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The English Mechanic

AND

MIRROR OF SCIENCE AND ART.

FRIDAY, APRIL 15, 1870.

HINTS TO ASTRONOMICAL STUDENTS. (Continued.)

WE

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 unvarying limit 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.

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620 placed on the centre of the lens in some degree changes the character of the aberration, diminishing the magnitude of the disc, and increasing the brightness of the rings, while it somewhat dimi nishes their magnitude. Such are the deductions of theory. It is now easy to see what 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 E have in the preceding papers discussed various preliminaries with which it seemed dimensions, and therefore demands the use of a certain amount of magnifying power, which must desirable that the young astronomer should be be determined in each case by experience, as it acquainted, but we have not yet advanced so far will vary with the aperture. If an insufficient as the actual trial of the instrument; nor indeed power is employed, the image of a large star will are we even now in a position to attempt it be a sparkling, flashing point, the real character of fairly; for before we can use satisfactorily even which will not be apparent for want of sufficient angular value. As we increase our power we the most suitable tests for the purpose, we ought shall gradually find the general blaze of light to be acquainted with the exact nature of teles- develop itself into a disc of appreciable magnitude copic definition. Unless we have formed correct in the centre of several rings, more or fewer ideas as to this point, so as to know the degree of according to the brightness of the star, and we perfection which we have a right to look for, we shall then be able to form an idea of the degree of perfection to which the workman has been able may either (as the writer was in his young days) to attain. Beginning with such stars as Sirius, be subjected to disappointment from the unreason-Wega, Arcturus, or Capella, we shall find, with a ableness of our expectations, or satisfy ourselves sufficient power, and in steady air, which is a very too readily with first appearances, which may not essential condition, a considerable spurious be borne out by a further acquaintance with the disc" with a succession of rings, which, if the materials are homogeneous, and the centering correct, will appear circular and concentric; any material irregularity in form would 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 refer 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 succession stars of less brightness, we shall find a This may appear strange, but it is a necessary decrease in the magnitude of the disc and the consequence of the undulatory nature of light, number of the rings, till at last with minute consisting of vibrations of exceeding but still objects the latter disappear entirely, and the former is reduced to a mere point. This, it will be at finite and even measurable minuteness. It might once seen, is in strict accordance with theory. be possible to form an idea of light which would The difference is simply a subjective one, that is, fulfil the condition of perfect telescopic vision, depending on the power of the eye to distinguish namely, that every point and line in the object feeble degrees of light. The phenomenon exists should be represented by an exactly correspond-alone possess light enough for us to recognise it alike in the case of every star, but the larger ones ing and similar point and line in the focal picture. in anything like its completeness; as the brilliancy But this would not be in accordance with the of the object diminishes, both the external rings actual law by which He who said Let there be and the edges of the disc become too feeble to be light, and there was light," has seen fit to govern distinguished, till at length the centre of the disc its emission and propagation. In consequence of alone retains light enough to affect the eye. It that law, as was very ably pointed out many is satisfactory to find so intelligible an explanation years ago in the "Cambridge Philosophical Trans- of what might have otherwise seemed very anoactions" by the present Astronomer Royal, every malous-that the telescopic discs of the stars pencil of light transmitted by a lens or reflected should appear of such very different magnitudes, by a mirror is affected in converging to a focus by though we know, from their gradual reduction what is called the "interference" of its undu- through passing clouds, and their instantaneous lations. This interference is the necessary result extinction when concealed by the moon, that of its having an external boundary, whatever the even the largest of them are still of no sensible form or dimensions of that boundary may be. In dimensions. practice it arises at the edge of the brass cell, but if 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 the direct result of the employment of a limit in any way to the pencil of rays. Hence it follows that every focal image, no matter how it may be obtained, must exhibit the effect of the interference of undulations. This effect, though very minute, is not beyond the reach of investigation by modern analysis, and the inquiry, as conducted by Professor Airy, has led to the following result.

