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MIRROR OF SCIENCE AND ART. FRIDAY. APRIL 15, 1870.

HINTS TO ASTRONOMICAL STUDENTS. {Continued.)

WE have in the preceding papers discussed various preliminaries with which it seemed desirable that the young astronomer should be acquainted, bnt we have not yet advanced so far as the actual trial of the instrument; nor iudecd are we even now in a position to attempt it fairly; for before we can u-e satisfactorily even the most suitable testa for the purpose, we ought to be acquainted with the exact nature of telescopic definition. Unless we have formed correct ideas as to this point, so as to knowthe degree of perfection which we hare a right to look for, we may either (as the writer was in his young days) be subjected to disappointment from the unreasonableness of our expectations, or satisfy ourselves

too readily with first appearances, which may not

be borne out by a further acquaintance with the

instrument

The peculiar character of telescopic vision is immediately dependent on the nature of light itself, the laws of which, as fixed by the Great Creator, impose an nnvaryinglimit on the degree of perfection to which the finest human workmanship can attain, for it is a demonstrable fact that even if the optician's skill could unerringly realise the formula of the mathematician, the focal image would not be an absolutely perfect

optical reproduction of the object from which it

is derived.

This may appear strange, but it is a necessary

consequence of the unduinfory nature of light

consisting of vibrations of exceeding but still finite and even measurable minuteness. It might be possible to form an idea of light which would fulfil the condition of perfect telescopic vision namely, that every point and line in the object should be represented by an exactly corresponding and similar point and line in the focal picture

in consequence of

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Us emission tnd propagation tiut law, as was

▼try ably pointed out umtiy £?/**•? J.n ^ "Cam°ri''l.'e Philosophical Transactions by the present Astronomer Royal, everv pencil of light transmitted by a lens or reflected oy a mirr..r is affected in converging to a focus bv what is caned the " interference" of its mentations. I his interference is the necessary result of its having an external boundary, whatever the lorm or dimensions of that boundary mav be In ?/**.'"" ,f """ea at the edge of ihe brass' cell, but u the cell were removed, it would be equally produced at the edge of the lens or speculum, and cannot possibly be prevented or avoided, being tne direct result of the employment of a limit in BDy way to the pencil of rays. Hence it follows that every focal image, no matter how it m»y be obtained, must exhibit the effect of the interference of undulations. This effert, though *ery minute, is n-Jt beyond the reach of investilatioii by modern annlysis, and the inquiry, as conducted by Professor Airy, has led to the following result.

The ima^e, whether reflected or refracted, of a luminous point—we will say the image of a star, for it ia well ascertained that this possesses no sensible nangnituric—is not a point, buta circular luminous disc of a certain calculable diume er, surrounded by a number of bright rings. The angular dimensions of the whole will depend on notliii g but the aperture of the telescope, and will be inversely as the aperture. The intensity of the light of the cental disc decreases rapidly from the centre: the intensity of the brightest part of

tl e first or innermost ring is of the centre of

57

placed on the centra of the lens in some degree changes the character of the aberration, diminishing the magnitude of the disc, and inoreasing the brightness of the rings, while it somewhat dimi nishes their magnitude. Such are the deductions of theory. It iB now easy to see wh«t we may expect to find in practice, as far as the trial of a telescope on the fixed stars is concerned.

The whole optical phenomenon is of small dimensions, and therefore demands the nse of a certain amount of magnifying power, which must be determined in each case by experience, as it will vary with the aperture. If an insufficient power is employed, the image of a large star will bo a sparkling, flashing point, the real character of which will not be apparent for want of sufficient angular value. As we increase our power we shall gradually find the general blaze of light develop itself into a disc of appreciable magnitude in the centre of several rings, more or fewer according to the brightness of the star, and we shall then be able to form an idea of the degree of perfection to which the workman has been able to attain. Beginning with such stars as Sirius, Weua, Arcturus, or Capella, we shall find, with a sufficient power, and in steady sir, which is a very essential condition, a considerable "spurious disc" with a succession of rings, which, if the materials are homogeneous, and the centering correct, will appear circular and concentrio; any material irregularity in form wonld probably indicate unequal density in the glass, or inaccuracy of workmanship, neither of which could be remedied; if the rings are merely thrown to one side, the fault may lie in the centering, and admits of being rectified. For a complete illustration I need only re'er to a woodcut in the number for February 11, 1870, occurring in one of the articles of " F.R.A.S." whoBe most important and valuable contributions are, I am happy to see, duly appreciated by the readers of the ENGLISH MECHANIC. As we proceed to examine in succession stars of less brightness, we shall find a decrease in the magnitude of the disc and the number of the ring", till at last with minute objects the latter disappear entirely, and the former is reduced to a mere point. This, it will be at once seen, is in strict accordance with theory. The difference is simply a subjective one, that is, depending os the power of the eye to distingnish feeble degrees of light The phenomenon exists alike in the case of every star, but the larger ones alone possess light enough for us to recognise it in anything like its completeness; as the brilliancy of the object diminishes, both the external rings and the edges of the disc become too feeble to be distinguished, till at length the centre of the disc alone retains light enough to affect the eye. It is satisfactory to find so intelligible an explanation of what might have otherwise seemed very anomalous—that the telescopic discs of the stars should appear of such very different magnitudes, th .ugh we know, from their gradual reduction through passing clouds, and their instantaneous extinction when concealed by the moon, that even the largest of them are still of no sensible dimensions. .

