1841. J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



91 



heuce, when both engines are in their most disadvantageous positions 

 on a curve, the sines of the angles they form with the xnih will be 



iiearlv 



^ 2"r + 



1 



336 



to 



10 



+ 



— , and 



480' 



when these angles are 



equal to each other, we have 2 

 1612S0X3 



+ 



1 

 33G 



= .^ + 



480' 



, or r 



288 



IbSO feet = 560 yards. That is to say, that supposin" 



the deviation from the true position of the engine, due to the play 

 between the wheels and the rails to be no more than a quarter of an 

 inch rn its length, the six-wheeled engine meets the rails at a more 

 favourable angle, and is consequently less likely to run off them on all 

 curves in which the radius exceeds u60 yards; on curves of a less 

 radius the four-wheeled engine begins to have the advantage. 



1 am. Sir, 

 Breretun, Your obedient servant, 



Feb. 6M, 1841. W. H. Barlow. 



IMPROVEMENT ON ECCENTRIC RODS. 



Sir — Among the numerous readers of your highly esteemed Journah 

 perhaps there are many to whom the subject of this communication 

 will appear of little importance, I therefore apologize for once more 

 imposing it upon your pages. 



In your present month's number (page 66,) I observe a communica- 

 tion signed H. E., in which your correspondent points out several in- 

 convenient conditions as inseparable from the system of two eccentrics, 

 in reply to which, with your permission, I beg to make the following 

 remarks. I will notice these conditions one by one after the same 

 order H. E. has pointed them out. 



First. I do not clearly see how it is possible to give the lead at all 

 either with or without a complication of levers unless the eccentric 

 precedes the crank in its action. Even supposing the working the 

 valve, when going forward, by the upper pin of the double lever to be 

 inseparable from the system, it has in my opinion a peculiar advantage, 

 in this respect, over the four eccentrics, the rods of which are kept iu 

 gear partly by their own weight, for instance. Suppose some de- 

 rangement to take place in the reversing apparatus of an engine fitted 

 .with with the four eccentrics ; the two suspended eccentric rods would 

 fall upon the lever studs of the valve motion, and very probably cause 

 a most serious crash. Now with the double ended eccentric rods the 

 case would be rather difterent; their falling from the upper to the lower 

 studs of the double levers would only reverse the action of steam upon 

 the pistons, and as the engine-man has always the power to shut oft" 

 the steam, he could tmtantly prevent the reverse motion of the engine. 



.'Second. The centre of the double lever shaft may be situated above 

 or below the line C, E, just as circumstances may require, but it is re- 

 quisite to fix the eccentric so that it shall be exactly perpendicular to 

 the centre of the shaft and crank axle, when the piston is at either 

 end of the cylinder. I do not see any just reason why this should be 

 considered as an inconvenience. 



Third. I beg to state the amount of lead is not dependant upon the 

 length of the eccentric rod, as H. E. has stated, but it depends upon 

 the angle at which this rod works with the centre of the lever shaft 

 and crank axle. 



Fourth. It is possible to construct the valve motion so as to give 

 the power of increasing or decreasing the amount of lead both ways, 

 but as this would cause an additional number of parts, and consequently 

 render the system more complex, I will admit of "the lead being de- 

 lermined must remain invariable." 



With the four eccentrics, providing they are all independent of each 

 other, that is, fixed on the shaft separately, you certainly have the 

 advantage of varying the amount of lead ; but the eccentrics are not 

 always independtnt of each other, they are very frequently cast all 

 together. In this latter case the lead, for both ways, is determined in 

 the eccentrics, and of course remains /.rtii, therefore, you cannot in- 

 crease it one way w ithout diminishing it the other, this H. E. has 

 pointed out to be the moit serious objection to the two immoveable 

 eccentrics. With the four independent eccentrics the lead may be 

 varied correctly both ways it is true, still this is a rather particular 

 point, and requires considerable time to effect the alteration accurately, 

 consequently, 1 am informed, is very seldom resorted to. I have 

 hitherto been totally unaware of what H. E. has stated in his eighth 

 paragraph. 



