model, must be materially altered when the design is executed at full size. When any of the parts are intended for motion a new element is introduced, from the inertia of the moving masses; and thus both the size and the velocity of our machinery are confined within definite limits. To extend these limits, it is often necessary to solve the most complicated problems of dynamics, and to follow the train of motion through an intricate series of action and reaction. We must simplify and reduce the number of moving parts, and so adjust the momentum of the inertia, that the resulting strain shall be neutralised, or reduced to a minimum: and where it is necessary that the direction of motion should be reversed, we must accomplish this object with no such sudden or violent shock as would dislocate the machinery. The difficulty of this attempt in many instances is proved by the heavy motions and hideous noises that accompany the working of almost all newly invented mechanism, and of the simplest machines found among nations less skilled than we are in the arts of construction. The approach of a Mexican waggon is announced at a distance of three miles, by the creaking of its wheels. It is only after repeated trials and improvements, that we reach the perfection of which so many striking examples are presented in our various manufactories and ateliers. When the first steamprinting machine was working off' the impression of the 'Times' newspaper at the rate of 2500 copies per hour, the noise could be heard through the silence of early morning nearly across Blackfriars bridge. At present, conversation proceeds in the very room where the type-loaded frame, of far larger dimensions than heretofore, is travelling to and fro beneath the cylinders, and perfecting between 5 and 6000 double sheets in the same time. Dr. Cartwright describes his first powerloom as requiring the strength of two men to work it slowly, laboriously, and only for a short period. We may now enter a single apartment in a Lancashire mill, and see 250 looms at full work, each throwing 150 threads a minute; while a single shaft carried along the ceiling communicates motion to the whole, and with a noise by no means overpowering. In the manufacture of

* While these sheets are passing through the press, Mr. Applegarth has succeeded in effecting a new improvement in the steam-printing machine. The chase,' or type-frame, no longer travels to and fro, but is curved into the segment of a circle, and the whole form' is placed round a cylinder, and works off the sheets by a circular and uninterrupted motion. This machine already completes 9,600 double sheets per hour; and with additional steam-power, which is in pre

paration, is expected to accomplish at least 12,000.

VOL. LXXXIX. NO. CLXXIX.

F

needles, the slender bars of steel are forged out by a succession of hammers, each one less in weight aud quicker in stroke than its predecessor. As the motion of the hammer is necessarily alternating, the dislocating effects of its momentum when thrown into rapid vibration would be enormous, but for the contrivance of giving the hammer a double face, and causing it to strike every time it rises against a block of steel placed above, from which it is thrown back upon the anvil, The vibration is thus produced by a series of rebounds, between two opposing surfaces; five hundred strokes can be made in a minute, while the power is materially economised, and the strain upon the stalk and axle nearly annihilated. But it is needless to multiply examples.

It is equally unscientific, and almost equally dangerous, to give too much strength to our constructions as too little. No machine can be stronger than its weakest part; and therefore to encumber it with the weight of a superfluous mass, is not only to occasion a costly waste of material, but seriously to diminish the strength of the whole fabric, by the unnecessary strain thus produced upon the parts least able to bear it. This fault is one which is most frequently discoverable in new machinery; and which when once adopted in practice, retains its hold with the greatest inveteracy. It requires no common powers of calculation, and not a little faith, for men to trust to the safety of structures which have apparently been deprived of half their former strength.

There can be no better proof of the difficulties which oppose the adoption in practice of any new principle of construction or configuration, than that exhibited in the history of Ship-building. In no creation of human labour was it more necessary to secure the greatest possible strength from the minimum of material; as none were required to possess such vast bulk in proportion to their mass of resistance, or were exposed to more violent varieties of strain and shock, in the natural course of their service.

The men who superintended the public dockyards were often well versed in mathematical science; and were certainly acquainted theoretically with the common axiom, that among right-lined figures, the triangle alone will preserve its form invariable by the rigidity of the sides, without depending upon the stiffness of the joints. Yet none until a recent period, worked out the axiom into its very obvious practical development. For centuries were our ships constructed on principles which caused the whole frame-work to be divided into a succession of parallelograms. Every series of the timbers, as they

were built up from the keel to the decks, formed right-angles with their predecessors and with their successors; so that the whole fabric would have been as pliable as a parallel ruler, but for the adventitious firmness given by the mortices, bolts, and kneepieces. At least three quarters of the available strength of the materials was possibly altogether thrown away. The safety of the whole was made to depend upon its weakest parts; and when decay commenced through process of time or the action of the elements, every successive stage in its advance made the progress more rapid, since the wear and friction increased in double proportion as the fastenings became weak and loose.