Another, and a very important modification of the result of interference has to be taken into account, and in so doing we shall find a fresh agreement between theory and practice. It has been already stated, as part of Professor Airy's result, that the proportional amount of interference will be greater as the aperture is less; or, which comes to the same thing, the diameter of the spurious diso will be inversely as-the diameter of the object-glass or specnlnm: and this is experimentally found to be true. The more we enlarge our aperture the smaller we shall find the discs, and hence arises the great superiority in separating close double stars, which, with equal perfection in other respects, large telescopes possess over smaller ones. We have a very remarkable instance of the accordance of theory with observation, as well as a striking proof of the extraordinary approximation to ihe limit of perfection which optical skill has now attained, in a series of experiments conducted by one of the first observers of the day, Mr. Knott, the fortunate possessor of a 7J-inch Alvan Clark object-glass. The following tablecontain* a n ean of numerous observations on several of the larger stirs, showing •f -nee the progressive increiso of disc with the

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diminution of aperture, and tie correspondence

The earliest acquaintance with this result of interference seems to have been obtained by Hevel (a name frequently Latinised into Hevelius). The worthy old burgomaster was, however, comj'letely misled by his own experiment. By stopping down his object-glass to a working aperture of tlio size of a large pea, he succeeded in expanding the diECS of the brighter stars into circles of notable diameter, and in his simplicity rejoiced in the idea that he had rendered their re.il dimensions visible. Who first pointed out this curious mistake I do not know; bnt it was not likely to pass long unchallenged, since a little acquaintance with focal images is sufficient to show that the smaller they are, the more perfectly they represent the object. He did, in fact, purposely that which every modern optician as studiously avoids—enlarging instead of reducing the spurious disc.

The Astronomer Royal's theoretical result, that the diameter of this factitious appearance depends upon aperture alone, has not been univcwally admitted. A few years ago, Steinheil, the celebrated German optician, who has attempted to rival the work of the Optical Institute in the same city of Munich, asserted that the magnitude of the spurious disc depended in part also upon focal length. This idea, however, has not been sectioned by English mathematicians; and ourgrcit observer, Dawes, than whom few mcu have been more practically acquainted with telescopes, has expressed himself as entirely satisfied of the certainty of Airy's conclusion. From this, of course, follows the known fact that, with equal materials and workmanship, the dividing power of telescopes, or their capability of separating very close double stars, varies ns their aperture. Some differences may be found among amateur astronomers on this head, which, if not duly taken into account, may perplex the student. He maypossibly hear assertions made in all good faith in some quarters which elsewhere may be looked upon with suspicion. But experience shou'd give a lenient tone to criticism. Many allowances require in fairness to be made among the observing brotherhood, who ought to look upon each other's assertions in a candid and generous spirit. We must bear in mind th it, first of all, differences of natural vision must enter into the result. I we'l remember one night when I had the especial pleasure of visiting Mr. Dawes'* observatory at Hopefield, provided at that timewith a splendid 8J-incli Alvan Clark achromatic, that his eagle-eye rapidly picked up Enceladus, the closest but one of the satellites of Saturn, in a spot where I could make out nothing ; and yet my own is a fair average sight. Then, again, differences in the sttte of the air influence very largely any result of this kind, as a little experience, or the perusal of the works of Sir J. Herschel' or Dawes, will abundantly prove. And, thirdly, there are differences in judgment. One obgtrver may consider division completely effected when he sees a dark line crossing an elongated disc; while another, following the guidance of Dawes, looks upon such a line ns a mere interference result, though of course the precursor of separation ; aud restrains the latter term to the appearance of black sky between uncompressed circular discs.

However, while we endeavour to practise in. this, as well as other matters, the mutual forbearance required from sensible and well-informed men, not to say Christians, there must of coursebe a limits—a maximum of performance—beyond which all claims must be rejected as the offspring of mistake or imagination. And this limit is supplied by the consent of two unimpeachable authorities; the one in the manufacture, the other in the use of telescopes. It has been asserted, and probably with correctness, that Dullmeyer's standard of performance is obtained by dividing4-33 by the aperture in inches, the quot ciit expressing in seconds the central distance of the stars which ought to be just divided. On th» other hand, Dawes tellsus that having asccr.ained abont five and thirty years before (this was written in 1867). by comparisons of several telescopes of very different apertures, that tl.e diame'ers of star-discs varied inversely as the apertures, he examined with a great variety of apertures a vast number of double stars, whose distances seemed to be well determined and not liable to rapid ohange, in order to ascertain the separating powers of those apertures: and he thus determined as a constant that a 1-inch aperture would just separate a 6 mag. pair at 4""5G of central distance (it will be observed that the consecutive run of these figures fixes them on the memory); and hence the separating power of any aperture a, in a moderately-favourable atmosphere, will be 4"56

expressed by the fraction . The quotient thus

a obtained concurs with Dallymeyer's result even more closely than could have been anticipated, when we bear in mind the influence of what astronomers call "personal equation," or the unavoidable differences in eyes and habits of obser vation. I once heard the celebrated Alvan Clark assert, in very exact agreement with these formula?, that a 4-inch object-glass ought to divide 6 mag. stars to 1*.

