holders have to look to for good dividends-in some cases, indeed, the only thing-is economy in working the costly monopolies they have secured to themselves,-every fact bearing on this point is deserving of the most serious attention. We have reason to believe that there is yet a great deal to be done in this way; that is to say, if railway proprietors, warned by the past, will only be true to themselves, and look after their own. What, for example, can be more inexcusable, or more easy of remedy, than that the cost of engine power should on one line be double what it is, under the same, or nearly the same, circumstances, on another? Or that, on the same line, the consumption of some engines should be twice that of others? Why should not the engines of every line be worked at the least possible cost, and none but the most cheaply worked engines be in every case employed? The replacing of old engines by new may require an investment of additional capital not always at the command of railway companies; but wherever no insuperable obstacle of this sort exists, a proprie tary cannot consult their permanent interests more surely than by getting rid of the wasteful gluttons of their engine establishment as speedily as possible. The savings of two or three years would, in most cases, more than suffice to replace all the capital required. We have been led to make these remarks by what an engineering friend of ours witnessed the other day, in the course of a professional visit to the Brighton line. He went down with one engine, and came up with another, by different makers. The first was stated to be of the ordinary sort, and exhibited nothing in its performance which would lead one to doubt the correctness of the statement. The second was represented as being the crack engine of the line, and is called the Satellite. She drew a well filled train of nine carriages, the gross load, including engine and tender, being about 75 tons; and on the heavy parts of the line from Brighton to the first summit, where the rise is about 20 feet per mile, or 1 in 264, she went steadily, and without any apparent straining, at the rate of 30 miles an hour. Our informant was told that, on other occasions, the engine had dragged no less than seventeen carriages over the inclined planes, at the rate of 28 miles an hour, and that in more than one instance she had gone from London to Brighton at the rate of nearly a mile a minute, for the whole distance. It is now nearly a year since the Satellite came into active service (23rd of December, 1841), and since that period she has gone nearly 30,000 miles without requiring any repairs whatever-going off duty, in her turn, one week in six, but merely for the purpose of overhauling and cleaning. Nor is the economy with which the engine is worked less remarkable than her power and speed. Her average consumption of fuel is only 20 lb. of coke per mile, with a train of eight or nine carriages (the average number), or about a quarter of a pound per ton per mile. Every person familiar with railway statistics knows that such rates of performances and of expenditure are exceedingly rare, if, indeed, they have been ever before equalled, for so long a continuance. The average consumption of fuel upon railways cannot be taken at less than 40 lbs. per train, per mile; for, although some of the engines of the best makers consume considerably less, there are many which require a great deal more. On this very Brighton line, for example, it appears, from the enquiries which our informant made, that the consumption of the Satellite is less by one half than that of any of the other engines employed upon it. The questions, then, which naturally arise out of this state of facts are theseWhy is the Satellite the only engine of the sort employed on the Brighton line? and why are there not engines as good as the Satellite on all the other lines? Is a saving of 100 per cent. in the fuel account -equal, probably, to about 5004. per engine per annum-a thing not worth caring about? The superiority of the Satellite, we understand, is not owing so much to any peculiarity or novelty of construction, as to the judicious arrangement of ordinary forms of construction, and to the excellent style of workmanship, in which the whole has been turned out from the workshops of the Messrs. Rennie, by whom she was built. The centre of gravity is placed low, and the back pressure is much less than usual. "I never," says our informant, himself a railway engineer of extensive experience, "have yet seen so well finished a locomotive engine, or one so well proportioned, and with the parts exposed to strain so admirably strengthened and disposed; nor did I ever ride on one so extremely steady at all rates of speed, from the lowest to the highest." PROGRESS OF THE SCREW SYSTEM OF PROPELLING IN AMERICA. We mentioned not long ago, (p. 181 of our present volume) that Capt. Ericsson was prosecuting with great success in the United States the adoption of the Screw Propeller, which he first brought out and patented in this country; but the extent of that success proves to be much greater than we then supposed. We stated that the number of vessels to which it had been applied was eight: but it appears from documents which have been submitted to our inspection, that the actual number is thirteen, including a government steamer of the first class. The vessels are: The ROBERT STOCKTON, tug boat, 70 feet long, 10 ft. beam, and 8 ft. hold, employed on the Delaware and Schuylkill. The CLARION, 250 tons, wrecked on the coast of Florida. The VANDALIA, CHICAGO, OSWEGO, VULCAN, 90 feet long, 21 ft. 6 inch beam, and 8 feet hold. Employed on the Lakes. 