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care and read it. It cannot fail to add a great galvanometers) to moke its working ports mov

аш1 additional zest to his contemplation of the most marvellous object in the celestial vault.

Note.—It is necessary to observe, with reference to the first paragraph of this article, on p. 5U5, that it was written to appear last month— July, but has been delayed through pressure on our space. The reader is further requested to correct the following errata :—lu column 3, page 505, line 15, for " structures " read " structure;" and in the same column, line 31 from the bottom, insert a full stop after the words "simultaneously though."

ELECTRICITY—ITS THEORY, SOURCES,

AND APPLICATION.

Bv J. T. Spbague.*

(Continued from page 482.)

Ш MEASUREMENT.—It has been already • remarked that there are three modes of ascertaining the force of a galvanic current—viz., by its magnetic, its chemical, and its heating powers ; of these the first is the most convenient, because it interferes very little with tho actual passage or work of the current, and is open to inspection at any instant, and thus gives full information as to any fluctuations which may occur. Instruments for this purpose are called galvanometers. They are based on the principle that a magnetic needle tends to place itself at right angles with a galvanic [current. The reason of this is to be found in sec. 35 and the diagrams accompanying it. Hence if over a magnetic needle at rest and in the same direction, we place a wire, and through it pass a current entering at the southern end, the needle turns with the N. cud to the left, or westerly; if the wire be below the needle it turns to the right. If the direction of the current be reversed, that is, if ¡tenters at the N. end, the actions are reversed. At present we need deal only with this as a fact, leaving the reasons for after consideration.

If the wire makes a turn round the length of the needle, it is evident all these conditions come into play at once, for the current entering at S. and passing above the needle, when the wire turns to the lower side, the current passes from the N., hence both the actions are the same and the needle is deflected to the left with double force; each turn has the same effect, varied only by altered distance and position, provided the same "quantity " is passed, which of course will not be the case if the wire is lengthened and no change luade in the battery. Hence the reaction of such a galvanometer is very complicated and can only be thoroughly traced out by an amount of calculation uusuited to these pages, and of little interest except to pure mathematicians.

The practical result is, however, that no definite value can be given to the deflections of an ordinary galvanometer by any means except direct measurement. Most people think only of the degrees of the deflection, and suppose a deflection of 60' to be double that of ¡K)°, which is entirely erroneous. There are only two forms of galvanometer whose teachings can be at once valued, as they are related in one to the tangent and in the other to the sine of the angles of deflection, but even this is only relative, and one galvanometer can only be compared with another by an actual measurement.

156. The Tangent Galvanometeb is of most consequence, because the simplest; it is an in strument which should be possessed by every one л\ ho has any desire for real knowledge or accurate observations: I will therefore go fully into the principles and construction of the instrument. The fundamental principie, however, it is not necessary to demonstrate mathematically,—it may be assumed as a known fact. It is that if a magnetic needle is placed at the centre of a circular electric current, to whose diameter it bears a very small ratio, the tangents of the angles of its deflections will be exactly proportional to the quantity of electricity circulating. The larger the circle and the shorter tho needle, the more absolutely true this ¡в ; however, a needle lin. long in a circle of a foot diameter is correct for all ordinary purposes; this is the proportion of the instrument used iu all the experiments in these papers, and the table and particulars to follow will show how nearly true it is, but a circle of 18in. or more would be better.

The instrument may be solidly built on its stand, but it is far better (as, indeed, it is with all

able around a fixed centre which curries the
needle, us this permits of exact adjustment in the
true magnetic N. and S. line; to effect this a brass
rod or strong tube is fitted with three brauehing
feet, each having a screw at its extremity for
levelling; over this slides a brass tube, tight
enough to be steady, but able to move freely, and
provided with a collar and set screw to hold it
when adjusted ; on this tube the instrument itself
is framed. First there is fixed to it at right
angles a table of wood or brass, a little lower and
proportionately shorter than the diameter of the
intended ring. The shape of this is of no great
moment, so that it is large enough in the middle
to carry tho graduated card and cover, but it will
be steadier if it tapers away from this to the ring.
For ordinary use tho simple card fixed to the
table, with its zero in the true middle line, will be
sufficient; but more accurate results will be ob-
tained by using a graduated circle of much larger
radius, and having a movable arm marked with
a line down the middle and its extremity adjusted
to the graduated circle; by bringing this arm
exactly beneath the needle its position is more
readily ascertained, the larger circle facilitates
the reading, and if the edge is properly graduated
the reading may be effected to minutes, or tenths
of a degree. This latter is effected by marking on
the arm u space from the middle line equal to 11°
on the circle, and dividing this into ten equal
parts, the one of these which is nearest to a degree
division indicates the measure required; the divi-
sion into minutes is similarly effected by dividing
(51' into CO equal spaces.

