1S41.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



•w 



or in part, to some error in his deductions from the data furnished by 

 observation. 



Mr. Parkes snys Cpage 261') : "The absolute resistance consists of 

 the weight wliioh performs the return stroke, plus the value of engine 

 and pitwork friction, and of the elasticity of the uncondensed steam." 

 To this shoulil be added, for each f ircing pump, the weight of a 

 column of water whose base is equal to the sectional area of the 

 plunger and altitude to the mean height of the bottom of the plunger 

 above the level of the water in the cistern whence it is drawn, and for 

 each lifting pump, the weight of the whole column lifted, from the 

 level of the water in the cistern; and we should deduct the amount of 

 assistance, though small, given by the atmospheric pressure on the top 

 of the piston rod. 



We are not informed of the value of any of these quantities, except 

 the last, from direct experiment, but we know that the weight which 

 performs the return stroke is necessarily slightly in excess of that of 

 the average column of water raised, augmented hv the friction of the 

 water and machinery, and the dili'erence between the atmospheric 

 pressure and that of the steam, during the return stroke, on the area 

 of the piston rod ; and the excess fwhich is ni'cessarv to set the engine 

 in motion with its load of water) is cnunlerbAlanced at the end of the 

 stroke by the cushion of steam which brings the engine to rest. 



Mr. Parkes substitutes for this weight, in his calculations, the water 

 load, which, he says, can alone be termed a positively ascertained 

 quantity; but in computing this load he commits two errors, which, 

 however, compensate each other. He calls the mean diameter of the 

 pumps 14-G2.T inches, instead of 14-9GS, which renders it necessary to 

 assume a cubic foot of water to weigh 65*47 f(j., instead of 62-5 lb., in 

 order to make the water load equal to ()SGGii44 tt).. in which he agrees 

 with Mr. Henwood, by whom the experiment was made. It is per- 

 mitted, in calculating the effective resistance on the piston, to use the 

 total height of the column of water, since it is equal to the sum of the 

 average heights mentioned above ; so that the absolute resistance will 

 be equal to the weight of the total column of water raised, j»/«s the 

 friction of the water in the pipes, twice the friction of the machinery, 

 and the elasticity of the uncondensed steam, minus the difference be- 

 tween the pressure of the steam during the return stroke, and that of 

 the uncondensed steam during the working stroke, on the area of the 

 piston rod. The diameter of the piston being SO inches, and that of the 

 piston rod 7 inches, the area of the former minus that of the latter, or the 

 effective area of the piston, is equal to 49S8"0S square inches, and tlie 

 resistance on the piston due to the water load is consequently 11"01 ttj. 

 per square inch. (Mr. Henwood by some mistake made it only 10"2 

 lb. per square inch, which he also called the whole resistance of the 

 engines). The elasticity of the uncondensed steam is estimated at 

 1-2 j lb. per square inch, and that of the steam in the cylinder during 

 the return stroke appears, by Mr. Kenwood's indicator diagram, to 

 have been about 0-4 lb. The difference between these two last quan- 

 tities, reduced in the ratio of the area of the piston rod to the effective 

 area of the piston, becomes 0*04; and we find the whole resistance 

 per square inch of the piston (assuming with Mr. Parkes that the fric- 

 tions, the actual amount of whi 

 cause a pressure of 5'75 lb. per square inch) equal to 



11-01 + 5-75 + 1-25 — 0-04= 17-871b. 

 We think, with Mr. Parkes, that this amount is by no means exag- 

 gerated, but more likely the reverse, particularly in the evaluation of 

 the frictions, and must therefore conclude that the error, if any, must 

 be in the calculation of the power from the indicator diagram. Now 

 we have satisfied ourselves that the mean elasticity indicated by the 

 diagram would not produce sufficient power, so that we have no alter- 

 native left but to prove the diagram false or to confess ourselves un- 

 able to account for the facts observed by Mr. Henwood. 



If we admit the pressures to have been as shown in the diagram, 

 and that the equilibrium value was closed when the piston was 9 

 inches from the end of the return stroke, we must either suppose the 

 unreasonably large space of GS-0 i9 cubic feet to have existed below 

 tlie piston at the bottom of its stroke, or, (if we allow thirty cubic 

 feet,) we must assume a waste of 7-4 per cent, of the water expended. 

 On the latter hypothesis, the volume of steam of 6-4 lb. pressure 

 discharged from the cylinder every stroke was 352'SSG cubic feet, 

 which is the capacity of the space below the piston when the equi- 

 librium valve is closed, and the volume remaining above the piston 

 was 25"979-|-c, calling c the capacity of the space above the piston 

 when at the top of its stroke, or the volume of the steam-cushion. 