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The image, whether reflected or refracted, of a luminous point-we will say the image of a star, for it is well ascertained that this possesses no sensible magnitude-is not a point, but a circular luminous disc of a certain calculable diame er, surrounded by a number of bright rings. The angular dimensions of the whole will depend on nothing but the aperture of the telescope, and will be inversely as the aperture. The intensity of the light of the central disc decreases rapidly from the centre: the intensity of the brightest part of

tle first or innermost ring is

1

57

of the centre of

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 disc will be 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 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 7-inch Alvan Clark object-glass. The following table contains a mean of numerous observations on several of the larger stars, showing at once the progressive increase of disc with the diminution of aperture, and the correspondence

2:00

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, completely misled by his own experiment. By stopping down his object-glass to a working aperture of the size of a large pea, 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 real 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.nce 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, 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 sanctioned by English mathematicians; and our great observer, Dawes, than whom few men 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 as 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 may possibly hear assertions made in all good faith in some quarters which elsewhere may be looked upon with suspicion. But experience should give a lenient tone to criticism. Many allowances require in fairness to be made among the observing brotherhood, who ought to look generous spirit. We must bear in mind thit, first upon each other's assertions in a candid and of all, differences of natural vision must enter into the result. I well remember one night when I had the especial pleasure of visiting Mr. Dawes's observatory at Hopefield, provided at that time with a splendid 83-inch 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 state 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 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 course be 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 by dividing 4:33 by the aperture in inches, the quot ent expressing in seconds the central distance of the stars which ought to be just divided. On the other hand, Dawes tells us that having ascertained about five and thirty years before (this was written in 1867), by comparisons of several telescopes of very different apertures, that the diameters 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 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 456 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 formule, 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 of server to form a fair opinion whether his instrument is up to the mark. But here, again, a caution must be interposed. Many test-objects, otherwise perfectly suitable, are variable in dis

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. 6in.; 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 O. 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 coun-
terpoise 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 en-
compassing the tower, at a distance of 52ft.
The movements, which had, I believe, been ar-
ranged 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-
nificent enterprise went far to disarm criticism.
It was, I think, understood at the time that the

takes place, the zinc is dissolved, and hydrogen gas is given off. Another effect is produced which is seldom set forth as it requires when this fundamental experiment is stated, as it of course is in every work on electricity-as the zinc dissolves, 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 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 zine decompose s water, HO, 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 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 chief fault lay in the spherical aberration, but taking its place in compounds; and thus in the case under consideration, say of zinc acting on that one or both of the lenses had been worked dilute sulphuric acid H2SO, the metal simply too thin to admit of further correction. If the displaces the hydrogen and converts the substance apparatus is still in good condition, might it not into ZnSO4, sulphate of zinc, instead of sulphate be practicable to replace the O. G. by a better of hydrogen. If we ask, Why does this occur? one, of the same focal length, but smaller aper- the ready reply is, because the affinity of zine for ture, say 12 inches? the cost of which, as well as of necessary attendance and repairs, might be sulphuric radical is stronger than that of defrayed by an extensive subscription of small hydrogen; but this is merely stating the fact individual sums, each subscriber having the privi- itself again in more high sounding words: it is no lege of making use of the instrument. This is, explanation, because all we know about affinity, perhaps, not a promising scheme; but the idea is as it is called, is simply that the facts it expresses thrown out in the hope that some one may improve occur; because if we ask Why is the affinity of upon it, and devise means for giving efficiency to zine greater than hydrogen? the only and usual the really valuable portion of so noble an under-answer would be that it is so because it displace taking. the latter, thus working in a vicious circle.

