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MIRROR OF SCIENCE AND ART.

РЕШАТ. APRIL 13. 1870.

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centre: that of the third ring

620

A patch

of theory with observation
Notices," vol. xxvii., No. 5.)

OXSTS TO ASTRONOMICAL STUDENTS
(Ccmiiiiueii.')

WE Ьате in the preceding paper» discussed
Tirions preliminaries with which it seemed
desirable that the young astronomer should be
u-quainted, but we have not yet advanced во far
si tb* actual «rial of the instrnment; nor indeed
ur we eren now in a position to attempt it
£»irlT; for before we can u'e satisfactorily even
the moat suitable teets for the purpose, we ongbt
to be acquainted with the exact nature of teles-
copic definition. Unless we bave formed correct
ideas ae to this point, so as to know the degree of
perfecüon which we have a right to look for, we
may either (a» the writer was in his young days)
he subjected to disappointment from the unreason-
atAeness of nur expectations, or satisfy our-clves
too readily «¡th first appearances, which may not
be borne out by a further acquaintance with the
instnxauDt.

Tbe peculiar character of telescopic vision is immediately dependent on the nature of light itself, tie laws of which, as fixed by the Great Creuor, impose an un varying limit on the degree of perfection to which the finest human workmifflhip can attain, for it is a demonstrable fact ti»! even it the optician's »kill could unerringly realice, the formula of the mathematician, the local image would not he an absolutely perfect oçUea\ reproduction ot the objet; lrom which it is derived.

Thia may appear strange, but it is a necessary сомеопепее of the nndalatorr nature of light, eoasutiag of vibrations o/ exceeding but still finit* and even measurable minuteness. It might be possible to form an idea of light which would fulfil the condition of perfect telescopic vision, nimtly, that every point and line in the object ibcrald be represented by an exactly correspond, in» «¡d »imitar point and line in the focal picture, hut dv.» woold not lie in accordance with the actual law \>y which He who Said "Let there be light, and tlier* was light,"has seen fit to govern in emi«*ioc tod propagation. In consequence of that law, as waj v.ry ably pointed out many /ear» aga in the •'Cambridge Philosophical Transseri>ns " by the present Astronomer Royal, every pencil of light transmitted by a lens or reflected by a mirror is affected in converging to a fucus by what is called the " interference" of its undulations. This interference is the neces-ary result of ita having an external boundary, whatever the form or dimensions of that boundary may be. In ftwtice it arise« at the edge of ihe brass cell, bnt ü the coll were removed, it would be equally pndeced at the edge of the lens or speculum, and eavaot possibly W prevented or avoided, being Redirect result of the employment of a limit a my -way to the pencil of rays. Hence it fc-fcws that every focal image, no matter how it an be obtained, must exhibit the effect of the aerfere-nce of undnlaiions. This effert, though r-rr minute, is n a beyond the reach of investigation by modern analysis, and tbe inquiry, as ewtidocted by Professor Airy, has led to the foiluwing result.

Tbe ixoage, whether reflected or refracted, of a

¡uminonit point—we will say the imsge of a star,

low i( la well ascertained that this possesses no

arasFible magnitude—is not a point, buta circular

lominona disc of a certain calculable dinmo er,

•unrounded by a number of bright ring*. Th«

angular dimensions of the whole will depend on

■otnii g but i be aperture of the telescope, and will

tW iuversrly as the aperture. The intensity of

Ье light of tuneen ral disc decreases rapidly from

tha centre: the intensity of the brightest part of

1

С • fin* or innermost ring is of tbe centre of

ST

placed on the centro of the lens in some degree
changes the chsracterof the aberrarion, diminish-
ing the magnitude of the disc, and increasing the
brightness of tbe rings, while it somewhat dimi
nishes their magnitude. Such are the deductions
of theory. It is now easy to see whst 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 use 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 valne. 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,
Weif a, Arcturus, or Capella, we shall find, with a
sufficient power, and in stead y sir, which is a very
essential condition, a considerable "spurious
disc" with a succession of rings, which, if the
maierials arc homogeneous, and the centering cor-
rect, will appear circular and concentric; any
material irregularity in form would probably
iudicate unequal density in the glass, or inaccu-
racy of workmanship, neither of which could be
remedied; if the rings are merely thrown to one
side, the fault may lie in the centering, nnd sdmits
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." whose 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 suc-
cession stars of less brightness, we shall find a
decrease in the magnitude of the disc and the
number of the rings, 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 on the power of the eye to distinguish
feeble degrees of light The phenomenon existe
alike in the case of every star, but ihe 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 feible to be
distinguished, till at length tbe 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 othorwi-e seemed very ano-
malous—that the telescopic discs of the stars
should appear of such very different magnitudes,
th .ugh we know, from their gradual reduction
through passing clnuds, 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 comee to the ssme thing, the diameter of the spuri<>ns diso will he inversely as-the diameter of the object-glass or speculum: 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 the limit of perfection which optical skill bas now attained, in a series "f 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 table con'nin- a ri ean of numerous observations on several of tbe larger st irs, showing at once the progressive incre.su of disc with the