 The whole quantity of steam in the cylinder of this elasticity is there- 

 fore equal to 37S.8G5-|-c, and its relative volume 3GG8. The space 

 it occupied before the equilibrium valve was opened was 34G-393+C, 

 its elasticity was 7 Itj. per square inch, and its relative volume 3377, 

 so that we can find the value of c from the following proportion, 



I we have no means of ascertaining, 



32-472 : 291 : : 34G-393-I-C : 3377, 

 whence 



e=30'438 cub. ft. 



The absolute volume of the steam which formed the cushion was, 

 before compression, r)li-417, and its relative volume 3GG8; after com- 

 pression its absolute volume was 30-438, which makes its relative 

 volume 1979, and its elastic force 12-48 tb. instead of lU-7, as shown 

 by the diagram. Mr. Parkes gives 9-I7G cubic feet as the value of c, 

 vihich would evidently increase the difference between the calculated 

 and the observed pressure of the steam-cushion. 



The volume of steam of 7 16. pressure in the cylinder just before 

 opening the equilibrium valve is 37G-831 cubic feet, and the volume 

 occupied by the same steam when the piston had described one fourth 

 of its stroke, and the admission valve was quite shut, was 117*030 

 cub. ft., so tliat the relative volume of the steam was then 3377x 



Q'T" Qi ^ 1049, and its elasticity 24-87 rt. per square inch ; accord- 



o/o-oo 1 



ing to the indicator diagram it was only 20-4 R. 



We have no mean<^ of testing the correctness of the pressures 

 marked by the indicator during the period when the admission vahe 

 was open, but the above calculation suffices to prove that the diagram 

 is far from furnishing an exact measure of the steam's elastic force 

 at least in the instance quoted, and that if the whole, or nearly the 

 whole, of the water expended passed through the engine in the form 

 of sleam, it was sufficient to produce, by its simple elastic force, a 

 dynamic effect equal to the work actually performed, particularly if 

 the volume of the steam-cushion was only 9-176 cubic feet as stated 

 by Mr. Parkes, and which accords with Mr. Kenwood's datum of the 

 volume of sleam used per stroke. 



Mr. Parkes has rendered some part of his paper rather difficult to 

 understand by an ambiguity of expression relating to the expansion of 

 the steam, accompanied in one place by an apparent contradiction in 

 the facts. He says (page 264), " it is evident that the effect of a 

 given weight of water as steam, consumed during a stroke, will be the 

 same, whether that steam be regarded as having been all enclosed 

 betW'Cen the piston and cylinder cover, before the piston were per- 

 mitted to move, when it would expand nearly uniformly from the 

 beginning to the end of the stroke ; or, whether it be considered as 

 lia'ving been admitted during a portion of the stroke, at some pressure 

 greater than the resistance, and then expanded through the remainder 

 of the stroke." 



What the author meant by this we cannot guess; taken literally, it 

 is obviously fdse, and that it was not intended to be understood so, 

 apppars by the calculation of the effective power in the sequel. He 

 continues: 



" But, the value of expansion consists, virtually, in the quantity of 

 action derived from the steam, after it forms an equilibrium nith the 



resistance By tracing it, first, through the space of the cylinder, 



where it would barely balance the resistance ; and secondly, through 

 the space during which it suffered expansion below that pressure, a true 

 measure of the respective and total quantities of action developed by 

 it, expansively and unexpansively, will be obtained." 



The expressions in italics imply that the expansive is separated 

 from the unexpansive part of the stroke at the point where the pres- 

 sure of the steam is equal to that of the resistance; but the numbers 

 quoted in the next page prove that such was not the author's meaning, 

 for he says, " w hen the ])iston of the Kuel Towan engine had passed 

 through '50-7 out of 120 inches, which vvas its total length of stroke, 

 the steam's elastic force and the resistance counterpoised each other." 

 Now we are informed that the resistance was 18-01 ft. on the square 

 inch, and the indicator diagram shows a pressure between 13 and 

 14 tb. at the point mentioned; but in the diagram of the steam's 

 action at page 294, a pressure of 18 ft. is marked at that point, and 

 the steam and resistance are said to be in equilibrio. We are unable 

 to account for this discrepancy. 



A series of well-conducted experiments with Cornish single-acting 

 engines would not only be very interesting with regard to the working 

 of these engines, concerning which so much doubt is still entertained, 

 but would doubtless throw a great deal of light on the general theory 

 of the steam-engine, since they afford facilities for making obser- 

 vations which double-acting engines do not admit of. 



J New Line of Atlantic Steamers.— The St. John's N. B., Herald informs us 

 that the English Govi rnment is about conlracling for an additional line ot 

 steamers to theNordi American colonies, vibich will give a weekly commu- 

 nication with England. The new line will be likely to run direct to St. 

 John's, such being Sir William Colebrook's wish, while the present line will 

 continue to run to Halifax. We presume the new line will be extended from 

 St. John's to this jjoit.— New Yorti Evenmg Post. 