A word in conclusion as to Dr. Ussher's excel

tance, 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 are not always of ready attainment. Corona, 36 Andromeda, and that formidable object Cancri (which I saw beautifully, though in very unsteady air, March 31, with 450 on my 9-inch mirror) belong to the changeable class: Bootis, 52 Orionis, Aquila, Boötis, n Orionis, and y Andromeda (here arranged in order of distance) are much more available as showing ap. parent 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 the 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 even twilight 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 ELECTRICITY-ITS THEORY, SOURCES gives off hydrogen and heat while forming the

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 formulæ of Dawes or Dallmeyer, regard must be had to the degree of magnifying, as well 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 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, 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 the 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 in 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. I

102. It is requisite to clearly understand that lent" Advice." I had no idea that these lines besides the material elements, force enters into proceeded from so venerable a source. the constitution of all bodies; all possess a They were given to me many years ago by Mr. Lawson, specific quantity of what we know as heat, and then of Hereford, subsequently of Bath, the pro- according to the molecular theories, the atoms of prietor of a very rine 7-inch Dollond achromatic, which all substances are composed are in a conbequeathed by him to the Greenwich Naval stant state of internal motion; the amount of School. Some of our readers may perhaps be that motion governing the physical state, as solid, able to give an account of the present condition liquid, or gaseous, and also the chemical relations; and employment of this instrument, one of the affinity is, in fact, a function of these motions; the finest and most expensive of its day, and which I less the motion, the nearer the atoms approach, have several times used with great pleasure in by- and the greater the attraction they exert on each gone years. But to return to the verses: my other. Hence, when what are called higher edition is somewhat different, and I think with an affinities come into action, the internal motions evident improvement in the sense; the last four are diminished; but, as a consequence, this lines running thusmotion becomes external, active and sensible, instead of internal or latent; and thus it is tha every act of chemical combination sets 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.

Not that imparted knowledge doth
Diminish learning's store,
But books, I find, if often lent,
Return to me no more.

T. W. WEBB.

AND APPLICATION.

BY J. T. SPRAGUE.*

(Continued from page 27.)

103. Thus when our zinc is dissolving it

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

IN order to examine intelligentlythe wide and die wred, Now, if de pla, but separate
interesting subject of Dynamic Electricity or phuric acid, copper and zinc, but separate from
Galvanism, probably the most convenient process each other, we see gas pouring off the zinc and
will be to commence with a few leading facts and not from the copper, but if we permit them to
touch a new phenomenon occurs. The gas ap
principles, then describe the various forms of
pears to issue abundantly from the copper; still
battery derived from them, the instruments if we examine the liquid we find that no copper
necessary to examine the actions of the current, is dissolving, while the zinc is dissolving faster
and by their aid trace out the general laws and than before. Instead of allowing the two metals
fundamental principles of the science, after which wire, and we find that this wire is suddenly en
to touch within the liquid, we connect them by a
the applications of the force of which the nature dowed with extraordinary properties; if it ap-
has been thus examined will become far more proaches a magnetic needle the earth's directive
intelligible than by piling up isolated facts, or power is superseded, and the needle no longer
describing mere processes, however practically points N. and S., but places itself across the wire,
valuable. For the same reason, matters of mere
and in different directions, according as it is above
history of discovery, however interesting in them-iron, it is endowed with powerful magnetic pro-
or below; if the wire be coiled round a piece of
selves, will be left unnoticed unless they throw perties; if the wire be cut in two, and its ends
light upon the subject itself.
dipped in liquids, it produces chemical changes in
many of these; lastly, the wire itself becomes
hot. But in proportion as these effects are deve-
loped, so does the dissolving zinc generate less and
less heat in the liquid. Here we have the expla-

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

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nation of the source of these external actions; there is no creation of force, nothing new occurs except that under the new conditions the force set free by the combination of the zine 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. The conditions under which the force takes this form are a development of those pointed out in Section 26 under Static Electricity, but The fundamental conmore plainly evidenced. dition is a complete circuit of molecules, and the whole of conducting substances; where the electricity is developed by chemical action, part of the circuit must be a liquid-an electrolyte, that is, a substance whose molecules will readily assume the condition of polarity, and break up into two distinct parts.

This action occurs under the influence of the zine, which, as it attracts the sulphuric 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

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strument where saline solutions are in contact with the zinc, it is an improvement in any case except the Daniells cell charged with salts instead of acid; and as zinc is used in almost all cells, its properties should be known before preparing any forms of battery.

