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whose centre of gravity is over this axis, will only produce a downward pressure upon the lower end of the pivot; but that if the centre of gravity be to one side, as at the moment when an engine is passing on to the table, a horizontal, or nearly horizontal force will act upon the pivot, tending to produce lateral motion in it

round its lower point as a centre, or to break it transversely. To counteract this latter force is the object of the upper collar and bearing collar; between these, into the annular space, left by reason of their different diameters, is dropped a circular wrought-iron frame, consisting of two rings of flat bar iron, carrying six turned

cast-iron rollers, revolving horizontally between these collars, upon wrought-iron pins which pass through and connect both wrought-iron rings. The weight of these rollers and rings is sustained by four small rollers, which are fixed as to position, and revolve vertically in pairs, cast in brackets, projecting inwardly from the turned bearing collar.

Now, if the platform be adjusted level, the pivot therefore vertical, the roller frame in its place, and the rollers themselves nicely filling the interval between the collars, and the hollow cylinder below filled with water, beneath the toe of the pivot or plug, it is obvious that the system will be in a condition to sustain a load, however disposed upon the platform, and that the latter can revolve with it, the only resistances being, the direct and secondary, or rolling friction of the collars and roller frame above, and the friction of the leather gland and fiuid support below. The lateral force, if any, is sustained by the rollers and gland; the vertical is supported by the fluid in the cylinder.

The resistance due to friction of the latter, or vertical support, is so small, that we have no experimental data to estimate it by; it may practically be considered as nothing; the resistance will therefore be the friction of the rollers, and of the gland, and this will be much less than in the ordinary table or vertical rollers; for, if the height of the pivot, or distance between the bearing collar, and lower gland, be equal to the radius of the table, it may be easily shown that the strain, either upon the collar or gland, can never equal the whole weight of the body, i. e., of the engine coming in upon the table; while, in the ordinary table, the pressure on the rollers is always equal to the whole weight, but the friction of the rollers is in proportion to the pressure, and hence, is under the most disadvantageous circumstances in favour of the new table; while, when the engine is fairly in upon the table, and in a condition to let it revolve, it is nearly balanced, and hence, when in motion, nearly all friction, by its pressure, is removed from the rollers-and also from the gland below. But further, supposing the force to produce rotation, applied in each case at the circumference of the table, the resistance due to friction of the rollers will be in proportion to the radius

of the roller frames, or distances from the common centre of motion, with equal pressures; but in the new table, the roller frame has only about one-third the diameter of the table, whereas, in the ordinary case, the roller frame is nearly equal to its full diameter.

Thus, force is economized, not only by having a smaller resistance to move, but this resistance to move through a smaller space; and thus it becomes easy, by the application of the bevel gear, shown upon the section as applied to the vertical pivot, &c., to transfer motion to the table by shafting from a distance, by the power of a single man acting upon a small hand wheel and vertical shaft, in the same way that the points, or switches, are usually turned on railway lines. The power of doing so would often be found advantageous, as economizing labour and time, and in crowded or important stations would have the further advantage that all the turn-tables might be attended to, and worked by one man, and at one spot.

The masonry of this form of table is not more costly than that of the ordinary one, nor would the weight of castings, or the amount of workmanship, be greater. There are only two points requiring adjustment from the effects of use, viz., the outer bearing collar, which is made adjustible by keys and bolts, between the diagonal struts, or may be made adjustible in other ways, and the lower gland, the adjustment of which has been already described. From the nearly absolute stanchness which Bramah's leather collar ensures, as seen in every hydraulic press under pressure ten or twenty times greater than here required, there is no reason to apprehend escape of water from the lower or sustaining cylinder; but should such ever be the case, it is capable of being replenished with ease by means of a small hand pump, temporarily applied to the small tube, with the screw-plug valve before alluded to; and this also gives the power of adjusting the table as to level, both at first, and subsequently, if required.

Fig. 2 gives on an enlarged scale a plan of the roller frame and six rollers. The lowermost flat wrought iron ring is quite fair and flush on the under side, the roller pins being countersunk, to allow the whole to traverse freely upon its four small supporting rollers, before alluded to.

In proposing this modification of turntable, I would observe, that its merits, if any, consist only in choice and adaptation of parts; the collar of rollers has been long used for heavy cranes, as in those made by us for the Limerick docks, and the fluid pivot was invented and applied by Mr. Bramah to his planing machine, still in use in Woolwich Dockyard for shaping gun carriages, &c.

ROBERT MALLET, Member Institution C. E.

Dublin, August 4, 1842.

Description of the Engravings. Fig. 1, a surface of table; bb, the ribs of frame; c, the hollow conical pivot; d, the collar; e, the outer, or bearing collar; f, the roller frame and rollers; g, the small supporting rollers; h, the struts; k, the vertical holding-down bolts;, the strut ring; m, the water cylinder, or fluid pivot; n, the leather gland; o, the supply pipe; P p, the moving gear; S S, the masonry.

