 

 20 Ibs. per square inch ; so that the total pressure on its 

 surface is 100 X 20=2000 Ibs. Now, considering CD 

 as the arm of a lever, of which a portion E D is loaded 

 with pressures distributed uniformly over it, we know 

 that their combined effect to turn the lever round its 

 fulcrum C is The same as if the whole pressure were 

 collected into one force at H, the centre of gravity of 

 the part D E. We have, therefore, effectively a lever or 

 arm of the length C H, pushed round C by a force of 

 2,000 Ibs. applied at H. If we take CE=9 inches, then 

 as E H = 5 inches (| of 10 inches), C H = 14 inches ; and 

 during one revolution, H passes over a distance equivalent 

 to the circumference of a circle having a radius of 14 

 inches that is to say, over 88 inches, or 7J feet. Hence, 

 the work done during a revolution is 2000 Ibs.X 7 3- feet 

 = 14667 Ibs. moved over 1 foot. Also the quantity of 

 steam required to fill the annular space passed through 

 by the piston is thus found, C E being 9 inches, and C D 

 being 19 inches : 



Area of outer circle (38 inches dia.) = 1134-12 sq. ins. 

 Area of inner circle (18 inches dia.) = 254-47 ,, 



Area of annular spaet 

 Multiply by breadth 



(difference) 



Volume of annular space 



879-05 

 10 inches. 



8796-0 cubic ins. 



Were we to apply this quantity of steam in an or- 

 dinary cylinder, whatever be its dimensions, we should 

 produce equivalent power. Let us, for example, take a 

 cylinder 1 foot diameter, having an area of piston 113-1 

 square inches, subjected to a pressure of 113 '1 square 

 >< x 20 Ibs. = 2202 Ibs., moving through a stroke of 

 < feet (a length which requires equivalent volume of 

 team), we find the work to be 22G2lbs. X 6-488 = 

 ; His. moved over 1 foot as before. Instead of 

 niii'x the whole revolution of the rotary piston, we 

 might take any portion of its revolution, and we should 

 find the power derived from the quantity of steam used, 

 exactly equal to that which would be produced by the 

 same quantity of steam, in any corresponding portion 

 cylinder where the piston moves rectilineally. 

 Without reference to any special numerical example, we 

 observe these general laws. Whatever bo the form of 

 the rotating piston DE, steam pressing uniformly on its 

 surface produces the .same effect to turn it round its 

 centre C, as if the whole pressure were collected into one 

 force acting at the centre of gravity H of the surface of 

 piston ; and the work done by the steam during any part 

 of a revolution, is equivalent to the total pressure in the 

 piston, multiplied by the distance traversed by the centre 

 of gravity H, or the portion of the circumference of a 

 circle of which C H is the radius ; and, further, the 

 volume of steam required during the given portion of a 

 revolution is (by the well-known law of mensuration of 

 annular solids) measured by the area of the piston, mul- 

 tiplied by the distance passed over by its centre of 

 gravity. In the case of a piston moving rectilineally, 

 the work done by the steam, and the volume of steam 

 used during any portion of a stroke, are measured in 

 precisely the same way, and bear the same relation to 

 each other. There is, therefore, no theoretical objection 

 to the application of steam in such a way as to produce 

 direct rotary motion. That there are considerable prac- 

 tical difficulties in the arrangement of the parts, and 

 their construction, so as to present steam-tight nibbing 

 surfaces, and to avoid undue friction and unequal wear, 

 is doubtless true ; but were these difficulties fairly sur- 

 mounted, we should be in possession of an engine where 

 simplicity, and economy of weight and bulk, might en- 

 able us to apply steam-power in many cases where it is 

 not now applicable without inconvenience. 



Before leaving the subject of rotary engines, we may 

 mention that steam has been applied successfully to pro- 

 duce rotary motion on the same principle as that of 

 Barker's mill, or the turbine applied to water-power. 