The ring may consist of a single band or rod of
copper for powerful currents, or a flat band or
stout wire may make several turns; both may be
united in one instrument by leading off connecting
wires, provided with connecting screws, for use,
as desired. My own instrument is composed of
four turns of No. 13 insulated wire, and to this
all the measurements refer, but a further series
of four similar turns is added for feebler currents,
and the deflections caused by these represent а
current about half the value of the same deflec-
tion produced by the first four only; several such
series may be useful.

From the middle of the fixed central tube rises the point on which the needle is to be placed just in the centre of the ring. The needle itself should be lin. long, but provided with a very light wire or sheet metal for on indicator, which will lengthen it to 2 J in. or 3in. For this purpose the best thing to use is aluminum, because it is the lightest metal, and is very stiff, and may be obtained in sheets of extreme thinness and rigidity.

157.—It may be well hero to make some remarks as to the needles, applicable to all kinds of galvanometers. They are usually mado far too heavy, aud nothing answers better than watchspring, softened to cut and form it into shape, and then hardened again. It is light, susceptible of the very highest magnetic charge, and easily obtained and worked. I have compared the action of this simple needle, lin. long, with the aluminum extension to 'Л'ш., on an ordinary galvanometer, fitted with a purchased 3in. needle. It deflects to a higher degree, aud swings to its point of rest in less than half the time, evidently because it has so much less momentum to exhaust, for of course a heavy needle first resists motion, and then has to exhaust the force it has acquired by its motion. It is, however, not generally known, or at all events little practised in instruments made for sale, that these vibrations, which render experiments so tedious, may be very greatly reduced by placing a stout copper plate under and abo\ e the needle. The reason is that a moving magnet generates an electric current in neighbouring conductors. This both means force absorbed to produce the current, and a reacting tendency to stop the magnet—therefore, all galvanometers should be provided with a stout copper plate under the needle, and of somewhat larger radius, and the graduation may be either on this plate or on a paper pasted on it. Additional effect may be gained by cementing a small circle of copper on the glass cover, if this is flat and close to the needle, or else by fixing one over the needle, in any convenient manner. It must be remembered, if a movable arm is employed as before described, and this is of metal, when it is moved it will for the same reason dis turlj the need!

the top of the fixed tobe, thejpoint for the needbbeing in the middle of tliis plate. Greater deli cacy may be obtained by suspending the needLby a fibre of unspun silk, either from a point con nected with the fixed stand, or from the ring itself, where this is not made capable of motion; but for ordinary purposes the other plan i* simplest, particularly if the needle is fitted irith an agate centre to diminish friction. Fig- 4:i (see next page), will, with this description, render the construction of the instrument easy, aud if so perfect an instrument is not needed, it is easy to adopt only the essential parts, a large conducting ring around a small needle.

158. The accompanying table will, if carefully

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etudied until fully understood, furnish a complot»? comprehension of the principles of measurement. Ac I have expended a great deal of labour up. ч it, because I have never yet met with a work in which those principles are thoroughly exploin<>,l and tend to pull it after it, audi and put before readers in a practical manner. It therefore its motion ehould take place be low a has already been stated, that in the tangent galstout copper plate. This may be effected by vnnometer the force of the current is proportional causing it to move freely round the fixed central to tho tangents of the angles of deflectior. tube, and fixing the copper pinto by a socket on to I Column I. is the angles; II. the natural tangents,

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ments of my own, but are given in order to explain principles. Column IV. gives the value of each degree of my tangent galvanometer, and therefore of every experiment recorded in these papers in absolute value of work. The principle on which I base the unit will be examined hereafter. It is, however, the unit I have used throughout—viz., the chemical equivalent, but as the measurement of "current" requires time to be taken into account, I make ten hours the basis; shifting the decimal point one figure to the left of course converts it into one hour. The way this column is calculated is this, and it is applicable to every tangent instrument :—I carried out several experiments by depositing copper at different rates and noting the degree shown by the instrument. Each experiment continued ten hours, or was reduced to that time, and the weight of copper divided by 31-7 (its unit), gave the number of units corresponding to the degree; for instance, at 16°, 1-749 units were deposited; dividing the ratio of this degree in Column II., 16-4318, we get 9-395 as the tangential value of unit, which, by inspection, is seen to be about one 9-3 degrees.

