270 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[August, 



engine had been registered while it was making about 179,000 strokes, the 

 mean result as stated by Professor Moseley. was so nearly that arrived at by 

 Mr. \A'ickstccd, that he had no doubt of the accuracy of the machine as a 

 good indicator of the real duty performed by the engine, tlie difference in the 

 result of the mean pressure of the steam, deducting the vacuum, or 0-73 lb., 

 ■was O-l'J lb., namely, according to Mr. Wicksteed's experiments 12'94 — 0-73 

 = 12'21 lb., and according to Professor Jloseley Ti'OOlb. ; this difference 

 might arise from a variation in the mean length of stroke during the two sets 

 of experiments — from a slight variation in the point at which the steam had 

 been cut off — from a variation in the level of the water in the pump well, or 

 other practical causes ; the difference however was so insignificant, that he 

 •would rely on the accuracy of Professor Moseley's Indicator, and allow the 

 possibility of a slight error in his own experiments. 



Mr. Farey observed that Professor Moseley's instrument must be influenced 

 by variations in the length of stroke, for whenever the piston makes a long 

 stroke, tlie cone and the train of registering wheels, must he turned farther 

 round, and would register a higher number than they would do in case of a 

 shorter stroke ; supposing the impelling force exerted by the steam to be 

 always the same. If the instrument could be really made to give its results 

 according to the actual length of all the varying strokes made during the 

 time of observation, by truly aggregating these varying lengths into one sum, 

 the results would be free from the usual uncertainty respecting an average 

 length of stroke. In the monthly reports of engines in Cornwall, the per- 

 formance is reckoned according to some reputed length of stroke, which had 

 been fixed upon for each engine, when it was reported, and it is afterwards 

 assumed that no departure from that reputed length has taken place, when, 

 in fact, such departure does often occur. 



It would be very desirable to have a moving card applied to the new in- 

 stnmient, in order to indicate the impelling force of the steam in the cylinder 

 by tracing curves on paper, like those by the ordinary indicators. This it 

 appeared might he done, with the advantage of causing the paper on which 

 the curve is drawn to travel onwards and bring fresh paper into its place, so 

 as to obtain a series of distinct curves for as many succeeding strokes. 



The form of the springs of Professor Moseley's instrument would be a de- 

 cided improvement if substituted for the spiral spring of ordinary indicators ; 

 Mr. Farcy had applied to an ordinary indicator a mode of exhibiting at a 

 glance, whether the engine was exerting more or less force than its ordinary 

 ap])ointed task ; the plan answered that purpose : but as it required the 

 indicator to be always in action, the spring of the indicator broke after work- 

 ing more than two days, he therefore abandoned it. The springs in the new 

 instrument were proved by the trial at Old Ford to be capable of enduring 

 continual exertion without breaking. The professor had stated that the 

 scale of flexure of the new springs was found to be exactly, according to 

 theory, equal divisions with equal forces ; this might be expected because 

 the flexure of the springs was small, and the bending force acted in a direc- 

 tion nearly at right angles to the length of the springs. In ordinary indi- 

 cators the scale should not always be equal divisions, because the wire of the 

 spring being wound spirally into a screw of small diameter, the spiral obH- 

 cjuity of the thread of such screw becomes more oblique to the direction of 

 the bending force, as the spring is stretched, and less oblique as tlie spring 

 is compressed, and hence the scale of pounds per square inch by which the 

 curve should be measured for summing up the results, ought to be a scale of 

 unequal divisions. 



The indicators originally used by Boultou and Watt were of a large size, 

 ■with a long and powerful spring curled into a cylindric form, as large in 

 diameter as could be included in the cylinder of the indicator, and the motion 

 allowed to the piston by the spring was very short ; such indicators were 

 judiciously projiortioned, and they do not show any sensible inequality of 

 divisions in their scale. But recently, indicators have been frequently made 

 without th; knowledge of their true principle, and the rules of proportion 

 are not oscrved, so that it will sometimes he found, on actual trial of such 

 instruments with weights, that their scale of pounds per square inch is not 

 in equal divisions, although it is usual to employ a scale of equal divisions 

 for suraniing up the curves traced by them. In Boidton and Watt's indi- 

 cators the scale of pounds per square inch was formed from actual trial with 

 ■weights, but such trials were paade when the indicator was cold, and dis. 

 mounted from its place upon the steam engine. A much better mode is to 

 apply the weights on the upper end of the piston-rod when the indicator is 

 placed on the cyhnder of the engine, while it is hot, its piston being supplied 

 ■with the same quantity of oil, and the spring being in the same state as when 

 it is in use. The depression of the piston by the weights is recorded by 

 drawing a line with the pencil of the instrument on the card itself, in the 

 same manner as the usual atmospheric line is drawn thereon. A series of 

 lines thus drawn with given weights, Ijccome so many original stages for sub- 

 dividing between them, to form a true scale for summing up the curve de- 

 scribed under the same circumstances and nearly at the same time. 