This standard, in conjunction with a table of the distances of double stars, will enable the young oftervor to form a fair opinion whether hft instrument is up to the mark. But here, again, a caution must be interposed. Many test-objects, otherwise perfectly suitable, are variable iu distance, some from binary character, others possibly from differences of proper motion ; and it therefore becomes important either to make choice of pairs whose relative fixity is pretty certain, or to depend only npon recent measures; which are not always of ready attainment. ;; Corona;, 36 Andromedie, and that formidable object £ Cancri (which I Faw beautifully, though in very unsteady air, March 31, with 450 on my !)-inch mirror) beloug to the changeable class: < Boiitis, 52 Orionis, it Aquiltc, X, Bootis, Ij Orionis, and ■y* Andromeda? (here arranged in order of distance) are much more available as showing apparent fixity. Another consideration,too, should not be omitted. The eye (a bad photometer) takes little, if any, cognisance of the progressive decrease of illumination from tbo centre of the spurious image, and regards the disc as a Hat luminous spangle, giving rise to the old comparison of an unequal pair in a good telescope to a shilling and a sixpenoe on a piece of black cloth. Yet that nn actnal diminntion of light towards the edg3 does take place, as required by theory, is shown by the fact that tho discs appear somewhat smaller and more separable on the background of a daylight or even^twilight sky, ns well as from their not being proportionally" enlarged with the increase of magnifying power. This latter circumstance, favouring the separation of close pairs, had been detected by the penetration of the elder Herschel as far back as 1762, though, as the theory had not then been investigated, he was probably not aware of the cause. This, however, is sufficiently evident. If we suppose a light of such an intensity that when reduced to one-fourth of its present brightness, it would cease to affect the eye, the doubling of the linear magnifying power, by spreading it over four times the surface, will "render it iuvisible; and thus the edges of the disc will gradually be diluted so as at last to be imperceptible. It must be admitted that this is not quite in accordance with the evidence of sense, which gives to the discs in a good telescope such an appearance of uniform brightness; but it is the only explanation to which we can have recourse. And from this it evidently follows that in employing the formula? of Dawes or Dallmeyer, regard must be had to the degree of magnifying, as wed as to the brightness of the stars to be examined.

The subject has grown upon my hands, but I trust the readers of the English Mkchanic are not yet quite weary of it, as it is still not exhausted. Before closing this paper I will add a few remarks on the great telescope at Wandsworth Common, as to which a correspondent made an inquiry some time ago. Of its present condition I know nothing; but when it was new, in 1852,1 went to see it, armed with an order from the late Mr. Gravatt, C.E., to whose good nature I was indebted on several occasions. I reached tbo spot in splendid moonlight late in the evening of October 27, when, as the instrument was then an object of attraction, I had expected to find a number of visitors ; I was, however, mistaken iu this; no one had come, the outer gate was locked, and the attendants were preparing for their rest; however, on the production of my order they were rery obliging, and showed me every attention.

The following were tho dimensions then given to me :—Height of tower, fi4ft ; diameter, 15ft.; thickness of walls, 1ft. 2in.; whole length of telescope (including dewcap), 85ft.; dewcap, 6ft. 2in.; metal tube, 76ft. 6in.; focal length, 77ft.; diameter of widest part of tube, 4ft.; weight, about 3- tons. It was >lung in a cbaiu at the thickest part, which was considerably nearer to the 0. G. than to tho eye-end j the chain passing over the roof, which turned in azimuth with the tube, and carrying a cubical iron box as a counterpoise on the other tide. The eye-end of the tube rested on a moveable frame running towards or from the tower on a wooden railway; the outer end of this in turn ran on a circular iron rail encompassing the tower, at a distance of 52ft. The movements, which had, I believe, been arranged by Mr. Gravatt, struck me as simple and easy, considering the bulk and weight of the monster. The O. G., 2ft. in diameter, had its centre stopped out by a 12-inch disc: and I saw no other powers in use but 120 and 240; the latter very bad from wrong adjustment. With tbe former, little could he said in praise of the O. G. The view of Saturn and his retinue was, of course, brilliant and impressive, but. on the whole the instrument could not be considered a success : at the same time, the spirit of such a magnificent enterprise went far to disarm criticism. It was, I think, understood at the time that the chief fault lay in the spherical aberration, but that one or both of the lenses had been worked too thin to admit of further correction. If the apparatus is still in good condition, might it not be practicable to replace the O. G. by a better one, of the same focal length, but smaller aperture, say 12 inches? the cost of which, as well as of necessary attendance aud repairs, might be defrayed by an extensive subscription of small individual sums,each subscriber having the privilege of muking use of the instrument. This is, perhaps, not a promising scheme; but the idea is thrown out in the hope that some one may improve npon it, and devise means for giving efficiency to the really valuable portion of so noble au undertaking.

A word in conclusion as to Dr. Ussher's excellent " Advice." I had no idea that these liues proceeded from so venerable a source. They were given to me many years ago by Mr. Lawson, then of Hereford, subsequently of Bath, the proprietor of a very tine 7-inch Dollond achromatic, bequeathed by him to the Greenwich Naval School. Some of our readers may perhaps be able to give an account »f the present condition and employment of this instrument, one of the finest and most expensive of its day, and which I have several times used with great pleasure in bygone yea/8. But to return to ibe verses: my edition is somewhat different, and I think with an evident improvement in the sense; the last fonr lines running thus—

Not that imparted knowledge doth

Diminish learning's store,
But books, I find, if often lent,

Return to me no more.