Exact accuracy is scarcely attainable, as an error even of a hair's breadth in reading the angle affects the result, but the average of several trials at different degrees coming close to this, I took the angular value of one unit as 9-4, and dividing Column III. by this gives the figures of Column IV., the value in units of each degree. For any kind of work we have now only to multiply these figures by the proper unit, and we know what is actually doing. For instance, in electroplating, the instrument marks 25°, the value is 2-843, the unit of silver 108, gives us 30-7 grains per hour being deposited. If magnetic work is being done, it is only needed to ascertain once for all the work the particular magnet does at any degree, to ascertain a unit or coefficient for that magnet applicable to all other cases. Columns V., W., and VII., are corresponding deflections of other galvanometers, and their object will be seen as I describe the other forms of the instrument.

It will be seen that the values of the deflections are approximately proportionate to the degrees up to 15 or 16, then the value of each degree begins to increase more and more, and thus, after about 30 degrees, accurate observation becomes diffi

eult, and at 60 one degree is equivalent to three of the lower degrees; still, results are obtainable quite near enough for practical purposes. Thus at 45°, 317 grains of copper were deposited in fifteen hours ; this is equal to fi-666 units in ten hours, and the table gives 6-537 or 310 grains in the time; a difference of 2 i per cent.

(To be continued.)

THE MICROSCOPE. —HOW TO CHOOSE AND HOW TO USE IT.

AS the optical principles of the microscope are essentially the same as those of the telescope, and have been so frequently treated upon in tho pages of this journal, it would be unwise to occupy further space than by saying that a microscope is simply a means by which the eye of tho observer is removed further from the object observed, whilst the telescope carries the eye nearer to the object in its field of view. The microscope may bo either "simple" or "com pound." The former class is commonly repre sented by the single lens of the botanist, but a simple microscope may consist of a combination of several lenses, arranged so as to act as a single one. Of the latter class are "doublets" and "triplets." The compound microscope neces sarily consists of at least two lenses, one of which is called the eye-glass, the other the object-glass the two or more lenses being usually connected by a tube of metal. By the use of the compound microscope we obtain a much greater amplifica tion than is possible with a simple lens, inas much as the eye receives a magnification of the image formed by the object-glass, and not the image itself. A microscope of this kind may be very easily made by any one possessing the least mechanical ingenuity, but when made will be useless. Every object viewed by its aid will be seen to be surrounded by a "beautiful " coloured fringe, and to be terribly distorted. These several defects, known as chromatic and spherical aberrations, were for a long time insuperable obstacles in the way of microscopic progress; but, thanks to ceaseless effort on the part of the fathers of our science, we may now say that our instrument is about as perfect as can be desired; and that the tale it tells is in the main true and faithful. It is the compound achromatic microscope, of which we intend to speak in these papers. To this aohromatic microscope there are two essential parts—the mechanical and the optical—i.e., the stand and the lenses.

We now come naturally enough to the enquiry— What constitutes a good microscope? Certain things must be essential. What are they? 1st, as regards the stand. This must be solid, heavy, so that it may be free from vibration, and well balanced. It must be capable of being placed in either a vertical, an inclined, or a horizontal position, and of remaining there without being clamped. "The stage should be su Bleien tly large to admit either edge of a gises slide, 2" in diameter, to be brought under the object-glass." Tho aperture in the stage should not be less than 11" or 2" in diameter, and the stage should be thin to allow the oblique pencil to bo thrown by the mirror upon any object on the stage. The stage may be either simple or mechanical. If the former be n either tho "magnetic stage," the "lever stage," or the "concentric rotating" stage will be found useful. The plan adopted by Messrs. Beck is useful and exceedingly simple, but with high powers is slightly tantalizing, as the focus is disturbed by every movement of the stage, which is merely a thin plate of metal held down by a double spring, the pressure of which may be regulated by a screw (in practice itTis advisable to screw this down tightly, as otherwise it has au awkward knack of flying in one's face, to the serious detriment of one's nerve, and possibly of the object), and this plate is doubled under the stage on one side, so ав to be grasped by the thumb and forefinger of the right hand. This stage is extremely useful to the working microscopiet, and after some years of use we are disposed to speak very highly of it. There is nothing to get out of order, and practised fingers will perform all needful movements quite as delicately as would be possible with the most elaborate "mechanical" stage. Below the stage should be fixed я diaphragm, which should be furnished with a series of holes, in order that a variety of apertures may bo available, and tho whole arrangement should be capable of being easily turned aside. Mr. Collins's "graduated diaphragm" is perhaps the best

possible. The mirror should be full sized and double (concave on one side, plane on the other), and should be capable of movement in all directions, as well as of adjustment nearer to or further from the stage. It is convenient if the mirror be carried by a jointed arm, as a more oblique illumination may be thus obtained.

