1840.J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



61 



the key which connects the piston-rod with the cross head is a mul- 

 tiple of the weight of the piston and piston-rod, and so on for the other 

 joints. It is, however, necessary to deduct first in each case the fric- 

 tion on the piston and other parts which may intervene between it and 

 the joint under consideration. 



Since the strain calcuUited by the preceding method is due simply 

 to the inertia of the piston, it is clear that, in oriler to find the whole 

 strain, it will be necessary to increase the former by as much as the 

 pressure of the steam against the surface of the piston may exceed 

 that of the waste steam on the opposite side. 



We have now to examine the maimer in which the force of the 

 shocks is diminished by the lead of the slide, and to what extent this 

 remedy is effectual. 



We shall confine our reasoning on this subject to locomotive engines, 

 in which, as we have already observed, the strain brought upon the 

 key of the piston by destroying the momentum of the latter, is very 

 slight in comparison with that which results from the pressure of the 

 steam on the piston, and which the key must necessarily bear during 

 some portion of the stroke ; for, supposing the effective pressure of 

 the steam to be .50 pounds on each square inch of the piston, the area 

 of the latter being upwards of 113 square inches when its diameter is 

 one foot, the total pressure of the steam on its surface is more than 

 5G00 pounds, while the strain due to the inertia of the piston, being 

 under seven times its weight ; if we suppose this to be 70 pounds, 

 (which we believe to exceed the truth) is less than 490 pounds, or less 

 than one-tenth part of the strain due to the resistance of the load, de- 

 duction being made for the friction of the piston. The connecting 

 key of the piston must therefore unavoidably bear a strain of more 

 than 5000 pounds while the steam is acting with its full force, besides 

 that due to the inertia of the piston, which amounts, at one quarter of 

 an inch from the end of the stroke, to 69 seventieths of the maximum 

 strain due to that cause ; and, since this is less than one-eleventh of 

 the total strain at the end of the stroke, when no lead is given to the 

 slide, the greatest amount which can be saved by cutting otl'the steam 

 and admitting it on the opposite side of the piston at a quarter of an 

 inch from the end of the stroke, is no more than one-seventieth of the 

 strain due to the inertia of the piston, or less than one 700th part of 

 the total strain at the moment of cutting off the steam. 



It is therefore evident that the sudden jerks experienced by the key 

 which connects the piston with the piston-rod, in consequence of the 

 rapid changes in the motion of the piston, in as far as they are due to 

 the inertia of the latter, do not afford a sufficient motive for giving a 

 lead to the slide ; and that this remedy is entirely ineffectual in 

 diminishing them, in as far as they are due to the alternate action of 

 the steam on the opposite sides of the piston, which is the immediate 

 cause of nearly the whole amoimt of the evil; so that, if the piston 

 could not be kept tight on the piston-rod without the lead, neither 

 could it be with a lead of a quarter of an inch, when the length of the 

 stroke is 18 inches. 



Regarding the second reason, namely, that the steam may be ad- 

 mitted into the cylinder, and be completely ready to begin the next 

 stroke when the piston is at the end of the cylinder, we are of opinion 

 that nothing at all is gained in that respect by meiins of the lead, but 

 that, on the contrary, it is attended with a slight disadvantage. Near 

 the beginning of this paper we observed that, without any lead, a 

 loss of pressure during a very small portion of the stroke ensues 

 from the necessity of filling the waste space at the end of the cylinder 

 with steam at the beginning of the stroke; but this loss is of very 

 trifling amount. By a lead of one quarter of an inch this loss of pres- 

 sure is avoided, for this gives sufficient time for the waste space to be 

 filled with steam at full pressure by the commencement of the stroke ; 

 but by this means the resistance on the opposite side of the piston is 

 increased, during the last quarter of an inch of the stroke, by whatever 

 pressure the steam has acquired at every instant of that portion of the 

 stroke. The amount of resistance so produced is greater than the 

 loss of pressure at the beginning of the stroke resulting from the above- 

 mentioned cause when there is no lead. We do not, however, attach 

 any importance to this circumstance, as tlie whole amount of loss either 

 way is perfectly insignificant ; we only mention it to show that the 

 lead of the slide is not requisite, nor even advantageous, for the second 

 reason assigned by the author of the paper above quoted. 