T. W. Webb.

ELECTRICITY—ITS THEORY, SOURCES
AND APPLICATION.

Br J. T. Si'baotje.*
(Continued from page 27.)

IN order to examine intelligentlythe wide and interesting subject of Dynamic Eleotricity or Galvanism, probably the most convenient process will be to commence with a few leading facts and principles, then describe tho various forms of battery derived from them, the instruments necessary to examine the actions of the current, and by their aid trace out the general laws and fundamental principles of the science, after which the applications of the force ef which the nature has been thus examined will become far more intelligible than by piling up isolated facts, or describing mere processes, however practically valuable. For the same reason, matters of mere history of discovery, howover interesting in themselves, will be left unnoticed unless they throw light upon the subject itself.

101. If we place a piece of ordinary sheet zinc in a dilute acid, we find that a tumultuous action

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take- plice, tie zinc is dissolved, and hydrogen gas is given off. Another effect is produced which is seldom set f irtli as it requires when this fundamental experiment is stated, as it of course is in every work on electricity—as the zinc diss lives, the liquid becomes heated. Now this last fact is the one of primary importance; for with all the similar facts in chemistry, it teaches us that whenever an action takes place spontaneously between two substances, heat or force is set free. Let us examine, though on'y cursorily, as it must be very carefully treated. hereafter, what occurs in this instance, and why it occurs. The old explanation, and one even now commonly given, is that the zinc decomposes water, H,0, gives off the hydrogen and forms oxide of zinc, ZnO, which is then dissolved by the acid, forming a Bait of zinc. Tho true explanation is far more simple ; the acids are substances in which hydrogen forms the base, united with a special acid radical; hydrogen, though a gas, and one which has never yet been liquefied has many chemical analogies with the metals, and indeed, there is good reason to believe that it is a true metal, and capable of assuming the solid metallic state in alloy with some other metals being then a conductor of electricity, and displaying the ordinary physical characteristics of metals. At all events, metals are capable of taking its place in compounds; and thus in the case under consideration, say of zinc acting on dilute sulphuric acid H2SOt the metal simply displaces the hydrogen and converts the substance into ZnSO«, sulphate of zinc, instead of sulphate of hydrogen. If we ask, Why does this occur? the ready reply is, because the affinity of zinc for sulphuric radical is stronger than that of hydrogen; but this is merely stating the faot itself again in more high sounding words: it is no explanation, because all we know about affinity, as it is called, is simply that the facts it expresses occur; because if we ask Why is the affinity of zinc greater than hydrogen? the only aud usual answer would be that it is so because it displace the latter, thus working in a vicious circle.

102. It is requisite to clearly understand that besides the material elements, force enters into the constitution of all bodies; all possess a specific quantity of what we know as heat, and according to the molecular theories, the atoms of which all substances are composed are in a constant state of internal motion; the amount of that motion governing the physical state, as solid, liquid, or gaseous, and also the chemical relations; affinity is, in fact, a function of these motions ; the less tbe motion, the nearer the atoms approach, and the greater the attraction they exert on each other. Hence, when what are called hig7ier affiaitie* como into action, the internal motions are diminished j but, as a consequence, this motion becomes external, active and sensible, instead of internal or latent; and thus it is thai every act of chemical combination sets free force in some form, usually as heat, while ev^ry act of chemical decomposition requires the supply of force tore-establish the internal motions, or latent forces, or, as it is usually expressed, to overcome the chemical affinities.

103. Thus when our zino is dissolving it gives off hydrogen and heat while forming the more satisfied compound, sulphate of zinc. If we use a piece of iron it does the same, but if we use copper no action occurs, at least to any appreciable extent, but if we use nitric acid the copper is dissolved. Now, if we place in the same sul phuric acid, copper and zinc, but separate from each other, we see gas pouring off the zino and not from the copper, but if we permit them to touch a new phenomenon occurs. The gas appears to issue abundantly from the copper; still if we examine the liquid we find that no copper is dissolving, while the zino is dissolving faster than before. Instead of allowing the two metals to touch within the liquid, we connect them by a wire, and we find that this wire is suddenly en dowed with extraordinary properties; if it approaches a magnetic needle the earth's directive power is superseded, and the needle no longer points N. and S., but places itself across the wire, and in different directions, aocording as it is above or below j if the wire be coiled round a piece of iron, it is endowed with powerful magnetic properties; if the wire be cut in two, and its ends dipped in liquids, it produces chemical changes in many of these; lastly, the wire itself becomes hot. But iu proportion as these effects are developed, so does the dissolving zinc generate less and less heat in the liquid. Here we have the explanation of tho source of these external actions; (here is no creation of force, nothing new occurs f xcept. that under the new conditions the force set free by tho combination of the zinc takes that form which we call electricity, instead of the other form we call heat, and is capable of manifesting itself by its magnetic, chemical, or calorific effects, thus furnishing the three natural divisions of the study of dynamic electricity.