Arrangements Fou Altering The Focus.— Every microscope should have a coarse and fino adjustment. The former may be obtained by a rack-and-pinion movement, by a chain and pulleys, or by a watch-spring band. The chain movement is peculiarly smooth and easy, and in practised hands entirely obviates the necessity for a fine movement. This latter is usually obtained by the action of a finely cut screw on a lever. The screw may be graduated so that the distance through which the object-glass passes may be measured and tho thickness of an object approximately obtained. The milled heads of all these adjustments should be so placed as to be conveniently accessible, and they mimt work smoothly or they are utterly worthless.

Secondly, The Optical Arrangement. — A student's microscope is usually furnished with two eyepieces, called A and B, being, as nearly as possible, in the following ratios, 1, 2: and with two objcct-glas6es—or, to nso henceforward the correct technical term—objective* of 1" and |" focus respectively (i. e. these lenses have tho same magnifying power as simple lenses of those foci). The range of these powers is about as follows:—55. 90, 210, 350 diameters. Theso should bo accurately centered and be perfectly corrected. Means of estimating the quality of these lenses shall be given later. A stage condenser and a stand, or bull's-eye condenser, for opaque illumination, will complete the instrument.

The next question that arises is, where shall we go for our instrument? Mr. E. Ray Lnnkester has lately written in praise of foreign instruments; but we do not see what there is to be gained by going abroad for that which may bo obtained better at home. English makers will beat the world for quality, and now—thanks to tho Society of Arts and some of our more enterprising manufacturers,—a really good English instrument may be obtained at about the same price as the continental ones. There is hardly any comparison between the convenience of the two classes of instruments. These remarks apply only to the stands.

In lenses, the Germans surpass us by far. Price being taken into account, although within the last year our English makers have contrived to turn out very decent lenses at less than half the prices formerly charged. We need only instance Crouch's or Swift's |" and Mr. Wheeler's \". We will, therefore, look at home for our stand. Where all are equally good it is a difficult (not to say an inviduous) task to instance the best. Those who wish a better-finished class of workmanship may either select their higher priced stands, or look over the catalogues of half a hundred makers and make their choice. The better plan is to select a good stand, capable of being increased as funds are available, and to add objectives and accessories from time to time. Such a stand will cost about £10 or £16 with two eyepieces. The price of objectives will vary with different makers. A fair English inch may bo purchased for £2 10s., and a good quarter for about £3. German lenses (Grnndlach, of Berlin) of these foci will not cost more than IN. 6d. and 21s. respectively, and are about equal in quality. Of course the firtt-class lenses of our best makers are unequalled by those of any continental maker; but Messrs. Beck's first-claBS \" costs £5 5b.—a sum as large as many can afford to spend on the whole affair. To euch we commend the German lenses.

