1841.1 



THE CIVIL ENGINEER AND ARCHITECT.S JOURNAL. 



.303 



newly built merchant ships, iii general, present no more satisfactory 

 aspects than their predecessors; and the proportions published as 

 those of the new steamers building in Scotland for Government, are a 

 further illustration of this position.t Unfortunately this defect has 

 influenced the docks intended for the reception of merchant steamers, 

 and unless the gates of those in this port were increased in width they 

 ■would not admit steamers of the same tonnage as those now using 

 them, but of greater beam. — Having trespassed at so great length on 

 your space, I beg to observe in conclusion, that my reason for having 

 cited no authority in support of my views is simply that I have met 

 ■with none taking the same ground; and of those treating on this sub- 

 ject under any aspect, the older ones are generally very vague and 

 general in their statements ; and the more modern, though entering at 

 gi'eat length and with much pains into particular forms, are so partial 

 in their manner of considering the matter, that of the two whose 

 opinions have latterly carried most weight, I have foimd one advo- 

 cating the construction of seagoing steamers twelve times as long as 

 they are broad, and the other predicting the time -when masts and 

 sails would be considered mere useless incumbrances, and our Itne-of- 

 battle ships be used only as coal transports for steamers. Such ideas 

 indicate any thing but a comprehensive mode of viewing this subject, 

 and not trusting much to the assistance to be derived from such sources 

 1 have preferred merely stating the result of my own observations, re- 

 lying on the candid consideration of those who are practically inte- 

 rested in this matter. 



I am, &c., 



H. P. H. 



ON LONG AND SHORT CONNECTING RODS. 



Sir — Perceiving in your excellent 



Journal for this month, an article 



respecting long and short connecting 



rods, wherein it is stated that the 



short connecting rod is as eftective 



"'- as the long one, I take leave to send 



',] you a diagram, which will perhaps 



W show that there is a greater amount 



of friction with the short connecting 



rod. 



The accompanying figure repre- 

 sents the direct connection with the 

 piston of a long and short connect- 

 ing rod. In this case it is clear that 

 the long rod c, b, is working always 

 nearer the parallel liiieoi the piston 

 than the short rod c, a, or that the 

 short rod is pushing against the 

 slide s, e, at a much greater angle 

 than the long one, and consequently 

 that there must be less friction at e, 

 than at s, therefore as the angle c, 

 a, d, is greater than the angle c, b, d, 

 so is the friction at s, greater than 

 the friction at e. The same results 

 will be found with common marine 

 and all other engines, only that 

 ■where slides are used there is al- 

 ways a much greater amount of 

 friction than upon centres. I am 

 however of opinion, that a short 

 connecting rod, with a direct con- 

 nection with the crank, might be 

 used with greater effect than a longer 

 one in the common marine engine, 

 where heavy side levers are kept 

 continually reversing their motion, 

 besides the extra weight which the 

 boat has to carry. 



J. F. 



I am, Sir, your most obedient servant, 



London, July 12. 



Sir — In your July number you have a letter from Mr. Daniel Clark 



* The Forth has lately arrived in Liverpool, anJ fully confirms the above 

 opinion ; proving with two steamers lately sent hiilier from Hamburg, that 

 the examples of the Liverpool, British Queen and President have not in- 

 fluenced their builders in assigning the proportions of breadth and depth. 



on "Long and short connecting rods," in which he arrives at the con- 

 clusion that "upon the whole, then, shore and long connecting rods on 

 the same length of crank must be equally effective, whatever pecu- 

 liarities there be." In this, however, I do not concur with him and 

 would recommend him to re-consider the subject and see if really the 

 force (see Mr. Clark's first figure) D C or E M be of no consequence ; 

 I think he will find it to be of the greatest consequence, and to be to- 

 gether with the analogous force of pressure on the other journals, the. 

 reason why long connecting rods always have been and always will be 

 preferable, and why, moreover, an engine, the connecting rod of which 

 bears to the length of the crank a ratio less than a certain quantity, 

 would not work at all. Mr. Clark would find it very interesting to 

 consider minutely the case in which the length of the connecting rod 

 is equal to the length of the crank, he will find that the strength re- 

 quired for the paddle-shaft, the connecting rod, all the journals, the 

 framing, and in fact the whole engine, is what may probably startle 

 him. 



I am. Sir, 

 Greenock, July 5. Your obedient servant, 



J. G. L. 



MR. 



PARKES NEW THEORY OF THE PERCUSSIVE 

 ACTION OF STEAM. 



Sir — Feeling some interest in Mr. Parkes' new theory of the per- 

 cussive action of steam in the Cornish engines, might I offer a few re- 

 marks on the former part of a paper which appeared in your Journal 

 for this month. The w'riter of that paper appears from his remarks 

 at the beginning, to have an opinion that Mr. Parkes has rather fa- 

 voured the Cornish engines, in considering that the percussive force 

 of steam is only developed in them; and he remarks that if such a 

 property does exist in the steam, we might expect to find it more 

 fully developed in the case of the locomotive engines ; for he says, 

 "though why he coiisiders it to operate in these engines only, we 

 know not; we are of opinion that if it obtains in them, it should obtain 

 ci fortiori in locomotives, where the density and velocity of the steam 

 entering the cylinder are so much greater." 



The object of the following remarks is to try to show that the Cor- 

 nish single-acting engines are the only ones at present in which the 

 percussive force of steam could act with any very great advantage ; 

 and that the locomotives are the very worst engines in which it could 

 be used as a moving force. 



We will first of all take the case of a common double-acting rotative 

 engine. In these engines the slide is so adjusted as to let the steam 

 into the cylinder when the piston is either at the top or bottom of its 

 stroke; and consequently, when the crank is just passing the centre. 

 Now this being the case, it is evident that any percussive force of the 

 steam striking upon the piston would be injurious rather than benefit 

 the engine, as it could not by any means have any effect in turning the 

 crank, but, on the contrary, only create an additional wear and tear of 

 the different working parts, on account of the violent jerk which would 

 be the eflfect of its striking upon the piston while the crank is in such 

 a position as not to let it recede before the blow. In this engine, 

 then, as at present constructed, we must not expect to find any very 

 great economy by bringing this force into action. 



In consequence of the rapidity with which the strokes of the piston 

 in a locomotive follow one another, it is found necessary to admit the 

 steam into the cylinder before the piston has finished its stroke, for 

 two reasons: 1st, it is found necessary to admit the steam into the 

 opposite side of the piston before it has finished its stroke, in order to 

 bring it gradually up to the stop, and to diminish the violent jerk that 

 would be occasioned by its motion being so rapidly changed, and 2ndly, 

 so that it may be ready to act as soon as the piston has finished its 

 stroke. This being the case, the percussive force of steam would act 

 still worse here than in the before-mentioned case, as it would, instead 

 of helping to impel the piston, actually impede if, if not stop it alto- 

 gether. In this case, as well as in the former, the percussive action 

 is altogether avoided by the gradual motion of the slide, for as soon 

 as the slide begins to open the steam way, the steam rushes into the 

 cylinder and strikes upon the piston, but with very little effect, on 

 account of its being so much wire drawn in consequence of the small 

 size of the opening at first. 



In order to render the percussive force of steam available to its 

 fullest extent as a moving povf er in single acting pumping engines, it 

 would be necessary to have some medium interposed between the 

 direct action of the steam on the piston and the pumps ; so as to con- 

 vert the ever-varying pressure on the piston into a regular and steady 

 pressure on the plunger of the pump. This I think will be clearly 

 seen, for if we suppose for an instant this medium not to exist, such as 