108. Amalgamation is readily effected by thoroughly cleaning the zinc with a strong acid solution, dropping a little mercury 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 washing with caustic soda, to remove grease. When plates are removed from a battery they should always be washed and brushed before putting away; and then is the best time to apply fresh mercury, if required. The brushings 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 distillation, 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 free The action can be traced by the ordi- again takes place, the molecules making a semi- saline solutions, cast zine is very commonly used. Zn + H SO must revolution, and resuming the position of line 2. In this case, great care should be taken to use nary chemical symbols. sidents first become Zn + SOH,, then Zn It will be seen that this view of the action in- good zinc cuttings, removing any parts with SO4 + Ha. In this case the atoms of hydrogen volves two exertions of force at each stage, first solder on them, and using a little nitre as a flux, are what is called nascent, but they instantly the mechanical semi-revolution of the molecules which will remove a portion of the foreign metals. form a free molecule, taking up and rendering on their axes; and, secondly, the overcoming the A very pure zinc might be removed from spent latent that portion of heat or force necessary to chemical attraction within the molecules; and battery solutions by first neutralising thoroughly convert them into a gas, but before this process this latter also involves two separate actions, the with zinc cuttings, precipitating with carbonate is completed they are in a condition of great acti- actual disruption which occurs only as to one of soda (washing crystals), and then drying and vity, and eager for combination, but as they are molecule of the chain, and the temporary disrup-fusing with powdered charcoal, thoroughly surrounded only by molecules, the nature of tion and reforming of all the other molecules in mixed; but the process would hardly pay. which they would not change-i.e., hydrogen com- each chain. I only indicate this now because it Rolled sheet zinc, from one-sixteenth to a pounds, they are compelled to become free, but presents itself, and because the more clearly these quarter-inch thick, suitable for cylinders and where this complete circuit of molecules capable various principles are seen, the more thoroughly plates, costs from 4d. to 6d. per pound. The of polarisation and discharge is provided, this the subject will be mastered; these various simplest way to cut it to size is to scratch a action is deferred to the last; molecule after actions, however, will have to be studied further groove with a steel point, such as a bradawl, run molecule is decomposed, and the hydrogen is not on, as they constitute together the "resistance" first acid solution, and then mercury along this set free till it reaches a point at which its nascent of liquids and the internal resistance of groove, and allow it to penetrate; then repeat energy is powerless to effect a decomposition, and the process on the other side; when the metal thus in the combination under examination, it is easily broken. Zinc possesses a peculiar pro reaches the copper plate before it becomes free, perty of softening with a moderate heat, so that and does not do so at all if it can help it, for if a hard and brittle as the metal is, it can easily be metallic salt is present at the copper plate, such bent up into small cylinders, if held in front of a as suphate of copper, it displaces the copper, good fire till too hot to handle with the naked which fixes itself in turn upon the superficial hand, and then bent round a piece of wood or molecules of the metallic plate, to which the metal. polarising force is transferred.

105. These two processes furnish us with a natural division of generators or batteries into two classes. 1. Those in which the hydrogen gas is set free. 2. Those in which the hydrogen is not set free, but displaces some other substance, and this latter class 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 porous diaphragm or partition.

batteries.

106. That this condition of polarization or strain 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 the tension which tends to produce it exists exactly as in the cases studied under static electricity. If the two wires are connected to any condensing arrangement, such as Fig. 22, p. 519, Vol. X., the two plates will be found to exhibit electric tension exactly as if they were connected to a machine, only to a degree so feeble that very delicate condensing electroscopes are required to trace it. This indicates the existence of the complete chain, the air or dielectric between the plates of the condenser being polarised; connect the plates by a conductor, and discharge and current are produced.

(To be continued.)

MECHANICAL MOVEMENTS.*
(Continued from page 53.)

(Illustrated on page 76.)

16. The external and internal mutilated cog. wheels work alternately into the pinion, and give slow forward and quick reverse motion.