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The earliest acquaintance with this result of interference seems to have been obtained by Hevel (a name frequently Latinised into He velius). The worthy old burgomaster was, however, completely misled* by his own experiment. By stopping down his object-glass to a working aperture of the size of a large pen, he succeeded in expanding the discs of the brighter stars into circles of notable diameter, and in his simplicity rejoiced in the idea that he had rendered their re . 1 dimensions visible. Who first pointed out this curious mistake I do not know; but it was not likely to pass long unchallenged, since a little acquaint .псе 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 universally admitted. A few years ago, Steinbeil, 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 als» upon focal length. This idea, however, has not been sanctioned by English mathematicians; and ourgrcit observer, Dawes, than whom few men have been more practically acquainted with telescopes, hn» expressed himself as entirely satisfied of the certainty of Airy's conclusion.

From this, of course, follows ;he known fact that, with equal materials and workimuship, tho dividing power of telescopes, or their capability of separating very close double stars, varies us 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 assenions 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 nnd generous spirit. We must bear in mind th it, first of all, differences of natural vision must enter into the result. I well remember one night wheu I had the especial pleasure of visiting Mr. Dawes'sobservatory at Hopefield, provided at that timewith a splendid 8J-inch Alian Clark achromatic-, that his eagle-eye rapidly picked up Eneeladus, 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 stite 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 observer 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 as a mere interference result, though of course tho precursor of separation ; aud restrains tho latter term to th« 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 end well-informed men, not to say Christians, there must of coursebe a limit—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 Dallmeyer's standard of performance is obtained bydividing 4-33 by the aperture in inches, the quot cut expressing in seconds tbe central distance of the stars which ought to be just divided. On the other hand, Dawrs tellsus that having asrer.ained about five and thirty years before (this was written in 1867). hy comparisons of several telescopes of

diminution of aperture, and tLe correspondence very different »perinrea, that tl.e diame'ers uf

star-discs varied inversely as the apertures, he examined with a great variety of apertnres а vast number of double stars, whose distances seemed to be well determined and not liable to rapid change, 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 n, in a moderately-favourable atmosphere, will be 4"-56

expressed by the fraction . The quotient thus

a obtained concurs with Dallymeycr'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-iuch object-glass ought to divide 6 mag. stars to P.

This standard, in conjunction with a table of the distances of double stars, will enable the young olfterver to form a fair opinion whether hit instrument is up to the mark. But here, again, a caution must be interposed. Many test-objects, otherwise perfectly suitable, are variable in 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 upon recent measures; which ore not always of ready attainment, n Coronas, 36 Andromeda:, and that formidable object ? Cancri (which I saw beautifully, though in very unsteady air, March 31, with 450 on my it-inch mirror) belong to the changeable class: < Bootis, 62 Ononis, я- Aquilre, £ Boö'is, n Ononis, 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 tho centre of the spurious image, and regards the disc as a flat luminous spangle, giving rise to the old comparison of an unequal pair in a good telescope to a shilling and a sixpence on a piece of black cloth. Yet that an actual diminution of light towards the edge, does take place, as required by theory, is shown by the fact that the discs appear somewhat smaller and more separable on the background of a daylight or even4wilight sky, as 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 1782, 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 invisible; 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 Mechanic are not yet quite weary of it, as it is still not exhausted. Before closing this paper I will add the 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,I 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 tho 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 very obliging, and showed me every attention.

The following were the dimensions then given to me :—Height of tower, 64ft ; diameter, 15ft.; thickness of walls, 1ft. 2in.; whole length of telescope (including dewcap), 85ft. ; dewcap, 6ft. 2in. ; metal tube, 76ft. (¡in.; focal length, 77ft.; diameter of widest part of tube, 4ft.; weight, about 3- tons. It was slung in a chain at the thickest part, which was considerably nearer to the 0. G. than to the eye-end ; 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 side. 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 the former, little could be 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 mag- nifioent 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 B better one, of the same focal length, but smaller aperture, say 12 inches? the cost of which, as well as of necessary attendance and repairs, might be defrayed by an extensive subscription of small individual sums, each subscriber having the privilege of making 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 upon it, and devise means for giving efficiency to the really valuable portion of so noble an undertaking.