104. Tho conditions under which the force tikes this form are a development of those pointed out in Section 26 under Static Electricity, but more plainly evidenced. The fundamental condition is a complete circuit of molecules, and the whole of conducting substances; where the electricity is developed by chemical notion, part of the circuit must be a liquid—an electrolyte, that i«, a substanco whose molecules willreadily assume the condition of polarity, and break up into two distinct parts.

This action occurs under the influence of the zinc, which, as it attracts the sulphurio radical, turns the hydrogen half of the molecule away from itself, and by diminishing the internal attractions of this first molecule disturbs those of others, if there be this complete chain provided along which the force can act; if not, the hydrogen simply escapes, and the heat is at once sit free. The action can be traced by the ordicarr chemical symbols. Zn + H2 S04 must evidently first become Zn -f SO'Hj.then Zn SO4 + H.. In this case the atoms of hydrogen are what is called nascent, but they instantly form 1 free molecule, taking up and rendering latent that portion of heat or force necessary to tontert them into a gas, but before this process is completed they are in a condition of great activity and eager for combination, but as they are J surrounded only by molecules, the nature of | which they would not change—i.e., hydrogen compounds, they are compelled to become free, but where this complete circuit of molecules capable of polarisation and discharge is provided, this action is deferred to the last; molecule after molecule, is decomposed, and the hydrogen is not set tree till it reaches a pointat which its nascent energy is powerless to effectadecomposition, and thus in the combination under examination, it reaches the copper plate before it becomes free, and i<oes not do so at all if it can help it, for if a metaHic salt is" present at the copper plate, such as suphate of copper, it displaces the copper, which fixes itself in turn upon the superficial molecules of the metallic plate, to which the polarising force is transferred.

105. These two processes furnish us with a Mtaral dirision of generators or batteries into two classes. 1. Those in which the hydrogen guisretfree. 2. ThoBe in which the hydrogen i» not set free, but displaces some other subrtance, and this latter clasB consists of two kinds, those in which one liquid fulfils all the requirements, and those in which two separate liquids are required, kept apart by a pbrous diaphragm or partition.

Before examining these various forms, it will be as well to explain various terms as to which there is much confusion in many minds. A» the action commences at the surface of contact of the zinc with the acid, the zinc is called the positive metal or element -, and hence the order of polarisation originated there in the liquid is such that the positive or -f- ends of the molecules are turned from tho zinc, and consequently all the negative ends, which are the acid radicals, »re turned towards it. This also corresponds with the terms of static electricity, and shows the wire united to the zinc plate and called its pole, in the same electrical condition as the rubber of a glass electrical machine — or negative. The current passing through the liquid to the copper or other collecting plate polarises its molecules with their — ends to the liquid, and their -(- or positive ends towards its wire. Hence we have the zinc, the positive metal plate, or element, but its wire, the negative or — pole; the copper is the negative plate or metal, but the wire proceeding from it, the positive or + pole. Fig. 30 shows this, together with one series of the reactions shown in their successive stages. Line 1 exhibits the arrangement before action, the molecules indifferent, the shaded part here, and In all future diagrams, representing the -+- or metallic or basic element or half; the white being tho — or acid half. In line 2 we see the molecules polarised under the attraction of the zinc; in line 3 the resulting discharge, the whole

chain simultaneously breaking up, one atom of zino forming a molecule of zinc sulphate; and at the other end of the chain, the two atoms of hydrogen, which are equivalent to one of zinc, are set free, when they satisfy oach other's attractions, I its properties should be known before preparing

strument where saline solutions are in contict with the zinc, it is an improvement in any case except the Daniclls cell charged with salts instead of acid; ami us zinc is used in almost all cells,

and together form a gaseous 111 decule of hydrogen. This step being reached, polarisation

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again takes place, the molecules making a semirevolution, and resuming the position of line 2. It will be seen that this view of the action involves two exertions of force at each stage, first the mechanical semi-revolution of the molecules on their axes; nnd, secondly, the overcoming the chemical attraction within the molecules; and this latter also involves two separ.ite uctions, the I actual disruption which occurs only as to on< molecule of the chain, and the temporary disrup 1 tion and reforming of all the other molecules in each chain. I only indicate this now because it presents itself, and because the more clearly these various principles are seen, the more thoroughly the subject will be mastered; these various actions, however, will have to be studied further on, as they constitute together the " resistance" of liquids and the internal resistance of batteries.

106. That this condition of polarization or Btrain tending to disruption really does occur in this case is manifest, because although actual disruption can only happen when the whole chain is composed of conducting materials, yet tho tension which tends to produce it exists exactly as in the cases studied under static eleotricity. If the two wires are connected to any condensing arrangement, such as Fig. 22,p. 519,Vol.X.,thetwo plates will be found to exhibit electric tensioa exactly as if they were connected to a machine, only to a degree Bo feeble thot very delicate condensing electroscopes are required to trace it. This indicates the existence of the complete chain, the air or dielectric between the plates pf the condenser being polarised; connect the plates by a conductor, and discharge and current are produced.