17 and 18. These are parts of the same moveBefore examining these various forms, it will 107. The force generated by a chemical action ment, which has been used for giving the roller motion in wool-combing machines. The roller to be as well to explain various terms as to which depending on the degree of that action, the which wheel F (18), is secured is required to make there is much confusion in many minds. As generating substance is best which has the one-third a revolution backward, then two-thirds the action commences at the surface of contact of greatest attraction for the radical of the acid, but of a revolution forward, when it must stop until the zine with the acid, the zinc is called the practical considerations limit us to iron and zinc another length of combed fibre is ready for positive metal or element; and hence the order as the cheapest; both, however, have the draw-delivery. This is accomplished by the grooved of polarisation originated there in the liquid is back that they maintain their action whether heart cam, C, D, B, e (17), the stud, A, working in such that the positive or + ends of the molecules we want the force they can give us or no; but the same groove; from C to D it moves the roller are turned from the zinc, and consequently all pure zinc is, however, very slightly acted on, backward, and from D to e it moves it forward, the negative ends, which are the acid radicals, except when the conducting circuit is closed, the motion being transmitted through the catch, This also corresponds while ordinary zinc is continuously dissolved. G to the notch-wheel, F, on the roller-shaft, H. are turned towards it. with the terms of static electricity, and shows the The reason of this difference is by no means When the stud, A, arrives at the point, e, in the wire united to the zinc plate and called its pole, clearly known; though it is usually attributed to cam, a projection at the back of the wheel which in the same electrical condition as the rubber of the presence of foreign metals, setting up little carries the cam strikes the projecting piece on a glass electrical machine - or negative. The local circuits; but it has been discovered that the catch, G, and raises it out of the notch in the current passing through the liquid to the copper common zinc, when amalgamated with mercury, wheel, F, so that, while the stud is travelling in or other collecting plate polarises its molecules is not to be acted on, and this seems to render the cam from e to C, the catch is passing over the with their ends to the liquid, and their or this explanation somewhat doubtful. However, plain surface between the two notches in the positive ends towards its wire. Hence we have a well amalgamated plate is scarcely acted on in wheel, F, without imparting any motion; but the zinc, the positive metal plate, or element, dilute sulphuric acid, but the presence of hydro- when stud, A, arrives at the part, C, the catch has but its wire, the negative or - pole; the copper chloric acid nearly, and of nitric acid, and metallic dropped into another notch, and is again ready to is the negative plate or metal, but the wire pro- salts, entirely does away with the protection, move wheel, F, and roller as required. ceeding from it, the positive or + pole. Fig. 36 which appears to depend chiefly on the adhesion 19, Variable circular motion by crown-wheel shows this, together with one series of the of a film of hydrogen gas to the surface, so pre- and pinion. The crown-wheel is placed eccentrireactions shown in their successive stages. venting contact with the liquid. When the cally to the shaft, therefore the relative radius Line 1 exhibits the arrangement before action, circuit is closed, the hydrogen is transferred to changes. the molecules indifferent, the shaded part here, the negative plate, and the protection is removed 20. The two crank-shafts are parallel in direcand in all future diagrams, representing the + or while the conditions of discharge bring fresh tion, but not in line with each other. The revometallic or basic element or half; the white actions into play. Amalgamation also renders lution of either will communicate motion to the being the or acid half. In line 2 we see the the zinc a better source of electricity, as it is more other with a varying velocity, for the wrist of one molecules polarised under the attraction of the positive than ordinary metal. Hence, though it zine; in line 3 the resulting discharge, the whole is not required for protection in any forms of in

Extracted from a compilation by Mr. H. J. Brown, Editor of the American Artisan.

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cak working in the slot of the other is con- | on its axis during the one revolution of B, and tinually changing its distance from the shaft of the latter.

21. Irregular circular motion imparted to wheel, A. C is an elliptical spur-gear rotating round centre, D, and is the driver. B is a small pini n with teeth of the same pitch, gearing 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 Cevolves 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 a 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 spur-gear moving on an eccentric 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.