Another correspondent (An Apprentice in Glasgow,) remarks that 

 " I have described the contrivance for working an engine with one 



eccentric as an invention of my own, although it has long been quite 

 common in that country." I certainly have described it as my own, 

 and I had every possible reason for doing so. I was not aware that it 

 had ever been applied succiss/idly, that is, exactly correct in ererif 

 point. But I am aware, and well aware too, that engines, for winding 

 purposes, have long been common in mining districts with one im- 

 moveable eccentric, and a double lever for reversing ; and I have been 

 informed that this contrivance has frequently been applied to engines 

 for marine purposes, but in both casts has failed in point of correct- 

 ness. This has been the consequence of not fixing the eccentric rods 

 at the proper angle, &c. 



Notwithstanding all that H. E. has said, he, together with the Ap- 

 prentice, appears to be in favour of the two immoveable eccentrics. 

 I remain, Sir, your's, very respectfully, 



J. C. Pearce. 



Leeds, Feb. 8, 1S41. 



ON THE CONSTRUCTION OF IRON BRIDGES. 



When we consider the superiority of iron bridges, says M. Polon- 

 ceau, in his notice of the new plan of iron bridges invented by him- 

 self, and of which the bridge of Erdre (at Nantes) affords a good spe- 

 cimen, we are astonished that so few have been constructed in France, 

 and even in England, where it is so much the custom to make use of 

 iron, and where it is so plentiful. If these bridges are compared with 

 stone bridges, it will be found that they are constructed with much 

 less difficulty, and that they are considerably less expensive, and that 

 when they have cast-iron roadways they are not inferior, if not su- 

 perior, to them in durability. In fact, cast-iron is more durable and 

 more strong than stone ; it is better adapted to bridges with large 

 arches, because the weight of an arcli in iron being much less than 

 that of an arch in stone of the same span, the destruction of the piles 

 and abutments is less to be apprehended, and on this account can be 

 constructed at less expense. 



Compared with wooden bridges, bridges of cast iron cost about a 

 half less than bridges of that kind which liave abutments in stone ; but 

 their duration is indefinite, and the keeping wooden bridges in repair 

 is attended with great expense, while the cost of repairing iron 

 bridges is a mere trifle. The difference of expense between solid iron • 

 bridges and that of well executed suspension bridges is not so con- 

 siderable as might be supposed. 



In endeavouring to explain the causes which have prevented these 

 kind of bridges from being more generally used, continues M. Polon- 

 ceau, we discover three principal ones which have been unfavourable 

 to their general adoption. 



First — The great expense of iron, and the uncertainty in the cast- 

 ing of the larger pieces, before the year 1830. 



Second — The great expense of the only two iron bridges con- 

 structed in France before that time. The cost of the Pont des .Arts 

 amounted to ?UO,OOUf., and that of Austerlitz to two millions and a 

 half, not including the approaches. 



Third — The accidents and repairs required by these two bridges. 



Those works of art were constructed on two entirely opposite 

 principles. In the bridge of Austerlitz the arches, and the triangular 

 pieces above them which support the roadway, are composed of por- 

 tions of the arcs in frame-work, and are attended with all the incon- 

 veniences consequent on this plan ; and further, these frame-work 

 pieces are small, much ornamented, and are of unequal thickness, and 

 to this may partly be attributed the accidents which take place. 



The plan of construction adopted in the Pont des Arts, which is 

 composed of large arches connected together by pieces of iron, is 

 more rational ; but the principal arches are not sufficiently strong, and 

 owing to the variations in the thickness of the castings, the metal con- 

 tracts and expands unequally. In each of these bridges durability has 

 been sacrificed to lightness' and elegance, which occasions frequent 

 fractures in the least durable parts, 



Southwark Bridge, in London, one of the most remarkable of the 

 kind, is composed of portions of arches, like the bridge of Austerlitz, 

 but those are plain, and are not carved, although they are more than 

 two metres high, and the method on which they are arranged is much 

 superior to that adopted in the bridge of Austerlitz. The strength 

 and entire preservation of the Southwark bridge is to be attributed 

 entirely to the great quantity of iron used, which was procured at 

 enormous expense, and amounted to more than fifteen millions of 

 francs. It is probable that the great expense of this beautiful struc- 

 ture has prevented its being imitated. 



The natural consequence of what has been stated is, that it is 



2 