107. The force generated by a chemical action depending on the degree of that action, the generating substance is beat which has the greatest attraction for the radical of the acid, but practical considerations limit, us to iron and zinc as the cheapest; both, however, have the drawback that they maintain their action whether we want the force they can give us or no; but pure zino is, however, very slightly acted on, except when the conducting circuit is closed, while ordinary zinc is continuously dissolved. The reason of this difference is by no means clearly known; though it is usually attributed to the presence of foreign metals, setting up little local circuits; but it has been discovered that

any forms of battery.

108. Amalgamation is readily effected by thoroughly cleaning the zinc with a strong acid solution,droppinga little niereury on it, and spreading it, and rubbing with a wad of tow or flannel. Sometimes spots are very hard to act on, and may require thoroughly scraping, or first well "ashing with caustic soda, to remove grease. When plates are removed from a battery they should always be washed nnd brushed before puttingaway; and then is the best time to apply fresh m.'rcury, if required. The brushing* should be collected (which is easily done by setting a jar apart for the purpose of washing), as they consist very largely of mercury, which can be removed by distillution, when a quantity is collected. A superficial amalgamation is given by immersing plates in water in which a little nitrate of mercury is dissolved, or corrosive sublimate may be used. •

Rolled zinc should always be used in preference to cast. The latter is very hard to amalgamate, and has less electro-motive power, but for rods for use in porous jars, and particularly with saline solutions, cast zinc is very commonly used. In this case, great care should be taken to use good zino cuttings, removing any parts with solder on them, and using a little nitre as a flux, which will remove a portion of tho foreign metals. A very pure zinc might be removed from spent battery solutions by first neutralising thoroughly : with zinc cuttings, precipitating with carbonate ,]?»>?,°e ! of f?oda (wa9ni"g crystals), and then drying and

fusing with powdered charcoal, thoroughly

mixed; but the process would hurdly pay.

ltolled sheet ziuc, from one-sixteenth to a quarter-inch thick, suitablo for cylinders and plates, costs from 4d. to (id. per pound. The simplest way to cut it to size is to scratch a groove with a steel point, such as a bradawl, run first acid solution, and then mercury along this groove, and allow it to penetrate; then repeat the process on the other side; when the metal is easily broken. Zinc possesses a peculiar pro • perty of softening with a moderate heat, so that hard and brittle as the metal is, it can easily be bent up into small cylinders, if held in front of a good fire till too hot to handle with the naked hand, and then bent round a piece o£ wood or metal.

(To he continued.)

MECHANICAL MOVEMENTS.*
(Continued from page 53.)
{Illustrated on page 76.)

1G. The external and internal mutilated cogwheels work alternately into the pinion, and give slow forward and quick reverse motion.

17 and 18. These are parts of the same movement, which has been used for giving the roller motion in wool-combing machines. The roller to which wheel F (lii), is secured is required to make one-third a revolution backward, then two-thirds of a revolution forward, when it must stop nntil another length of combed fibre is ready for delivery. This is accomplished by the grooved heart cam, C,^D, B, e (17), the stud, A, working in the same, groove -, from C to D it moves the roller backward, and from D to « it moves it forward, the motion being transmitted through the catch, G to the notch-whocl, F, on the roller-shaft, H. When the stud, A, arrives at the point, e, in the cam, a projection at the back of the wheel which carries the cam strikes the projecting piece on the catch, G, and raises it out of the notch in the

common zinc, when amalgamated with mercury, 1 wheelj F> s0 that) whilo the stud is trave)iiug in is not to be acted on, and this seems to render | the cam from e to C, the oatch is passing over the this explanation somewhat doubtful. However, | plam surface betweeu the two notches in the a well amalgamated plate is scarcely acted on in 1 wheel, F, without imparting anv motion; but dilute sulphuric acid, but the presence of hydro- when stud, A arrives at the part, C, tho catch has chloric acid nearly, and of nitric acid, and metallic , dropped into another notch, and is again ready to salts, entirely does away with the protection, j move wheel, F, and roller as required, which appears to depend chiefly on the adhesion 19. Variable circular motion by crown-wheel of a film of hydrogen gas to the surface, so pre- and pinion. The crown-wheel is placed cccentriventing contact with the liquid. When the cally to the shaft, therefore the relative radius circuit is closed, the hydrogen is transferred to changes.

the negative plate, and the protection is removed; 20. The two crank-shafts are parallel in direcwhile the conditions of discharge bring fresh ' tion, but not in line with each other. The revoactions into play. Amalgamation also renders lution of either will communicate motion to the the zinc a better source of electricity, as it is more other with a varying velocitv, for the wrist of one positive than ordinary metal. Hence, though it . Extraoted^ »compilation by Mr. M. J. Brown, is not required for protection in any forms of in- Editor of the American Artisan.

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c:a k working in the slot of the other is contimiRlly changing its distance from the shaft of tie latter.

21. Irregular circular motion imparted to wheel, A. Cite an elliptical spur-gear rotating round centre, 1), and is the driver. B is a small pini n with teeth of the same pitch, gazing with C. The centre of this pinion is not fixed, but is carried by an arm or frame, which vibrates on a centre, A, so that as C "wolves the frame rises and falls to enable pinion to remain in gear with it, notwithstanding the variation in its radius of contact. To keep the teeth of C and B in gear to u proper depth, and prevent them from riding over each, wheel, C, has attached to it a plate which extends beyond it and is furnished with a groove, g, h, of similar elliptical form, for the reception of a pin or small roller attached to the vibrating arm concentric with pinion, B.