Howto Choose A Microscope.—It will have been seen in what we have said that the essentials of a stand arc steadiness in all positions of the body, ample stage room, and proper adaptation for tho reception of extra apparatus. The appearance of an instrument is of secondary importance. To test the steadiness of the instrument use the J" power, focus carefully, and get some one to walk sharply round the room whilst you observe an object. If there be excessive tremor, reject the instrument at once. Next, try the adjustments, and see that they work smoothly and without loss of time "—i.e., that they "answer" promptly to the slightest movement of the milled heads in either direction. Use the 1" and J" objectives, and also а 3", and see that a small object remains truly in the centre of the field of each power, and that there is no "twist" or sideway movement on altering the focus. So far for the mechanical portion of the instrument. We will add that a short-bodied microscope having a draw-tube for elongation when increased power is desired, is much to be recommended, on the ground that it is far easier to work with. The corrections of the lenses must be carefully tested, and unless the tyro go to a good and well-reputed maker, we would advise him to get some experienced friend to select his lenses for him. The lenses of even the best makers vary considerably, so that it is possible for an experienced man to select a far better lens than might fall to his lot. The power of 1" should not be less than 30 diameters with the A eyepiece. It should give a large, clear field, free from colour, and with a clean, sharp, circular margin. For chromatic aberration the severest test is said to be a racial section of fir. The glandular markings in this should be well shown, and be free from colonr with the C eyepiece* For flatness of field the section of an Echinus spine is useful. For definition the pollen of mallow. For J" a good test for definition is the scale of the Podura or the j'riutules of Pleurositjma hippocampus. The markings on either of these should be clearly resolved. Dr. Carpenter specially recommends Mr. Lealand's preparations of muscular fibre as giving a fair test for lenses of from l-10th" to l-5th" focus. Every objective should be tested with a series of eyepieces, as a glass will often perform well with a shallow eyepiece, when a deeper one will render manifest the most atrocious defects. centre or " boss." B B projecting arms, the back raises of which a a are slanted away from the plaue of the paper, which presents an end view. A xidc view, Fig. 95, shows the sliiut. The number of arms varies; there must be two (this is 11 very common sort); there may bo 3, 4, 5, or more. The shape, both as to outline and slant, is undergoing changes every day. Of these changes a good idea may be had from the excellent series in tho English Mechanic, Vol. IX., pp. 51, 102, &c. by Mr. P. N. Burgh, C.B.

Having selected our instrument, we will proceed to use it. Before us lie slides of Echinus, of Foraminifera (mounted as opaques), of Diatoms, and the eye of a fly. Having taken our instrument oat of its case and put it in order (the maker of each instrument will put the purchaser " in the way of doing this ''), we will select a table having a good light. If in the daytime, we will avoid direct sunlight as having too much glare, and select a position in which we can receive light from a white cloud. The microscope should be placed in an inclined position, and the mirror adjusted so as to throw, an equable light upon the slide, neither too intense, or too much the reverse. Careful use of the diaphragm apertures and focussing of the mirror will give us any variety. We now take our slide of Echinus, and baring an inch objective on, place the slide on the stage and in the field. We run down the rack motion until the objective is brought within j" of the slide, and then, with our eye to the eyepiece, focus back until we obtain a clear definition. Having turned aside the diaphragm, we proceed to tilt the minor into different positions, in order to get various degrees of obliquity of the illuminating pencil. We will substitute a slide of Foraminifera for the Echinus spine, and proceed to examine it as an opaque. Having closed the aperture of the diaphragm, we throw a good light on the object by means of the bull'seye, varying its angle of obliquity until we gain the best effect. The working of the J" is essentially the same, bnt unless we have the aid of accessories, "transparent" objects alone can be used with it, and greater care must be paid to the focussing, &c, of tho mirror. Wo cannot urge too strongly upon our readers the importance of paying special attention to this vital, but, to the beginner, seemingly unimportant, matter of illumination, as truthful interpretation almost entirely depends upon it. The best possible light for microscoping is daylight from a white cloud, but as most of our readers (the author amongst the number) are compelled to work almost solely by night, it is encouraging to know that good results may bo obtained from the use of a candle, even if it be protected by a glass shade, and it be not more than lOin. or 15iu. from the instrument. The author has used for some years a small paraffine lamp, costing at the outset about eighteen pence, and has found it to answer every purpose, and to be most convenient, inasmuch as it permits a vast variety of "dodges " in illumination to be tried with little trouble. And here let us whisper to the readers of the Mechanic, " beware of opticians, unless you have a long purse: make what you want when possible—that is nearly always."

We have now, wo think, gone through our A B C. We have seen what our tool is— wuut arc its essentials, and bow to put it through its A B C. Bnt that is not learning howto use the

* I ura indebted for ninny valuable hints on this matter to Dr. Carpenter's vuinahle "Microscope and its Revektionff," and ProfeSBor Beale's " How to Work with the Microscope."

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through which the micrometer screw A should be moved Bo as just to touch the top of the curve, gave the measure of the curve. The medium of comparison used was some perfectly plane surface. For a concave lens, the spherometcr feet and screw being made perfectly level on the plane, and the height of the micrometer accurately noted from the divided bar A, the amount of depression of the screw required to just touch (when the instrument was transferred to the lenB) was indicated by the bar A and the micrometer plate.

