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hence, when both engines are in their most disadvantageous positions on a curve, the sines of the angles they form with the rails will be nearly as ~ to — -j- , and when these angles are equal to each other, we have 2 r + ~r =: ^ + -rvr,> or r = ’ 33d 2 r 480 161280X3 -——— =r 1080 feet = 5G0 yards. That is to say, that supposing 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 in 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 560 yards; on curves of a less radius the four-wheeled engine begins to have the advantage. 1 am, Sir, Brereton, Your obedient servant, Feb. 6th, 1841. W. H. Barlow. IMPROVEMENT ON ECCENTRIC RODS. Sir—Among the numerous readers of your highly esteemed Journal) 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 in 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 different; 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 instantly 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. 1 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, 1 will admit of “the lead being de termined 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 independent 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 fixed, therefore, you cannot in crease it one w r ay without diminishing it the other, this H. E. has pointed out to be the most 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, I 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 successfully, that is, exactly correct in every 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 cases 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, 1841. 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 arch 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 nave 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 execnted 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 Pout des Arts amounted to OOOjOOOf., 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 dts 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, winch 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 O 2