100 ft. long, 23 ft. IRONSIDES, beam, and 7 ft. hold. ANTHRACITE, River Delaware, RaBLACK DIAMOND, ritan Canal, &c. PROPELLER, the St. Lawrence and Rideau Canal. 100 ft. long, 18 ft. 6 inches ERICSSON, beam, 6 ft. draught, Philadelphia & Baltimore station PRINCETON, Government steamer, 680 tons, building at Philadelphia. iron vessel not yet named, of 80 ft. long, 14 ft. 4 inches beam, and 6 ft. hold, building by the Messrs. Worthington of New York. Arrangements are also stated to have been made (October 1842,) for building two more vessels similar to the Ericsson, which are to be placed "next spring" on the same station. The engines of the Princeton Government steamer, as well as the screw propeller, have been constructed from designs furnished by Capt. Ericsson, and under his immediate superintendence ; and in a letter which we have now be fore us from the makers, Messrs. Merrich and Towne of the South wach Foundry, they are thus described: "The engines consist of two semi-cylinders laid side by side; the piston shafts lay horizontal, and upon them is fixed a piston or leaf, which is to vibrate 90 degrees. The semi-cylinders are 8 feet long and 3 feet radius. The main shaft passes under and between the semi-cylinder, through a stuffing box in the dead wood of the ship. The propeller is fixed on the end of the shaft in a space left between the dead wood and the stern post. The two piston shafts have fixed on their outward extremity each a crank which vibrates with the piston, and are connected by a pitman with the crank of the main shaft. The centres of the three cranks form a triangle. "These engines are nearly finished, and are now being put together for inspection in our establishment. "The vessel has been delayed from circumstances beyond Captain Ericsson's control, but it is expected that she will be ready to launch in the early part of next summer.' A letter, or rather certificate, dated the 27th of October last, by Mr. Peter Hogg, of the firm of Messrs. Hogg and Delamater, steam-engine manufacturers of New York, furnishes the following additional particulars respecting the boilers of the Princeton and her propellers. "The boilers lately manufactured by the said firm for the steam frigate Princeton, are three in number, each of 26 feet in length, 7 feet wide, and 9 feet 6 inches in height. Agreeable to a contract made with the United States Government, the firm are also manufacturing the propeller for the said steam frigate according to Capt Ericsson's drawings, and under his direction, which said propeller is made entirely of composition metal, (nine parts of copper to one of tin,) and measures 14 feet in diameter." Messrs. Merrich and Towne make some observations in their letter respecting the peculiar application of stern propellers to coast and canal navigation, which are also well worth quoting. "The success of this vessel (the Ericsson) and four iron vessels, built by Capt. Stockton last spring, with the propellers, has satisfied us, and many well-informed parties, that the introduction of the "Ericsson Propeller" will in a short period completely revolutionize our coast trade, by the introduction of steam instead of sails. "The peculiar formation of the coast, being filled with deep indentations connected The attention of the student is next directed to the "Strength of the Materials" of which machines and buildings are composed. The " elasticity" of bodies is shown to depend on this law, that "the force necessary to keep a body extended or compressed is proportional to the amount of the extension or compression; so that each equal increment of the extending or compressing force produces an equal increment of its extension or compression." From an extensive series of experiments made by Mr. Barlow, on iron bars of different qualities, he deduced the conclusion, that a bar of iron of mean quality may be assumed to elongate by 100 millionth parts, or the 10,000th part of its whole length, under every additional strain of one ton per square inch of its section. The French engineers of the Pont des Invalides assign 82 millionth parts to this elongation; but Professor Moseley thinks it probable that these experiments were made upon iron of an inferior quality. Cables of iron wire elongate, according to Vicat, by 91 millionth parts; and bars of oak, according to Minard and Desormes, by 1,176 millionth parts. The strain, however, in all these cases, is supposed to be applied by equal increments; for it is further established, that if the whole strain, corresponding to any particular degree of elongation, be put on at once, twice as much work will be done upon the bar as is expended on its elasticity. Of this Mr. Moseley gives the following striking illustration, to which we would beg to invite the particular attention of all concerned in the working of steamengines. "The mechanical principle involved in this result has numerous applications; one of these is, to the effect of a sudden variation of the pressure on a mercurial column. The pressure of such a column varying directly with its elevation or depression, follows the same law as the elasticity of a bar; whence it follows, that if any pressure be thrown at once, or instantaneously, upon the surface of the mercury, the variation of the height of the column will be