Fig. 2, c c, the hollow conical pivot; e e, the outer or bearing collar; ff, the roller frame and rollers.

ON THE CONSTRUCTION OF FURNACES AND BOILERS. BY C. W. WILLIAMS, ESQ.

[Continued from page 136.]

In my last communication I referred to the two main causes for the little advance that had been made in obtaining some fixed and satisfactory data as to the principles, scientifically considered, which should govern us-first, in the construction of furnaces with the view of obtaining the greatest quantity of heat by the most perfect combustion of the fuel employed-and, secondly, in the mode. of testing the relative efficiency of such furnaces, and the degree in which they were aiding in producing such perfect combustion. As to the first-the construction of furnaces-those who have written on the subject, (and Tredgold in particular,) have confined themselves to the giving empirical rules, which have no better foundation than some supposed relation between the dimensions of their several parts, but which relation could be traced no farther, or more satisfactorily explained, than by reference to some inaccurate and often discordant experi

ments. The inferences drawn from such were therefore, necessarily, as vague and various as the circumstances under which they were made.

What was thus done with respect to furnaces, and the rules laid down for their construction, also took place in what regarded boilers. The rules for the latter, made without a knowledge of, or even reference to, the chemical process of vaporization, or the natural laws which govern the conduction and transmission of heat through metallic or liquid bodies, were, like the former, based on some imaginary or assumed relation of proportions among the several and respective surfaces and arcas, mechanically considered.

In both cases, too, this doctrine, or implied harmony of parts, being deduced from what was called " practice," became thus clothed with the supposed character of certainty, or matter of fact, and thence was received as infallible, in contra-distinction to the dictates of science, which were rejected as being merely spe

culative or theoretic.

Yet this furnace and boiler practice-this very thing which demanded improvement and regulation-was in fact nothing more than the rudest and most slovenly operations, conducted without reference to chemical principles, and often in direct opposition to the best established chemical truths. Nevertheless, the results and inferences drawn from such have been, by several writers, worked up into rules, and issued with all the authority of established practice.

With respect to the second point above referred to the mode of testing the principles on which furnaces may be constructed, and the degree in which such construction may be found to aid or obstruct the natural process of combustion -I stated, that the only test hitherto applied was, strange to say, not anything connected with such furnace, or the processes to be carried on in it-with the quantity of heat generated, or temperature of the products from the fuel-but a test drawn from a totally different process, conducted in a totally different class of vessels, namely, the process of evaporation, carried on in an iron boiler. This boiler, be it also remembered, with all its good or bad evaporative powers and peculiarities, was itself as little understood, and is to this hour as much the

subject of examination and improvement, as the furnaces themselves, of the relative value, efficiency, and economy of which, however, it was assumed to be a certain and safe test and standard.

Referring this latter branch of the subject to a future communication, I will on the present occasion confine myself to the examination of the first of these sources of error and confusion, that is to say, to the "rules" laid down for the construction of furnaces and boilers, and the dangerous, though prevalent mistake of basing such rules on mechanical and mathematical, rather than on chemical considerations and principles.

It may here be asked, in limine, what connexion or necessary relation there is between mere mechanical proportions, as regards furnaces, and the processes to be carried on in them, so as to authorize our considering their construction as dependant on the former, rather than the latter? Yet we have abundant references to such details, and to the necessity for accuracy in such relative proportions, but none which examine or explain the really desirable object, namely, their connexion, chemically, with those processes. And are not combustion and evaporization two processes peculiarly referable to chemical, rather than mechanical considerations? Would it not be a more rational proceeding to construct the vessels, in which these distinct processes are to be conducted, with reference to their uses rather than their forms?

Where shall we find, in the whole range of authorities, any chemical investigations or manipulations decided by reference alone to the sizes, surfaces, or cubical contents of the retort or other apparatus to be employed, rather than to the nature of the ingredients employedtheir solid, liquid, or gaseous charactersand the conditions under which their union or action are to be effected? Yet combustion, is essentially and exclusively chemical action and chemical combination.

Indeed, we find none hardy enough to dictate or recommend such an absurd proceeding in the laboratory; but the moment the same operations and processes are to be carried on in the manufactory, and on the large scale, the reasonings and inductions of science are to be at once discarded; then, every tyro or ignoramus, down to the common

stoker, is supposed to be a safer guide, and have a sounder opinion, than those who go into the inquiry with caution, and respect for chemical authorities and truths; and, by examining the laws and conditions of nature, as regards the fuel or other materials to be employed, are enabled to judge how far the sizes, proportions, and details of the furnaces or other vessels bear on such laws, and can be made instrumental in their development.