Sir Robert Seppings at length succeeded in vindicating the claim of the shipbuilder to be ranked among the members of scientific professions. By the introduction of the diagonal 'truss,' the innumerable parallelograms formed by the hull and frame timbers were converted into triangles: And the limits of the magnitude, the strength, and the durability of the wooden walls of England were thus largely extended. The faults of hogging,' and 'sagging,' which had formerly revealed the weakness of the fabric, often at the first moment of its launch, were almost annihilated; and the huge machines no longer bent under the strain of their masts or the weight of their batteries. But Seppings, after all he had done or projected, could have formed no conception of the vast advance which was ere long to be effected in his favourite art by the introduction of a new material. No possible combination of science and skill could enable him to give to his timber-built ships the magnificent proportions of the Great Britain, together with strength sufficient to encounter the billows of the Atlantic. Still less could he have conceived it possible that such a vessel might be consigned, through a series of mistakes and mischances, to the inhospitable keeping of a storm-vext Irish beach, throughout an entire winter, and yet afterwards be dragged from its shingly bed, and towed into port with only a net result of very reparable damage.

Among the properties of matter are some that we may term subsidiary or incidental: qualities which we may be said to discover rather than to comprehend; and whose agencies are of a secret, and as it were stealthy character, so that we cannot always predict their recurrence or calculate their force.

Fluid and gaseous bodies present many instances of these perplexing phænomena. While investigating the conditions under which solid substances enter into solution; the rise of liquids through capillary cavities; the motions of camphor and other bodies when placed on the still surface of water; the

phænomena of crystallisation; the condensation of gases in charcoal; or the inflammation of hydrogen when in contact with minutely divided platinum,-in these and similar cases, we encounter on every side a series of anomalies which as yet baffle all our efforts to group the incoherent facts into a consistent theory. For the present, therefore, we must content ourselves with the functions of empirics and registrars. We must observe and collect the facts which may hereafter furnish a clue to the labyrinth; confident that when that clue is once seized, every step will not only bring us to some result of practical utility, but will reveal yet another example of the divine symmetry of nature.

Upon this point, Paley has allowed himself to be betrayed, by his course of argument, in his Natural Theology,' into a singularly false assumption. In his day the four ancient elements, Earth, Air, Fire and Water, still in quaternion 'ran,' although philosophers had already seen that it was high time that this category should be reformed. Notwithstanding which, like so many other benevolent writers, he was anxious to console men for their ignorance; and consequently he declared that of these elements, as it was not intended so it was not necessary, and might not be useful, for us to know anything further. Referring then to one of them, Water, whose decomposition and constituent elements were at that moment making some noise in the world, he says: • When we come to the Elements, we take leave of our mecha'nics; because we come to those things of the organisation of which, if they be organised, we are confessedly ignorant. This 'ignorance is implied by their name. To say the truth, our 'investigations are stopped long before we arrive at this point. But then it is for our comfort to find that a knowledge of the • constitution of the elements is not necessary. For instance,

as Addison has well observed, "we know water sufficiently, "when we know how to boil, how to freeze, how to evaporate, "how to make it fresh, how to make it run or spout out in

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any quantity or direction we please, without knowing what "water is." The observation has even more propriety in it now, than at the time it was made: for the constitution and 'the constituent parts of water appear to have been in some measure lately discovered; yet it does not, I think, appear that we can make any better or greater use of water since 'the discovery, than we did before. Or, in other words, that the discovery of the chemical constitution of the fluid would not prove useful, because it had not been immediately followed by any mechanical application of extended and striking use.

It

should not have required the splendid contradiction which time has given to this assertion, to have satisfied such a man as Paley how unphilosophical was his deduction, even from his own assumed premises.

The various questions which suggest themselves relative to these properties of fluid and solid bodies, are finally resolvable into a single inquiry, touching the absolute nature and condition of a constituent atom. Hitherto the ultimate atoms of bodies have eluded all our attempts at identification. Our most powerful microscopes have failed to render them perceptible: nor are we able, by any process or contrivance, so to separate an individual from the mass as to be entitled to pronounce positively that it possesses any definite form, weight, colour, or magnitude; or indeed any single quality, either chemical or mechanical. Not one of its properties can we discover directly. A few we have inferred-but even of our inferences we assume neither their certainty nor their correctness. Hypothetically we speak of the atom as a minute sphere; perfectly indivisible and consequently unchangeable in form, and incompressible in substance; because the deductions from a multitude of observed facts render the supposition of these properties a matter of necessity. We must moreover conclude that in no known substance are the contiguous atoms in absolute contact; because we have never yet ascertained the limit of condensation from decreased temperature or mechanical pressure.

To follow out this hypothesis, we must then imagine every atom to be surrounded with no less than three consecutive strata or atmospheres of antagonistic forces, extending nevertheless in the aggregate to a distance altogether inappreciable. The innermost stratum consists of a force of repulsion so enormous in its strength that no two atoms can be forced into actual contact; around this is a stratum of attractive force, of very finite action; giving their power of cohesion to all the visible particles of matter: and, last of all, is an outside stratum of repulsion, which prohibits the parts when once separated from again cohering (except under particular conditions) even when forcibly pressed together. The extreme tenuity of these strata may be inferred from the fact that two surfaces may be brought so closely together as to render the interval imperceptible by any of our senses; and yet as no cohesion takes place, it is evident that the atoms cannot have been brought within the circle of the exterior atmosphere of repulsion.

Under the influence of an increasing temperature, the two external strata of repulsion and attraction appear to become modified and diminished until, when a certain point of heat is