1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



271 



an equilibrium between the power anJ resistance ; and a further force of 

 2J lb. per square inch was necessary in order to urge the mass at about the 

 half of its ordinary velocity. Mr. Enys stated that less pressure was exhibited 

 when the engine was brought in slower, and i-ice versa; and tliis was con- 

 sistent with everyday experience. Now, though the Old Ford engine had 

 not to overcome so much frictional resistance as a deep mine engine, yet, 

 having a weight to raise, according to Mr. Wicksteed, alone equal to 11-8 lb. 

 per square inch on the piston, Mr. Parkes was of opinion that this could not 

 be effected at the usual working velocity, with less than H or 15 lb. pressure 

 of steam per square inch, ne would suggest to Mr. Wicksteed, to repeat 

 Messrs. Enys and Loam's experiments, and also to work his engine with 

 steam reduced nearly to such pressure in the boiler as would barely suffice to 

 bring the piston down. Such experiments would confirm or invalidate the 

 results given by Professor Moseley's instrument, and probably lead to the 

 discovery of its imperfections, should any exist. 



It appeared that Mr. Wicksteed conceived a greater amount of elastic force 

 to be required to perform a stroke in proportion to the degree of expansion 

 given to the steam in the cylinder. He would quote the pressures deduced 

 by Mr. Wicksteed under five cases of expansion, as they exhibited some 

 curious anomalies. They were taken from Table VI. of Mr. Wicksteed's 

 Treatise. According to his table, when the steam was stopped at 6 ft. of the 

 stroke, the mean force exerted during the stroke of 10 ft. was 13 lb. per 

 square inch ; at 4} ft., 13.^ lb. ; at 4 ft., 14| lb. ; at 3i ft., 15^ lb. ; and at 

 3.f^ ft., Id^ lb. It must be observed that in every one of these eases, the 

 resistance, as appreciated by Mr. Wicksteed, amounting to 13 lb. per square 

 inch, was a constant quantity ; so that, for some unexplained reason, an 

 invariable load appeared to require a variable force to overcome it ; a sUding 

 scale of power, given as the measure of a constant resistance. It was possible 

 that some small difference might have existed in the velocity of the stroke, 

 in these cases ; it was also possible that some error existed in the method 

 used for determining the pressures, or in the evaporative quantities. How- 

 ever this might be, it was clear there was an error somewhere, as it could not 

 be granted that an effect deemed constant in all these cases could require a 

 varying cause for its production. 



As these appreciations differed widely from each other, and still more so 

 from the indications of Professor Moseley's instrument, Mr. Parkes hoped 

 that Mr. M'icksteed would re-consider and verify this part of the subject. In 

 corroboration of his opinion that the acting force recorded by the instrument 

 Is too low, he would draw attention to the circumstance that, during the 

 period of its application (28 days), the duty performed by the engine for 

 each 94 lb. of coal amounted only to about C8i millions, and it would not 

 be unreasonable to expect that at least 150 millions should have been the 

 result under the different circumstances of mine and water-works engines, as 

 100 millions had been performed for some months by mine engines under a 

 water load equal to 12 lb., and a mean steam pressure of 18 lb. per square 

 inch of the piston. Whereas, if the water-load of 1 1 lb. at Old Ford vras 

 overcome by an amount of force little exceeding 12 lb. per square inch, a 

 proportionate increase of duty ought to have resulted, but such was not the 

 case. 



Mr" Parkes then proceeded to comment on the phrase " effective power," 

 •which he understood from Professor Moseley, as signifiant of the force of 

 the steam, or piston pressure, measured by his instrument. He thought 

 that phrase more strictly applicable to the a:'.iount of power given off by the 

 engine, when ascertained, as it might be in the case of pumping engines, by 

 the weight raised ; or, as it could only be determined on rotative engines 

 driving machiiiery, by a dynamometer applied at the extremity of the crank 

 shaft. He would illustrate this by an example. There was at Birmingham 

 a corn-mill belonging to Mr. Lucy, worked by an excellent engine of 40 

 H.P., made by Messrs. Boulton and Watt. This engine had a fly-wheel 

 ■weighing 24 tons, and nine pairs of stones were driven, besides dressing- 

 machines. Mr. Lucy had taken out a patent for an apparatus as a substitute 

 for the fly-wheel, which had been removed. The engine so altered now 

 drives ten pairs of stones, under the same pressure of steam, and with the 

 same consumption of fuel as before. Thus, what he should denominate the 

 "effective power" of the engine was increased, by this simple change, eleven 

 per cent. Yet Professor Moseley's instrument, or any other indicator, would 

 have exhibited, both before and after the alteration, the exertion of a pre- 

 cisely equal force on the piston. Neither did the Professor's instrument 

 register the absolute, or what Professor Whewell had denominated the 

 " labouring " force of the steam on the piston of an engine, as it made no 

 deduction of the amount of force, whatever it might be, which was neces- 

 sarily expended in overcoming the resistance opposed by the uncondcnsed 

 .-team. Its construction permitted it only to record the difference of these 

 amounts. For these reasons he could not regard the instrument as likely, 

 even when made trustworthy, to become of that utility to engineers which 

 ■was the Professor's aim and hope. There was a greater need of an accurate 

 dynamometer capable of showing the eft'ective power of an engine ■whilst in 

 regular work, and he was happy to say that this desideratum had been sup- 

 plied by Mr. Davies of Birmingham, by whose permission he would take an 

 early opportunity of describing to the Institution the construction and effi- 

 cacy of this instrument. It was very important that a self-registering ma- 

 chine should be made, capable of recording the mean steam pressure ope- 

 rating throughout a stroke of the engine, but it was still more important 

 that this registration should be accurate ; and he hoped that the remarks 

 which had been made would only urge Professor Moseley to farther investi- 



gations, and induce him to enlarge as much as possible the useful powers of 

 the instrument. 