For convex lenses, the method was reversed. First the lens was touched by screw and feet, then the height to which the screw had been raised measured upon the plane. Once the height of the curve was known a simple formula supplied to the optician any other "element of the lens he might desire. At present, the spherometcr is made with three feet, and is chiefly used to know or compare the thickness of very delicate plates, or else to examine very minute effects of expansion or contraction in the same body from various causes. For these purposes, the feet arc placed upon a plane surface (Fig. 92), and under tho screw is laid the plate, &c, to be measured. The screw is turned down until the point just touches the plate X, and the height of the divided circle C on the index A is exactly noted. Tinscrew is raised a little to allow the removal of the plate X. and then again turned down until the point touches the plane; the difference of position indicated on A shows the thickness of

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the plate X. So delicate is the measurement el the spherometcr, that it will indicate the inert*.-* of thickness in a plate of glass when examined . second time after having been slightly warms! by the contact of a finger.

Screw Siui-app. II.—The DrviDrac MicunrE. —The micrometer screw is the basis of the invaluable machine by means of which rules. *alr;, verniers, ifcc, can be divided most delicately ad most exactly. Theoretically, the dividing michi^ is a very simple apparatus; but practically, asajt perfected, and especially aB adapted for divi&a of a curved line, it is considerably complictd. For the present, only the explanation of tisimple form employed for dividing a straight it A few words will describe the machine. J micrometer screw set in a frame, nioves. bytsi of its nut, either the holder of the '■ graving1* or the table on which rests the rule, &c,. > divided. When one division has been ret. the number of degrees on the micrometer "pa shows how much the screw should be & before again applying the "graving tool" toa the second division, and so onto the third, fuc. nth division. In Fig. 93, AB is a L-s

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of metal, the top edges planed level. On them edges slides tin- table T. which fields the measure ra, V <■., to be divided. To the back at the frame a joined the tool-holder C, which has a horizontal motion by which the tool (a) cats the div and a perpendicular motion to lift off the tool, when tho table must be movtA tar the vrext division. When the work is circr.VnT,for instance the dividing of a glass tube or the like, the wort (the tube, etc.) is moved round under the gravuiE tool. With this simple form of the machine, much is left to the operator,—the number o! turns, the short strokes on the measure, the lorn the medinm strokes, &c. All this is done by tit machine itself in the perfected arrangcnin:' which shall be explained in the next section a "parallel forces."

Screw App. IV.—TnE Screw PropeluiThe moving of ships by what is caUed a -p peller" has proved to be so successful as at cation of the screw that it must be treat exceptional attention. Without, howevei.tsi into too many particulars, it can be fully exptt-l in two points—1st. What the propeller is. H I low its action is that of a screw. The fa point is easily settled—it requires bnt a pUi statement of fact. The second, of itself not at si clear, becomes quite evident by means of a It I principles and practical considerations.

1st. The propeller has no appearance of hcit: a screw. Hence the early notions of it in xh minds of many were very far from the realitr.

A most learned professor at —• noxrreTtd

to his audience the impression that the propeller w«s n long screw, extending from stem to stem of tho ship; and exemplified his explanation bv

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Tho general form of the propeller, not so easily shown in a diagram, is very clearly represented by a. cardboard model. Cut a bit of cardboard for stiff paper) into the form of Fig. 00. A is the "boss." B B the vanes. Now bend away from their plane the points a and a—as the arrows indicate, or the reverse—taking care that the bend is in opposite directions; if a be brought out «' is to be pushed in, or rice vena. A little d elicate manipulation of the arms will give them a graceful slant, which, however, diners from the strict notion of tho propeller, inasmuch as the slant of the paper model varies from the boss to tThe point a a'. In the propeller, strictly speakLug, the slant on the " boss" is the same as at tho points. Practically, the paper model mounted on a shaft will convey to llic eye an excellent idea of tho propeller, and by the knowledge of its deficiency all danger of error is avoided of considering the enrves of the paper to represent exactly the curves of the propeller.

2nd. To Bee distinctly the screw action of the propeller it is only necessary to recall some of the essentials of a screw, with some of their necessary " consoquences," and apply these consequences to the correction of defects which at once appear in an ordinary screw used to produce quick motion, especially in water. Tho essentials of a screw are, that it is an inclined plane, the "height" of which is the "pitch "of the screw. Hence it follows (1) that the cylinder on which the plane is wound may be reduced to have only the proportJMiB of a shaft, i.e., as compared to the " brcadtBj of a plane. ThuB in Fig. 07

turning it with one hand and marking it with the thumb nail of the other. Having produced a few turns, begin again, but at a point on the end of tho cylinder diametrically opposite to the starting point of the first spiral. Trace as before. Examination proves that the second spiral nowhere meets the first. In the same way, any other starting point may bo taken to begin the spiral, and as to the number of spirals or "threads" tho strength of the material is the only limit. A screw thus formed with 1, 2, ifcc, . . . n threads, is called an Archimedean screw, as Archimedes is said to have been the first to investigate the properties of a many-threaded screw. To these conclusions one more may be added, which having been fully explained need only be recalled. And this conclusion is that each portion of the screw acts independent of the rest; a small portion will act exactly as the whole screw, provided in that portion tho "elements " of the plan are not changed.