A word in conclusion as to Dr. Ussher's excellent " Advice." I had no idea that these lines proceeded from so venerable a source. They were giren to me many years ago by Mr. Lawson, then of Hereford, subsequently of Bath, the proprietor of a very fine 7-inch Dollond achromatic, bequeathed by him to tho Greenwich Naval School. Some of our readers may perhaps be able to give an account of 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 yeaft. But to return to the verses : my edition is somewhat different, and I think with an evident improvement in the sense; the last four lines runniug 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.

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ELECTRICITY—ITS THEORY, SOURCES
AND APPLICATION.

Br J. T. Spbague.»
(Continued from page 27.)

IN order to examine intelligentlythe wide and interesting subject of Dynamic Electricity or Galvanism, probably the most convenient process will be to commence with a few leading facts and principles, then describe the 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 of which the nature has been thus examined will become far more intelligible than by piling up isolated facta, or describing mere processes, however practically valuable. For the same reason, matters of mere history of discovery, however interesting in themselves, will be left unnoticed unless they throw light upon the subject itself.

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

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takes pl<ee, the zinc is dissolved, and hydrogen gas is given off. Another effect is produced which is seldom set forth as it requires wh< fundamental experiment is stated, as it ojEj is in every work on electricity—as th dissolves, the liquid becomes heated. N last fact is the one of primary importance with all the similar facts in chemistry, it teaches us that whenever an action takes place spontaneously between two substances, heat or forre is set free. Let us examine, though only 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, HtO, gives off the hydrogen and forms oxide of zinc, ZnO, which is then dissolved by the acid, forming a salt of zinc. The 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 s 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 H2SOi 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 fact 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 and 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 i 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 the motion, the nearer the atoms approach, and the greater the attraction they exert on each other. Hence, when what are called higher afflnitie» come into action, the internal motions are diminished; but, as a consequence, this motion becomes external, active and sensible, instead of internal or latent; and thus it is that every act of chemical combination sots free force in some form, usually as heat, while every act of chemical decomposition requires the supply of force to re-establish the internal motions, or latent forces, or, as it is usually expressed, to overcome the chemical affinities.

103. Thus when our zinc 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 sol phuric acid, copper and zinc, but separate from each other, we see gas pouring off the zinc and not from the copper, but if we permit them to touch a new phenomenon occurs. The gas »>p-; pears to issue abundantly from the copper; still, Л if we examine the liquid we find that no copper ""V is dissolving, while the zinc is dissolving faateajs/'*1! than before. Instead of allowing the two metals», so to touch within the liquid, we connect them by jL "^wire, and we find that this wire is suddenly esv}^""í domed with extraordinary properties; if it Fit- To proaches a magnetic needle the earth's directive ,'J", power is superseded, and the needle no. lougs».."* points N. and S., but places itself across the wiroyw"' F and in different directions, according as it is abo«« .**. or below; if the wire be coiled round a piece a" A, iron, it is endowed with powerful magnetic pro-. _"<»i per ios; if the wire be cut in two, and its ends". '"''J dipped in liquids, it produces chemical changes m4"'4< many of these; lastly, the wire itself becomes '* hot. Butin proportion as these effects are de ve- > Ц loped, so does the dissolving zinc generate less and ■• i less heat in the liquid. Here we have the expía- *'*.¡,

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nation 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 к working in the slot of the other is continually changing its distance from the shaft of tLe latter.

21. Irregular circular motion imparted to wheel, А. С is an elliptical spur-gear rotating round centre, 1), and is the driver. В is a email piui n with teeth of the same pitch, geajfing with С The centre of this pinion is not fixed, but is carried by an arm or fraim which vibrates on a centre, Л, so that as С ¡evolves the frame rises »nd falls to enable pinion to remain in gear with it, notwithstanding the variation in its radius of tontact. To k»ep the teeth of С and В in gear to a proper depth, and prevent them from riding over each, wheel, C, lias 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 la-t figure an ordinary spu'-gear moving on an ecccntiic ceutre of motion be substituted, a simple link connecting the centro of the wheel with that of the pinion with which it gears will maintain proper pitching of teeth in a more simple manner th in the groove.

23. An arrangement for obtaining variable circular motion. The sectors are arranged on different planes, and the relative velocity changes ■according to the respective diameters of the sectors.

24. This represents an expanding pulley. On turning'pinion, d, to the right or left, asimilar motion is imparted to wheel, c, whiob, by means •of cur>ed-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.iou of the arm carrying a pawl.