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 turaing pinion, d, to the right or left, a similar motion is imparted to wheel, c, 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 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 othersone on the shaft, F. and the other on the same hol'ow shaft with C, which turns loosely on F. The 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 the 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 on B would cause the frame, A, also to receive one revolution, and as this frame carries with it the gear, D, gearing with E, one revolution would be imparted E; but if the gears on the hollow shaft, C, were replaced, D would receive also a revolution

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 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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power as heretofore applied, and to operate the propulsion by a motion of the body of the rider similar to that made use of in riding a horse. The patentee proposes to apply to the cranks connecting rods ascending to pivoted junctions on oscillating levers set on bearings on the socket, through which the stem of the guiding wheel or wheels is passed, and to which socket the main bar, bars, or frame forming the body of the machine are fixed, wrought, or attached. In a bicycle this bar would extend to a fork in order

to contain the trailing wheel, but in tricyc'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 and motion of the body. velocipede by the cranks with the mere weight 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 machine, the stirrup levers work.ng one on each side, and the angles of their cranks may be made to correspoud, if desired, so that both feet inay 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 fare set; g is the saddle, anhh the connecting rods to the three-throw crauk i, and k is the connecting rod from the crank to the stirrup lever f. In Figure 2 the form of this connecting rod is shown consisting of a bracket, through which the inain bar passes, whereby a free vertical motion is afforded to the stirrups or foot bearers fi. The bracket or compound rod k' is capable of adjustment on the lever f by means of the sliding socket and bolts and screw nuts, in crder to set the stirrup bearer further from or nearer to the rider's seat. The bearer is also capable of adjustment in the slot 1, Fig. 1, to suit the rider. The wheels may be made with iron spokes doubly dished, and set in an iron or metal nave or box, as shown, or the spokes and nave may be of wood. m is the

steering handle. On the same principle can also be made a tricycle with a three-throw crank, causing the two side cranks to be worked by duplieate stirrap levers and connecting rods, one on each side of and below the main bar, the stirrup bearer connecting the two transversely; the saddle lever is then connected by one rod to the central crank. The method of working is the 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 consequently 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 bring two sets of connecting rods or levers thereto; one set from the saddle bar or lever, and the other from the stirrup bearers, the operation of working being the same.

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HIS blowpipe consists of two concentric copper cylinders A B FE and MRSN, which are brazed together. A small tube E F T W runs upon the outside of the outer vessel (as

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PLAN

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SELF-PROPELLING VEHICLES.

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by which he propels is automaton carriage are | must undergo in the course of its long revolution, shown in the drawing; they are, A shaft of the while hidden from the sight of the inhabitants of driving spring; B winding up movement; C this little sphere. Were it admissible in a short 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; K connecting rods; L cover of spring barrel to prevent accident should the spring break; R driving spring. The shaft A of the driving spring 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 K K, which in their turn drive the revolving axle H. The pinion on the steering lever J gears 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 NOZZLE carriage. The whole of the mechanism is on the fore carriage, by the driver's srat. The carriage must be provided with an ordinary brake, to be used when required.

THE

HALLEY'S COMET.
(Continued from page 51.)
BY 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 Courdoux, 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 LETTERS PATENT have been pris, for the till that generation should long have passed

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-PROPELLING VEHICLES.

Ferdinand Constant Colney, of Paris, for the invention of "improved mechanism for the propulsion of vehicles." The patentce 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 as this winding may be effected while the carriage is in motion, it causes no interruption of the journey. The pieces composing the improved mechanism

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

The

PARIDELION

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.
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 its 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 are shown in the diagram.
All honour is due to the pro-
found intellect and persevering
industry of those who have 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 con-
sidered the problem, and, briefly, the means that
have been employed to solve it.

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APHELION

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 the successful competitor. Before proceeding to givean account of the method pursued by Baron Damoiseau, it is necessary to say a few words concerning the advance of analytical science 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 sciences, in 1780, and

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