Professor Moseley's instrument had two cylinders and pistons operating in 

 concert on the same piston-rod, and springs of peculiar construction to indi- 

 cate the uidjalanced pressure exerted by the steam to impel the piston of the 

 engine. The elastic force wherewith the steam acts above the piston (called 

 the positive pressure or plenum) is shown by a common indicator, but the 

 clastic force wherewith the uncondensed steam is at the same time reacting 

 beneath the piston (called the negative pressure, or imperfect exhaustion or 

 vacimm) is not shown ; hence the observations are limited to two odd halves 

 of the stroke made by the piston, those halves being commonly the plenum 



during the descent, and the exhaustion during the ascent of the piston ; it is 

 taken for granted that the other two odd halves are the same as those which 

 are observed, although such assumed parity is not always the true state of 

 the case. In the new instrument the indication that it would make by draw- 

 ing on a card, would be that of the difference subsisting between the iilenum 

 above, and the exhaustion beneath the piston of the engine, during its de- 

 scent, and ascent, wherefore it would indicate on one caid as much as two 

 ordinary indicators can do on two cards, if they are applied one to the top 

 and the other to the bottom of the cylinder of the steam-engine; in that 

 case each indicator shows on its own card what the elastic force of the steam 

 is during the plenum, and what it is during the exhaustion, but the required 

 result (which is the ilifference between the two) must be obtained by com- 

 bining together in the computation those distinct curved lines which are 

 drawn on two separate cards. Professor Moseley's combined indicator pis- 

 tons, acting on the same springs, would at once indicate such difference by 

 the cux\e which it would trace on the one card. 



In answer to a question from Mr. Parkes as to whether the new instru- 

 ment had been put to any other test than its apparent agreement with Mr. 

 Wicksteed's estimate of the resistance overcome ; and whether the common 

 indicator had been applied to the engine at the same time. Professor Moseley 

 said that he had not compared the instrument with any other, but had sub- 

 jected Mr. Wicksteed's calculations to a rigid investigation, and felt quite 

 satisfied that they approximated closely to the truth. He relied upon them 

 as corroborations of the accuracy of the instrument. 



Mr. Parkes observed that it would have been more satisfactory to engineers 

 to have been assured that every means had been taken to demonstrate the 

 truth of the results recorded by an instrument which had such important 

 functions in view, lie wished to know in what manner the pressures de- 

 noted had been ascertained — whether by weights or by comparing them with 

 a mercurial column, lie had found the latter mode more exact than weights, 

 in verifying the scale of the common indicator, as the instrument being 

 heated was then in precisely the same state as when it was in use. He had 

 found that a certain amount of correction was frequently necessary, as both 

 the spring and the amount of piston friction were affected by heat. 



Professor Jloseley replied that the instrument had not been compared 

 with the mercurial column, but that the resistance of the springs, and the 

 friction of the piston and instrument generally, had been ascertained by very 

 accurate experiments, so that he had full confidence in the results. 



Mr. Parkes said that notwithstanding the respect and deference he felt for 

 Professor Moseley's attainments and ingenuity, his past experience would not 

 permit him to place entire confidence in the results afforded by the instru- 

 ment ; indeed he considered them to be altogether fallacious as representing 

 the force acting on the piston of the Old Ford engine. He could not admit 

 that the apparent near identity between Mr. Wicksteed's computations of 

 resistance, and the constant indicator's registration of force, amounted to 

 proof of the instrument's accuracy ; for it seemed to him that Mr. Wicksteed 

 had omitted to take into account one important item of force, without the 

 exertion of which a piston could not be brought from a state of rest into a 

 state of motion. He referred to the force required to give velocity to the 

 piston. Mr. Wicksteed had weighed the greater part of that resistance which 

 might he called ponderable, and had estimated the remainder, assigning 

 about 12 lb. per square inch on the piston as the total amount, after deduct- 

 ing the resistance to the piston's descent arising from uncondensed steam. 

 Thus, an elastic force of 12 lb. on the piston would counterpoise a resistance 

 of 12 lb., but motion would not ensue until a superior force were applied. 

 Considering the number of strokes made by this engine per minute, Mr. 

 Parkes could not estimate the velocity of the piston at less than from 300 to 

 400 ft. per minute, which was very great for this enormous mass, and would 

 require the exertion of proportionate power. He regarded the piston of the 

 engine, loaded with 12 lb. per square inch, as he would aMjall of 12 lb. weight 

 in a gun balanced by a fluid of a corresponding elastic force ; but such ball 

 would remain motionless unless it were propelled by some additional force. 

 This state of things had been denominated by Professor Barlow " the pre- 

 paration for motion." Now as Mr. Wicksteed's estimate proceeded no fur- 

 ther than the production of this state of things, and as Professor Moseley's 

 constant Indicator recorded the mean pressure of the steam in the cylinder 

 as barely equal to it, he could not assent to the accuracy of either method of 

 determining the total resistance overcome by the steam. 



Mr. Parkes would cite the experience of others as to the quantity of force 

 actually expended in giving velocity in a Cornish engine, over and above that 

 necessary to balance the weight at the opposite end of the beam, friction, 

 &c. included. In the fifth part of Vol. HI. Trans. Inst. C. E., Mr. Enys has 

 reported some experiments made at Mr. Parkes' suggestion, on several 

 engines. He would cite those of the Tresavean, as the cylinder was of the 

 same diameter as the one at Old Ford, viz., 85 inches. The water load was 

 equivalent to 12 lb. per square inch on the steam piston, and when about 

 half the usual velocity was given to the piston in the in-door stroke, a 

 pressure of about 17.i lb. was denoted throughout the stroke, by a mercurial 

 column connected with the cylinder. Mr. Loam had since transmitted to 

 him the following abstract of experiments made on the same engine, January 

 28, 1842. " Two indicators were used at the same time, and an open mer- 

 curial gauge ; the engine was held in-doors until the mercury became sta- 

 tionary, in order to ascertain the muiimum quantity of steam power necessary 

 to produce motion. The pressure was 15 lb. per square inch, when tlie water 

 load was 12 lb." Here, then, a force of 3 lb. was found requisite to establish 