And what is the operation, in respect to which the furnace is to be constructed, but an extremely complex chemical one, namely, combustion, or the chemical union of a combustible with the "supporter," or means of combustion, the oxygen of the air; and the involved considerations under which such union is to be effected?

Again, what is the operation, and the object sought, as regards the boiler, but the conduction or transmission of the largest quantity of heat, in obedience to those natural and chemical laws which govern such transmission, (itself a most difficult branch of science,) and the absorption of such heat by the liquid? And are all these involved and complex laws and processes to be set at nought, and the whole resolved by reference to mechanical proportions alone? Is it rational or safe to follow the suggestions of men, (though calling themselves "practical,' as every mere fireman or bricklayer does,) who are manifestly unacquainted with those processes, and the various and complicated conditions under which they are effected, in determining on the character or construction of the apparatus employed, or the mode in which the manipulations are to be conducted? we to reject the science of Leslie, and Davy, and Dalton, when investigating the laws which govern the transmission of heat, the cause and character of combustion and flame, and the quantities and laws of the diffusion of the gases, the very sine qua non of perfection in the operations of the furnace? Are we to consider these great men as having written and laboured in vain, or their authority as dangerous, and to be set aside, to give place to the "instinct of ignorance" of the "fireman and operative engineer," as we are gravely and offensively recommended to do in Mr. Armstrong's Essay on Boilers?

Are

But, further, it is not merely the chemical processes of combustion in the furnace, and vaporization in the boiler, and the difficult, though all-important branch of the subject which the venerated Dalton has investigated, that are to yield to this scale of proportions, mechanically and mathematically considered. No; even their joint relation to the power of the steam-engine itself is to be determined, to a fractional part, by this doctrine of proportions. Now, although this "instinct of ignorance" has had its sway and influence long enough, the absence of any determined scale of relative proportions between the furnace, the boiler, and the steam-engine power is as notorious as ever, and the practical world still calling out for information; and this, although Mr. Armstrong himself, with an admirable contempt of common sense and chemistry, informs us that the whole is an extremely simple and welldefined mathematical process, and that all we have to do is to make the number of horses' power, a geometrical mean between certain defined proportions of fire-grate and certain other defined proportions of boiler surface. Well may it be said, that

"By line and rule

Works many a fool."

But the utter discordance which prevails among mechanical engineers, even of the highest standing, (of which I will herafter give some illustrations,) notwithstanding all are still active and anxious in search after improvement and perfection: this absence of all system or accordance should long since have shaken public confidence in these dicta and "rules," which, however, have no better foundation than the kind of practice here referred to. "The best way," says the Essay on Boilers, "is to make the area of the fire-grate such, that the number of horses power of the boiler may be a geometrical mean between that area, expressed in square feet, and the area of the effective heating surface expressed in square yards. This proportion gives a convenient rule for ready application in practice." Can we, after this, wonder at the want of connexion between "practice" and chemistry in what regards the furnace and its chemical condition and processes? This "best way" is indeed a summary squaring of difficulties: for here we have the chemical process of

combustion-the conductive power of the metals-the evaporative action of fluids, and the dynamics of the steamengine, all resolved, harmonized, and dove-tailed, as it were, in the most approved method; and, secundum artem, brought into the smallest possible compass-suited to the meanest capacitypeculiarly adapted to the "instinct of ignorance"-condensed into a very nutshell. So compact, indeed, that, as we are in the same page informed, these formulæ express all the relations of those quantities, sufficiently convenient to be inscribed in a corner of a two-foot rule!" How edifying, yet how concise! Can we,

after this display, be surprised at the little advance that has been made since the days of Watt, while such pretenders and book-makers are suffered to be our guides, and such absurdities are palmed on the public under the false colouring of practical accuracy and mathematical truth?

But the error of considering the construction of furnaces and boilers, as resolvable by reference to mechanics and mathematics, rather than to chemistry, must further be pointed out before we can see our way to a remedy, and avoid the labyrinth of difficulties into which this so called "practice" leads us. Many instructive proofs of this erroneous practice may be found in Tredgold and Armstrong. I will here refer to one furnished by the latter writer, in which he draws his inferences, and takes as the basis of some notable calculations and formulæ, some loose experiments made with a "common furnace pot, or boiler of cast iron, such as are usually set in kitchens."

Of this boiler pot and furnace, with their respective functions and results, it is worth speaking,-seeing that they are taken as representing the "unit standard of steam-boiler power," and the standard by which the manufacturing world are to solve all questions of comparative combustion and evaporation. The proportions of this "boiler pot" and its cubical contents its heating surface-area of furnace-extent of draught and weight of fuel used, are given with admirable precision. These details should be given in the author's own words since they are to become the foundation of "rules" by which to determine practically, 1st, All that belongs to the heating powers of the

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