Steam of considerable pressure, passing through open- 

 ings on the sides of several tubular arms mouuted on an 

 axis, causes them to revolve in the direction opposite to 

 that in which the steam issues, on the same principle as 

 the movement of a rocket, where the issue of the elastic 

 gases, generated by the combustion of the charge at one 

 end, leaves an unbalanced pressure to act on the other 

 end, and thus to force it onward through the air ; but 

 we believe this modo of applying steam-pressure, though 

 exceedingly simple, is by no means sufficiently economical 

 to warrant its general adoption. 



SUPER-HEATING STEAM. For several years after 

 the adoption of steam-power for marine propulsion, it 

 was almost exclusively confined to vessels destined to 

 convey passengers over short distances, along coasts or 

 in rivers. The fuel required to maintain the steam- 

 power was to be obtained easily and cheaply, and, as the 

 distance was not great, the space required for the stowage 

 of coals was not so large as to trench inconveniently on 

 the accommodation of the vessel. Under these circum- 

 stances, engineers devoted their attention less to economy 

 of fuel than to speed. But since steam-vessels have 

 begun to make long voyages, lasting for several weeks 

 since the invention of the screw-propeller has rendered 

 them even more servicable as sea-going vessels for carry- 

 ing cargo than sailing vessels could be, and especially 

 since the application of steam-power to ships of war, 

 the economy of fuel has become a matter of the first 

 importance. Many modifications in the construction of 

 the boilers and of the engines have been introduced for 

 the purpose of saving fuel, and still greater changes will 

 probably be effected in the processes by which the com- 

 bustion of fuel is applied to the production of motive 

 power. Among these changes, which begin to receive 

 general adoption, may be noted surface-condensation, to 

 which we have already alluded (page 873), and ni/ier-heat- 

 ing iteam. The latter process appears to promise great 

 results in economy of fuel, especially when applied in 

 connection to combined engines with turface-cotuiensers, 

 and we will therefore endeavour to describe how the pro- 

 cess is effected, and what are the special advantages 

 which it secures. On looking to the Table of the Tem- 

 perature and corresponding pressure of steam (page 84.">), 

 it will be seen that a very small reduction of temperature 

 produces a very considerable diminution of pressure. 

 Thus, steam at a pressure of 4 atmospheres, or exert- 

 ing on every square inch a pressure of 45 Ibs. above that 

 of the atmosphere, hits a sensible temperature of li!)U. 

 On diminishing the temperature by 15 5 , the pressure is 

 reduced to three atmospheres, or a loss of pressure is 

 incurred amounting to 15 Ibs. on the square inch. At 

 higher temperatures the loss is proportionally greater. 

 Thus, steam at 344, having a pressure of 8 atmospheres, 

 when cooled down to 320, has its pressure reduced to 6 

 atmospheres, losing 2 atmospheres, or 30 Ibs. per square 

 inch, by a reduction of its temperature to the extent of 

 24". 



It may be readily understood, then, that steam, in its 

 passage from the boiler, where it is generated, to the 

 cylinder, where its elasticity is required, exposed, as it is, 

 to the cooling influence of an extended surface of pipe 

 passages and cylinder, all made of rapidly conducting 

 materials, must sustain a loss of effective pressure, which 

 is absolute waste of power or of the fuel used to pro- 

 duce it. Nor is the loss of pressure the only evil icsult- 

 ing from the reduction of temperature. A cubic foot t' 

 steam at 290 of temperature and 4 atmospheres ol 

 pressure, on having its temperature reduced to 275, 

 loses, as already stated, 1 atmosphere of pressure ; but, 

 as it has still 1 cubic foot of volume, and as, by Mar- 

 riotte's law, the density is always proportional to the 

 pressure, the loss of pressure is accompanied by a corre- 

 sponding diminution of density. That is to say, in the 

 case we are examining, one-fourth part, or 25 per cent, of 

 the steam, becomes actually condensed into water, leaving 

 the remaining three-fourths, or 75 per cent., occupying 

 the volume of 1 cubic foot at the reduced density and 

 pressure. This water accompanies the steam into the 

 cylinder, where it is of no assistance to the steam on the 