22. If for the eccentric wheel described in the last figure an ordinary spiK-gear moving on an ecccnliic centre of motion be substituted, a simple link connecting the centre of the wheel with that of the pinion with which it gears will maintain proper pitching of teeth in a more simple manner than the groove.

2M. An arrangement for obtaining variable circular motion. The sectors are arranged on different pln-ics, and the relative velocity changes ■accoiding to the respective diameters of the sectors.

24. This represents an expanding pulley. On tur,ring pinion, d, to the right or left, a similar motion is imparted to wheel, e, which, by means of curved slots cut therein, thrusts the studs fastened to arms of pulley outward or inward, thus augmenting or diminishing the size of the pulley.

25. Intermittent circular motion of the ratchetwheel from vibratory mo.ion of the arm carrying a pawl.

26. This movement is designed to double the speed by gears of equal diameters and numbers >■( teeth—a result one generally supposed to be impossible. Six bevel-gears are employed. The 5,'ear on the abaft, B. is in gear with two others— one on the shaft, F, and the other on the same hol'ow shaft with C, which turns loosely on F. The gear, 1), is carried by the frame, A, which, being fast on the shaft, F, is made to rotate, and (here"ore takes round D with it. E is loose on 1 he shaft, F, and gears with D. Now, suppose the two gears on the hollow shaft, C, were removed and D prevented from turning on its axis ; one revolution given to the gear onB would cause the frame, A, also to receive one revolution, and as this frame carries with it the gear, D, (rearing with E, one revolution would be imparted lu E; but if the gears on the hollow thai. C, were rejil .ccl, D would receive also a revolution

MECHANICAL MOVEMENTS.

on its axis during the one revolution of B, and thus would produce two revolutions of E.

27. Represents a chain and chain pulley. The links being in different places, spaces are left between them for the teeth of the pulley to enter.

28. Another kind of chain and pulley.

29. Another variety.

30. Circular motion into ditto. The connecting-rods are so arranged that when one pair of connected links is over the dead point, or at the extremity of its stroke, the other is at right angles; continuous motion is thus insured without a fly-wheel.

(To be continued.)

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to contain the trailing wheel, bnt in trirye'es, to which this invention is mere particularly applicable, a curved or semicircular extension is to be made, the extremities of which are caused to rest on the crank shaft by suitable journals or bearings. One of the oscillating levers is provided with a saddle or seat; this lever is borne above the second, which has a stirrup or foot support, suspended therefrom. The rider raises himself from the saddle and rests all his weight on the stirrup, thereby throwing down the crank connected to its lever, and thus when the opposite crank has risen and is passing over its centre the rider seats himself in the saddle, which has risen to an elevated position, and consequently the crank connected to its lever is in turn forced down, and so on successively, thus propelling the velocipede by the cranks with the mere weight and motion of the body. Mr. Fairbairn also proposes to employ a three-throw axle crank to machines to be worked on the above principle. In bicycles the connecting rods would be bent or arched from the oscillating levers, and then extend vertically or obliquely to the cranks, so as to freely pass one on each side of the driving wheel, but in tricycles they would extend in a direct line to the crank shaft. The guiding handle may be of any convenient form, the transverse double handle being preferable as a steadying support. The saddle lever would be of course central in the three throw m ichine, the stirrup levers work ng one on each side, and the angles of their .-ranks may be made to correspond, if desired, so that both feet may operate with equal power at each forward motion, instead of slightly in advance the one of the other, in which case, which is preferable, the stirrup rod or bearer is in one and the same line. Fig. 1 represents a side elevation, and Fig. 2 a plan of a tricycle constructed on the last-mentioned principle The hollow stem a through which the guiding pillar b is passed receives the main bar or frame c of the tricycle, above which on pivoted joints d d the saddle lever e and stirrup lever/ arc set; g is the saddle, an' A A the connecting rods to the three-throw crauk t, and k is the connecting rod from the crank to the stirrup lever /. In Figure 2 the form of this con nee ing rod is shown consisting of a bracket, through which the main bar passes, whereby a free vertical motion is afforded to the stirrups or foot beareis/t. The bracket or compound rod k' is capable of adjustment on the lever / by means of the sliding socket M and bolts and screw nuts, in order to set the stirrup bearer farther from or nearer to the rider's seat. The bearer is also capable of adjustment in the slot I, Fig. 1, to suit the rider. T"0 wheels may be made with iron spokes doubly dUba 1, and set in an iron or metal nave or box, as shown, or the spokes and nave may be of wo id. m is the i steering handle. On the same principle can also

I be made a tricycle with a three-throw crank,

f. canning the two side cranks to be worked by dnpli

I eate stirrup levers and connecting rods, one on

I etch side of and below the main bar. the stirrnp

# bearer connecting the two transversely; the

I saddle lever is then connected by owe rod to the

f central crank. The method of working is the

t same but in some cases the latter arrangement

would be preferred, because the pressure of the

feet on the stirrup bearer would be thrown

equally on the two external cranks instead of on

the central crank, as first described, and conse

nuently a greater steadiness of travel would be

effected In the application of this invention to

bicycles it would be necessary to apply a double

crank on each side of the driving wheel, and to

brine two s»ts of connecting rods or levers

thereto ; one set from the Baddle bar or lever, and

the other from the stirrup bearers, the operation

of working being the same.