26. This movement is designed to double the ►peed by ge.irs of equal diameters and numbers • f teeth—a result one generally supposed to be impossible. Six bevel-gears are employed. The gear on the shaft, 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. Tli6 gear, D, is carried by the frame, A, which, being fast on the shaft, F, is made to rotate, and there "ore takes round D with it. E is loose on ill» shaft, F, and gears with I). 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 on В would cause the frame, A, also to receive one revolution, and as this frame carries with it the gear, D, j:»nnnir with E, one revolution would bo imparted I" E; but if the gears on the hollow shaft, C, Wvre rejil iced, U 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 planes, spaces are left between them for the teeth of the pulley to euter.

28. Another kind of chain and pulley.

29. Another variety.

30. Circular motion into ditto. The connecting-rods are so arrauged 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 bo continued.)

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to contain the trailing wheel, bat in trioye'es, to which this invention is mere particularly aypli. cable, 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, nnd 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, thui 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 doable handle being preferable as a steadying support. The saddle lever would be of course central in the three throw- michine, the stirrup levers work ng one on each side, and the auglee of their cranks 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 с of the tricycle, above, which on pivoted joints d d the saddle lever e and stirrup lever/are set ; g is the saddle, an I А Л the connecting rods to the three-throw crank i, nnd It is the counectiug rod from the crank to the stirrup lever /. In Fignre 2 the form of this connecing rod is shown consisting of a bracket, through which the main bar passes, whereby a free vertical motion is afforded to the stirrups or f^ot beareis/i. The bracket or couipouud rod h' is capable of adjustment on the lever / by means of the sliding socket i? and bolts and ecrtw nut«, iu crder to set the s'irrup bearer further 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. Tue wheels may be made with iron spokes doubly di-bed, and set in an iron or metal nave or box, as shown, or the spokes a\d nave may be of wo id. m is the

a 15, IM

V

handle. On the same principle can also nue a tricycle with a three-throw crank, eg the two side cranks to be worked by dnplittirrtip levers and connecting rods, one on «de of and below the main bar. the stirrup вгег connecting the two transversely; the Idle lever is then connected by one rod to the central crank. The method of working is the •an.i' bnt 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 consequently a greater steadiness of travel would be effected In the application of this invention to bicTcles it would be necessary to apply a double crank on each side of the driving wheel, and to brin<* two sets of connecting rods or levers thereto ; one set from the saddle bar or lever, anil the other from the stirrup bearers, the operatiou 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; В winding up movement; С
small pinion; D large driving wheel; E fly
wheel ; F fork; G shaft; H revolving axle on
which the carriage wheels are keyed ; I toothed
steering gear; J steering lever; К connecting
rods; L cover of spring barrel tn prevent acci-
dent should the spring break ¡ R driving spring.
The ehaft A of the driving npring R gives motion
to the toothed wheel D, which governs the pinion
C. and thus turns the shaft G. This shaft
governs the connecting rods К К, which in their
turn drive the revolving axle H. The pinion on
the steering lever J ¿ears 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 siat. The carriage
must be provided with an ordinary brake, to be
usid when required.

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shown at D in plan) and passing along the inner »arfare of the bottom nf the outer cylinder, is turned upwards, as at W T. The space between the two cylinders bold» 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 •Ш heat the spirit in the place between the cylinders, and give a powerful jet of flames at X.

SELF-PROPELLING VEHICLES.

LETTERS PATENT have been granted to
Ferdinand Constant Colney, of Pai is, for the
invention of "improved mechanism for the pro-
pulsion 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 epring
requires winding up bul 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.)

By Omicron.

THE comet did not attain a suffieient elonga-
tion 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 toward* the
end of the month. In the meantime, however,
the observers of Lisbon, Toulouso, and various
other places had secured valuable observations,
and while the comet did not rise to the observera
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
Cœurdoux, at Pondicherry. In the beginning of
May its motion again brought the 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 tho 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 spice,
at any moment of its revolu'ion, 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
email circle representing the
earth's orbit. The problem is
to determine, from the know-
ledge of its motion in this
short arc, the remaining pari
of the curve, subject, as it is,
to alterations at every moment
from the various planets whose
arcs are shown in the diagram.
AU honour is due to the pro-
found intellect and persevering
industry of those who have

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devoted their energies to the solution of the problem, and brought their investigations to a successful issue. Let us proceed to mention those who have considered 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 eubject for their prize, and the late Baron Damoiseau wis tho successful competitor. Before proceeding to giveau account of the method pursued by Baron Damoiseau, it is necessary to say a few wordsconcerning the advance of analytical science sincethe days of Clairaut'e 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 the dffficult problem. "Doubtless," says Puntecoulant, "one might wish for a method of determining these perturbations in which tho numerical application could be more simple, but by the very nature of the difficulties that the question presents, it appears to me doubtful whether it will ever be attained, and thai for a

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