(To be tontinued.)

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there is no change made in the screw A by cuting away the cylinder—dotted line—and producing a broad thread a a a windinground a shaftB. The inclined plaue is not changed, it is only made wider: none of its " elements" have been affected in their action. In the samowaythe "thread" may be increased in breadth to the dimensions of a " vane " without making any change in the inclined plane. The strew A, Fig. 98, in its form a a a a remains the same screw in its form b b', except that the plane (the thread) is wider in the latter form. It follows (2) that a body joined to a screw moving in a fixed nut will advance in a straight line (at each' turn of the screw) through a space equal to the height of the inclined plane, or "pitch " of the screw. The greater the pitch, therefore, the greater the advance. No modification of the pitch can affect tho essentials of the screw: the "elements" of tho inclined plane are changed (in their direction, See.), but not destroyed; their action but not their existence is affected. The "pitch," therefore, may be given according to the required motion. It follows (3) that two, three, tto.... H inclined planes may be wound round the cylinder without in anyway interfering one with another. This verv important conclusion, apparently perhaps difficult to understand, is seen by a simple experiment. Hold in one hand a cylinder of wood, say a pencil or penholder, and trace upon it a spiral, commencing from the end of tho cylinder. This spiral may be traced in many ways; it is done very simply by holding the cylinder lightly in both hands,

CHAPTERS ON CURIOUS CATERPILLARS.

SEPTEMBER.

Bv J. R. S. Clifford.

OOME of our most remarkable native catertO pillars are comprehended in the family of tho C'nspidatcs, formerly classed with the llombyces, which in several particulars they nearly resemble. The grotesque forms of several are exceedingly unlike what we are accustomed to regard as the ordinary type of caterpillar, and from these moths are produced which are not notable for any marked singularity of appearance. Since the days of quaint old Izaak, of piscatorial memory, the history of that abundant species by river or streamlet, known as the Puss Moth (Dicranura vinuia) has been recorded in black and white. Fancy the worthy angler, sitting expectant under tho branch of a drooping willow, with one eye on his float, and the other surveying the general aspect of nature, of which he was an ardent lover; a twig comes within his i each, and glancing at it, he sees to his astonishment a caterpillar of a totally different kind to any he has before observed. Home he goes, his day's sport done, and amongst other things he does not forget to jot down the outlines of tho appearance of this devourer of willow leaves. "He is markod with a St. Andrew's cross, his lips and mouth are somewhat yellow, his eyes black as jet, his forehead purple, his feet and hinder parts green, his tail forked and black, and his whole body stained with red spots." Not a scientific description certainly; yet the reader can surmise from the accompanying figure that it is tolerably correct, or, if he chooses, can compare for himself, for tho living caterpillar is to be found through part of September on willow and poplar. Friend Izaak's description, we must

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remark, applies specially to the full-grown caterpillar; when young, it is almost Booty in hue, and gradually, through successive changes of skin assumes its more ornamental appearance. On the head, when young, there are two little prominences, which look like ears, and when in a position of repose they look remarkably like lilliputian black cats, hence probably was suggested the vernacular name. After the last change of skin we find that the head is brown, shading into black at the sides. Tho second segment forms a sort of recess, into which the head can be drawn back; this is pink, with a spot on each side, which, though eyelike, is not an organ of vision, but merely a surface mark. Behind this the body slopes upward to tho fourth segment, which is decidedly humped. A white gtripe Pa8SCS on eftCB Bi&e, from the head