Professor Moseley observed that when a body passes from a state of rest, 

 through a state of motion and into a state of rest again, or from a state of 

 motion at a given velocity through a state of motion at a different velocity, 

 and back to its first velocity again, then is the work which must be done 

 upon it by the moving power the same in amount, whatever may have been 

 the velocity thus intervening between the two states of rest or of equal 

 motion of the body, provided that the resistance opposed to its motion and 

 the space through which that resistance is overcome be in all cases the same.* 

 In Mr. Wicksteed's engine the resistance thus opposed to the motion of the 

 piston, and the space through which that resistance is overcome at every 

 stroke, are thus constantly the same (or in other words, the work done upon 

 the resistance is the same at every stroke of the engine), and the pistoiv 

 passes at every stroke from a state of rest, to a state of rest again ; it follows 

 therefore by the above well-known principle of " vis viva," that the work 

 done by the steam, as the moving power upon the piston of the engine whilst 

 it completes a stroke, is the same, whatever may be the velocity communi- 

 cated to it, and to the mass which it carries with it, at any period of the 

 stroke. It is true that to put the piston, and the mass carried along with it, 

 at first in motion, a pressure greater than the resistance is required, and there- 

 fore greater tlian the mean pressure necessary to complete the stroke ; a 

 piessure equal to the resistance would only bring it into the state of rest 

 bordering upon motion ; to cause it to pass from tliis state of rest to a state 

 of motion, more pressure is required, and the more as the velocity to be 

 acquired, whilst it m.oves through a given space, is greater ; or in other 

 words, in order to communicate any given velocity to a body whilst it moves 

 through any space, there must be an excess of the work done by the driving 

 pressure through that space, over tliat expended upon the resistance through 

 that space ; but all this excess is accumulated, and unless the steam pressure 

 be afterwards made less than the resistance, or unless the steam be after- 

 wards expanded through a distance dependent on the amount of this accu- 

 mulated work, so that it may expend itself in overcoming the surplus resis- 

 tance through that space, then the piston will strike upon the cyhnder 

 bottom. This principle may be illustrated by an example ; suppose that the 

 load upon the piston of an engine is 10 lb. per square inch, and that the 

 steam is admitted at a pressure of 15 lb., it is erident that by reason of the 

 excess of 5 lb. pressure of the steam above the load, the velocity of the 

 piston will be made continually to increase until the steam is cut off, and 

 afterwanls, so long as the steam pressure exceeds the load, or until by its 

 expansion the steam pressure is reduced to 10 lb. per square inch. Up to 

 this point the velocity of the piston and of the mass moving with it wLU con- 

 tinually have been increasing, a great momentum will therefore have been 

 acquired by it, and this momentum will carry it on to the completion of the 

 stroke -, although after this position is passed, the steam pressure will be less 

 than the load, and would by itself be insufficient to remove it. In other 

 words, the work done by the steam upon the piston will have continually 

 exceeded that expended on the load up to this period of the stroke, and the 

 surplus will have been accumulated in the moving mass,t which surplus work 

 will carry on the piston to the end of the stroke when a cyhnder full of 

 steam will be delivered of greatly less pressure than the load. If the steam 

 had been worked at full pressure, it is erident that at every stroke a cylinder 

 full of steam would have been discharged of the same pressure as the load. 

 In this consists, therefore, the advantage of working expansively. It is evi- 

 dent that the piston acquires its maximum velocity at the point where the 

 steam pressure becomes equal to the load, and that the engineer, by the 

 manipulation of the steam valves, produces that adjustment by which the 

 velocity acquired by the piston at this point (or the work then accumulated 

 in it) is caused to be just sufficient to carry on the piston to the end of the 

 stroka, but without striking the cyhnder bottom ; it is moreover evident that 

 the greater this maximum velocity can be made, the farther the piston will 

 be carried beyond the point where the steam pressure is equal to the load, 

 and the less will be the pressure of the cylinder full of steam discharged at 

 the completion of every stroke, or the greater the economy of the steam 

 power. . 



A second illustration of the same principle may be drawn from the effect 

 produced by a pressure suddenly thrown upon a spring. Suppose a spring 

 which would rest dcllected through an inch under a pressure of one pound. 

 If when this spring is in an undeflected state this pressure of one pound be 

 suddenly thrown upon it, it is certain that the spring will, at first, deflect 

 considerably beyond that distance of one inch in which its deflection will 

 eventually, after many oscillations, terminate. In fact, if it is thrown on 

 with mathematical suddenness, the first deflection will be two inches. To 



* This is a principle well known to mathematicians as resulting immediately 

 from the principle uf " vis viva," and is as old as the days of D'Alemberl j it 

 appears first to h'tve been applied to questions of practical m>-chanics by 

 M.M. Coriolis and Poncclet. 



1 The number of units of work thus accumulated is represented in 11>. one 



foot high by i - D! ; whence w represents the number of lb. in the Meiyht of 

 the moving mass, t) its velocity in feet per second reduced to the piston 

 g = Z2i lb. The expression - i" is said to represent^the vis viva ; so that the 

 accumulated work is equal to half the vis viva. 