With respect to the third reason, we do not think that so ranch can 

 be gained as the author appears to suppose, yet, if there is any advan- 

 tage in the lead, it is probably in beginning to get rid of the waste 

 steam before the commencement of the stroke, so that, when the piston 

 commences its stroke, there is but little waste steam before it to resist 

 its progress, the steam beginning to be let out of the cylinder before 

 it has driven the piston to the end of the stroke. Now there is clearly 

 tliis advantage in beginning to let out the waste steam before the end 

 of the stroke, that, supposing the time occupied in getting rid of the 



vvfhole of it to be the same as without any lead, the portion of the 

 stroke traversed by the piston during this time is less, because its 

 velocity is on an average less : besides which, the resistance of the 

 waste steam during the first portion of the time, namely, at the end of 

 the previous stroke is thereby avoided, though at the expense of a 

 part of the useful effect ot the steam in the latter part of the stroke : 

 indeed, by as much as the pressure of the waste steam at the beginning 

 of the stroke has been diminished by the eduction port having been 

 already some time open, by so much must its efiective pressure have 

 been reduced at the end of the previous stroke. We have also already 

 mentioned the resistance of the steam let on to the front of the piston 

 before the end of the stroke, which of itself nearly compensates the 

 saving of part of the resistance of the waste steam at the beginning. 



The preceding reasoning is only intended to prove that there is 

 little or no reason, and certainly no necessity to give a lead to the 

 slide in locomotive engines ; for-other descriptions of engine it is need- 

 less to say any thing, as no one would ever think of giving a lead in 

 any but a locomotive engine. It might however be advantageous to 

 give a lead to the eduction only, as by that means the saving of re- 

 sistance at the beginning, would not be counteracted by the additional 

 resistance of the steam admitted into the cylinder before the end of 

 the stroke. 



To return to the action of the steam in the cylinder. The whole 

 effect produced during an indefinitely short period of time is equal to 

 the pressure of the steam on the whole area of the piston multiplied 

 by the distance travelled by the piston during that time, the pressure 

 of the waste steam being considered as a part of the resistance, or 

 total effect. This is true, although at some moments the resistance 

 may apjiear less than the pressure of the steam, and at others infinitely 

 greater ; for the compensation is perfectly made by the momentum of 

 the moving parts, which serve as reservoirs of power, absorbing, as it 

 were, the excess at one time by receiving an increase of velocity, and 

 giving it out again at another time, when the pressure of the steam is 

 inferior to the resistance. But although the pressure of the waste 

 steam is strictly a part of the resistance, yet we shall, in the following 

 investigation, deduct its amount from the gross power of the steam, 

 and consider the balance as the gross power of the engine, which will 

 then be equal to the useful effect, ^/«s the friction and other resis- 

 tances ;;( the engine. In our next paper we shall commence this inves- 

 tigation with the low pressure condensing engine, for which the cal- 

 culation is the most simple, and then extend it to the other varieties 

 of engine. 



ARCfflTECTURAL COMPETITION. 



Sir, — The subject of Architectural Competition is one, which at 

 this moment, should be more than usually interesting to members of 

 the profession. I do not, therefore, hesitate to request your inser- 

 tion of the following correspondence, which I think, it will be con- 

 fessed, exposes as unsatisfactory a case as any of those recently so 

 nuich commented upon. 



In the early part of this year, a committee formed for building a 

 new church at Cardiff, advertised for plans, offering premiums of 20/. 

 and 10/. for the first and second best designs. In conjunction with 

 my partner, Mr. Brandon, I submitted plans, with a specification and 

 estimate. On the 11th June, we received the following letter: — 



Cardifl' Vicarage, 10th June, 1839. 

 Gentlemen, — The premium of 20/. offered for the Ijcbt plan and design for 

 a churcfi in this town, having been awarded to you, I have great pleasure in 

 forwarding you from the committee, an order for that amount on the Loudon 

 and \\estimnster Bank, of which I shall be obUged by your acknowledging 

 the receipt. 



I am. Gentlemen, your very obedient Servant, 



T. Stacev. Hon. Sec. 



In the course of two or three weeks after the receipt of that letter, 

 we heard it rumoured that a Mr. Foster, of Bristol, was to be em- 

 'ployed as archttct to this church. Being at a loss to reconcile this 

 statement with the announcement that our's was "/fc best plan and 

 design," we wrote to say, that if their subscription fell short of the 

 contemplated amount, we should be happy to submit sketches for a 

 building on a reduced scale. On the 'JSth June, we received the fol- 

 lowing Tetter. 



Cardiff, 24th June, 1839. 



Gentlemen, — I fear I have been gudty of an omission in my last commu- 

 nication, that has occasioned yon some misconception relative to the pro- 

 ceedings of the committee for building the new church here. Had it 