twice that which it would receive from an equal pressure gradually ac cumulated. Some singular errors appear to have resulted from a neglect of this principle in the discussion of experiments upon the pressure of steam, made with the mercurial column. No such pressure can, of course, be made to operate, in the mathematical sense of the term, instantaneously; and the term gradually has a relative meaning. All that is meant is, that a certain relation must obtain between the rate of the increase of the pressure and the amplitude of the motion; so that, when the pressure no longer increases, the motion may cease."Page 490. The general rule laid down by the Professor is, that the work expended on the elongation of a bar should vary only as the square of the strain and the length of the bar, and inversely as the area of its section; and, by applying this rule, the engineer may always determine the amount of work expended prejudicially upon the elasticity of the rods used for transmitting work in machinery under a reciprocating motion-pump-rods, for instance. A sudden effort of the pressure transmitted in the nature of an impact may make the expenditure of work double that which the above rule shows to be necessary. In treating of rupture by elongation, Mr. Moseley is led to a discussion of the theory of Suspension Bridges. As these structures have been hitherto commonly designed, the chains have had one uniform section. The author demonstrates that this is "false in principle;" and that, if it is required to build a bridge of "uniform strength," and therefore with "the greatest economy of material," the area of the section of the chains should "increase from the lowest point towards the points of suspension where it is greatest." The readers of our work will instantly recognize, in this important conclusion, the distinguishing principle of the suspension bridge invented by Mr. Dredge, which we have so often had occasion to bring under their notice; and will, doubt less, be as much surprised as we have been, (a surprise, on our parts, not unmixed with sorrow,) to read the following note, which Mr. Moseley has appended to his enunciation of the new principle. "This variation of the section of the chains is exhibited in a suspension bridge recently invented by Mr. Dredge, and appears to constitute the whole merit of that invention."-Page 543. The tone of this note is palpably slighting and disparaging; and, exactly to the extent which it is so, is most unjust. True, Mr. Dredge did but discover that the section of the chain should diminish from the highest to the lowest point; but, in discovering that, he discovered all that is confessedly of most importance in the erection of structures of this class. He discovered this principle, too, before any thing similar had been evolved, either by the practice of engineers or by the cogitations of mathematicians. What one of the first of engineers, Telford, missed, in the greatest of all his works, a person wholly unknown before to the engineering world has had the good fortune to find; what all the profound learning of the Whewells and the Powells of the schools, had failed to bring to light, has been revealed to the world through the humble medium of the self-taught experimenter of Bath. It may be that the happy thought came to him, not through a long vista of mathematical symbols, (as, in truth, but few happy thoughts come,) but simply from contemplating the taper form of his fishing rod; but surely it is not for a follower of the illustrious observer of the fall of the apple to sneer at Mr. Dredge on that account. Mr. Moseley does not fail to point out in his Preface, (page xvi.,) the beautiful simplicity which the principle of the diminishing section has introduced, for the first time, into the theory of the suspension bridge; and common justice, if not gratitude, demanded at his hands a frank recognition, in the same conspicuous place, of the claims of its discoverer. We say gratitude, and repeat the word emphatically; because it is a curious fact, that, as long as engineers continued to make the chains of their suspension bridges with one uniform section, the mathematicians never discovered that there was any thing wrong in that practice, but, on the contrary, made it the foundation of all their theories on this subjecttheories which, proceeding on false data, were, of course, good for nothing; and because, if Mr. Moseley has been able to produce a simpler and truer theory of the suspension bridge than any of his predecessors, it is entirely owing to the individual whom it is the tendency, if not the object, of the foot-note we have quoted to discredit and injure. We gladly pass from the case of Mr. Dredge and the Suspension Bridge to another, in which, though strikingly and essentially alike in all its circumstances, Professor Moseley has seen fit to pursue a directly opposite course. We allude to that of Mr. Eaton Hodgkinson, and his experiments on the Strength of Columns. Here, as in the case of the Suspension Bridge, the "Mathematics and Mathematicians" were all at fault, till" Practice and Practicians " came to their aid. "The hypothesis," Mr. Moseley admits, " upon which it has been customary to found the theoretical discussion of the subject, is so obviously insufficient, and the results have been shown by Mr. Hodgkinson to be so little in accordance with those of practice, that the high sanction it has received from labours such as those of Euler, Legrange, Poisson, and