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by which he propels his automaton carriage are shown in the drawing; they are, A shaft of the driving spring; B winding up movement; C small pinion; D large driving wheel; K fly wheel j F fork; G shaft; H revolving axle on which the carriage wheels are keyed ; I toothed steering gear; J steering lever; K connecting rods; L cover of spring barrel to prevent accident should the spring break; B driving spring. The shaft A of the driving spring B gives motion to the toothed wheel D, which governs the pinion C, and thus turns the shaft G. This shaft governs the connecting rods K K, which in their turn drive the revolving axle H. The pinion on the steering lever J iiears with the piece I and permits the carriage being guided in any desired direction. The use of the forked piece F is for starting and stopping the machine, and the fly wheel E serves for backing or advancing the carriage. The whole of the mechanism is on the fore carriage, by the driver's Brat. The carriage must be provided with an ordinary brake, to be used when required.

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shown at D in plan) and passing along the inner surface of the bottom of the outer cylinder, is turned upwards, as at W T. The space between the two cylinders holds the spirit or naphtha to be burned, which is introduced at A. A small quantity of spirit is then placed in the inner cylinder, as at P Q. This spirit being ignited will heat the spirit in the place between the cylinders, and give a powerful jet of flames at X.

SELF-PBOPELLING VEHICLES.

LETTERS PATENT have been granted to Ferdinand Constant Colney, of Paris, forthc invention of "improved mechanism for the propulsion of vehicles." The patentee asserts that his automaton carriage, set on four wheels, will carry at least three persons, may travel from nine to ten miles an hour, the driving spring requires winding up but once an hour, and asthis winding may be effected while the carriage is in motion, it causes no interruption of the journey. The pieces composing the improved mechanism

HALLEY'S COMET.

(Continued from, page 51.)

Br Omicron.

THE comet did not attain a sufficient elongation from the Sun to admit of observation till the last week of March, 1795. La Nux, at the Island of Bourbon, detected it on the 26th, and Messier on the 31st of March. On this occasion Delisle permitted Messier to give notice of his discovery, and a formal announcement of the reappearance was made on the 1st of April. The Southern Declination soon became too great to admit of observation in the latitudes of Paris, and observations were discontinued towards the end of the month. In the meantime, however, the observers of Lisbon, Toulouse, and various other places had secured valuable observations, and while the comet did not rise to the observers of Europe, La Nux, as already mentioned, in the Island of Bourbon, devoted his attention to it; and its place was also noticed by Father Cceurdoux, at Pondicherry. In the beginning of May its motion again brought tho comet within the reach of European observatories, and Messier and others continued to watch it till the 3rd of June, when it finally vanished, never to reappear till that generation should long have passed

away.

If our readers will take the trouble to add on seventy-seven years, which is about the average length'of a revolution, they will find that about the year 1835 its reappearance might be again anticipated. Several mathematicians who were already celebrated for their analytical researches, proposed to themselves the task of computing the amount of planetary disturbance that the comet

SELF-PROPELLING VEHICLES.

must undergo in the course of its long revolution,
while hidden from the sight of the inhabitants of
this little sphere. Were it admissible in a short
paper like the present, it would be a pleasant
task to dwell upon the power of analysis that is
evidenced by the capability of following a comet
in its tedious course round the sun, in a path
that renders the comet so long invisible,- from
a few observations made at the time that, it is
within sight, and to be able to predict with cer-
tainty the place that the comet occu pies in space,
at any moment of its revolution, the time of its
return, and the position it will
then assume. A glance at the
accompanying figure will, how-
ever, show to those that have
honoured me with their atten-
tion, the difficulty of the case
far more plainly than I can
put it in a few words. The
ellipse represents the path of
the comet, the small circle at
the perihelion extremity, the
course of the Earth round the
Sun. The comet was, perhaps,
observed (roughly, as we should
consider now) a short dis-
tance on either side, beyond the
small circle representing the
earth's orbit. The problem is
to determine, from the know-
ledge of ita motion in this
short arc, the remaining part
of the curve, subject, as it is,
to alterations at every moment
from the various planets whose
arcs areBhownin the diagram.
All honour is due to the pro-
found intellect and persevering
industry of those who have
energies to the solution of the
brought their investigations to a successful issue.
Let us proceed to mention those who have con-
sidered the problem, and, briefly, the means that
have been employed to solve it.

So early as the year 1817, the interesting question of the perturbations of Halley's Comet began to interest the astronomers of that period. The Academy of Sciences of Turin proposed the successful investigation of this subject for their prize, and the late Baron Damoiseau was tho successful competitor. Before proceeding to give an account of the method pursued by Baron Damoiseau, it is necessary to say a few words, concerning the advance of analytical Bcience since the days of Clairaut's investigation. The theory of perturbations of comets received its greatest improvement at the hands of Lagrange, in a masterly memoir, which gained the prize proposed by the Academy of Sciences, in 1780, and which may be regarded as finally settling tho difficult problem. "Doubtless," says Pontocoulant, "one might wish for a method of determining these perturbations in which the numerical application could be more simple, but by the very nature of the difficulties that the question presented, it appears to me doubtful whether it will ever be attained, and that for a.

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devoted their problem, and

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