Navier, can no longer establish for it a claim to be admitted among the conclusions of science." Again" for all the knowledge on this subject, on which any reliance can be placed, the engineer is indebted to experiment." And farther-" In treating of the strength of columns, I have gladly replaced the mathematical speculations upon this subject, which are so obviously founded upon false data, by the invaluable experimental results of Mr. E. Hodgkinson, detailed in his wellknown paper in the Philosophical Transactions for 1840." For that paper the Royal Society, with excellent judgment, awarded to Mr. Hodgkinson the Royal Medal; and possibly it may be owing to that circumstance that Mr. Moseley sees a merit in the "Practician" Hodgkinson, which is so dimly discernible to him in the clever, but unmedalled "Practician" Dredge. While adverting to the services which practicians have rendered to science, we must not here omit to quote the very proper notice which Mr. Moseley takes of the share which another eminent individual of that class had in Mr. Hodgkinson's experiments. "The experiments were made at the expense of Mr. Fairbairn, of Manchester, by whose liberal encouragement the researches of practical science have been in other respects so greatly advanced."-Page 578. Let us now see which are the chief practical results for which we are indebted to Messrs. Hodgkinson and Fairbairn. Leaving out the mathematical formulæ in which the Professor has, (for his scholarship's sake, we suppose,) enveloped them, they are these: "In all cases the strength of a column, one of whose ends was rounded and the other flat, was found to be an arithmetic mean between the strength of two other columns of the same dimensions, one having both ends rounded, and the other having both ends flat. "The above results apply only to the case in which the length of the column is so great, that its fracture is produced wholly by the bending of its material; this limit is fixed by Mr. Hodgkinson, in respect to columns of cast-iron, at about 15 times the diameter, when the extremities are rounded, and 30 times the diameter when they are flat. In shorter columns, fracture "takes place partly by the crushing, and partly by the bending of the material. * * * "It was found that the strength of columns of cast-iron, whose diameters were from one-and-a-half times to twice as great in the middle as the extremities, were stronger, by one-seventh, than solid columns containing the same quantity of iron, and of the same length, when the extremities were rounded; and stronger by one-eighth, or oneninth, when the extremities were flat, and rendered immoveable by discs. * * * "Calling the strength of the cast-iron column 1000, the strength of the wroughtiron column will, according to these experiments, be 1745; that of the cast-steel column, 2518; of the column of Dantzic oak, 108.8; and of the column of red deal, 78.5. * "It results from these experiments, that the strength of short columns of wet timber to resist crushing is not one-half that of columns of the same dimensions of dry timber."-Page 579. (To be concluded in our next.) WOOD-PAVING. Sir,-To arrive at truth, it is necessary to look at every side to a question; and if the letter of "Junius Redivivus" were left unanswered it might lead us from that desirable point. He states that in 1834 he gave three several reasons for doubting the success of wood-paving, founded on the assumption, that none but the hexagon block would be used; all which reasons have proved erroneous. How, indeed, should they prove otherwise? What can the swelling of blocks, or the rotting of blocks, or being stolen like farmers' fences, (the three reasons,) have to do with the shape of the blocks? He finds, after a lapse of eight years, there are good reasons why wood-paving should become general; and he is right in his conclusion, but not so in his views as to carrying out the improvement; and as every error in so important a question is calculated to do some harm, I will trouble you with a few observations. The alleged drawback of the slipping of horses, has proved as fallacious as all other fears on this subject. It is a fact now rarely questioned, that horses used to wood can run as securely as on stones; nor will it be difficult to understand this, if we recollect our own awkardness, when for the first time trying to walk the ice on skates. Besides, the alteration in the construction of the shoes is at this moment keeping pace with the paving. The swelling of the timber, stated by your correspondent to be an evil, is proved to be the very contrary. In the instance of St. Giles' Church, the primary evil was, that the patentee depended on the curb-stones for abutments, to form a sort of arch, leaving no room for swelling. Such disruption has never occurred in any other instance, nor will it ever occur again unless from a similar cause. I believe no one has more studied this subject than myself, and I can safely assert that London does not contain a yard of paving, on any principle, that has not been improved by the swelling in the working of the blocks to their final settlement and adjustments, in assisting to fill up and form a mass that can be obtained by no other means. In fact, it has proved a friend to wood-paving most desirable, and certainly most unexpected. The principal error, however, of your correspondent consists in stating that the most approved plan now is with an angle of 45° to the horizon. It is a fact which |