to this hump, then bends to the spiracle on the eighth segment, and turning up again slightly, terminates at the anal horns. This stripe forms a boundary of the two shades of ground colour; above it the body is whitish, streaked with purplish brown j beneath it all is yellowish green, except a blotch on the eighth segment of a brown hue, which sometimes connects itself with the white stripe, and is sometimes altogether absent. The "leading feature " of this caterpillar's singularity is, however, not at the head, bnt at tho tail, which bears a couple of tubular horns, which have a bristly surface, and within these are contained rose-coloured filaments, which can be 2>rotruded or withdrawn at the will of the caterpillar, until it is very near its change into the chrysalis, when it ceases to exercise this power. When annoyed the caterpillar of the "Puss" bends these horns forward, and displays the inner horns, but does not, as far as we have observed, attempt to strike with them. In fact, it is evident that they have no power of wounding, nor is any fluid propelled from them; and the statement that the caterpillar, by means of them, drives off the parasites which molest it, is probably a myth. The real defensive apparatus is to be found beneath the head. There is a small transverse slit, only noticeable in the full-grown caterpillar, from which, when provoked, it can squirt a pungent fluid of an acid nature to some distance; and we aro assured by an entomologist of experience, that the caterpillar can not only manage to direct this at its pleasure, but also, when annoyed by an iuqnisitive biped, it takes aim at his eye, as he (the observer) had experienced more than once to his sorrow! Removed from its original habitat, and fed up in confinement, the Puss caterpillar loses this remarkable means of defence, and when touched this slit will be seen to quiver, yet no fluid issues from it. The position assumed by the caterpillar when "standing at. ease " is not unlike what is so frequently noticed amongst the Sphinyina. The head and the front segments are raised in the air, the centre of the body fixed by means of the eight claspers, and the tail and its appendages also elevated. The cocoon which is constructed by this creature for its winter abode is composed of a gummy substance, mixed with portions of the bark of some tree, and it so nearly resembles the trunk on which it is formed, as often to escape notice.

Closely connected with tho preceding, though much less in size, are tho species called tho Poplar and the Sallow Kittens. This is one of the singular instances which occur amongst Lepidoptera, wherein two species have very different caterpillars, and yet the moths so nearly resemble each other as to be with difficulty isolated. Of the two the Poplar Kitten (Dicranura bifida) is rather the larger, and occurs pretty generally in England and Ireland. The females seem to select shrubby plants upon which to deposit their eggs. The caterpillars remain at rest during the day, spinning a silken pad on tho leaf, to which they attach themselves very firmly. The colour of the body is various shades of brown, grey, and yellow, the latter running into green on the fourth segment, which, as in the Pnss, is crowned with a hump. The anal horns are green, with two brown lings, tho inner horns or filaments being black. The Sallow Kitten (Dicranura furcula) appears of late years to have become more scarce. This is still handsomer, and has the second and third segments flattened, tho latter rising into a ridge, and giving the caterpillar a singular appearance. The ground colonr is white on the back, and an apple green on the sides, striped longitudinally with purplo and a deeper green; there are also numerous purplish dots, with a white point in the centre of each; on tho second segment are two brownish blotches, and thero are faint orange patches on the seventh and eighth segments. The anal horns are white, tinged with purple, the head is a pearly grey. This caterpillar, when not feeding, is fond of retiring to some withered leaf, and thus is occasionally missed by the collector. Both tho Kittens are to be found in the caterpillar stage throughout the summer, but rarely later than the beginning of September.

The caterpillar of the Barred Hooktip (Platyptcrux unyuicula) is not uufrequent on pollard beeches at this time, occurring in various places near London, and in different parts of the south of England, but not in the north. The head is slightly notched, and the crown of it rises above the level of the segment behind it, the body tapers towards the extremity, and terminates in a point; it is studded with small warts. Tho head is pale brown, tho body a darker brown; a pale stripe runs from the head to tho sixth segment, from which another light brown stripe passes to the tenth sequent. The uniier-siile, legs aud claspers, are of a pale green. When oí full size, this caterpillar "spins a web between two leaves, and therein changes to a chrysalis. Of its habits Mr. Newman observes that when not feeding " it rests in a nearly straight position, but with both extremities slightly raised, and not touching the objeet on which it maybe. When roughly touched, or picked off, it very frequently hangs by a thread, and thus suspended, begins twirling round and round, at first slowly, aud afterwards with great rapidity." A* species very partial to woods is that colled the Iron Prominent (Solodonta dromedariiu), the caterpillar feeding on birch, and sometimes on oak. The head is broad, and cleft above; it is shining and brown. Along the back there are five humps, four of which point backwards, the last one only being erect; the body is of a yellowish green colour, chequered with purplish brown. The cocoon is formed on the ground, and the caterpillar usually attaches one side of il to a fallen leaf; the chrysalis state lasting until tho following June. Still more remarkable, from the attitude it assumes in repose, which gives it an extraordinary .appearance, is the caterpillar of the Pebble Prominent, called iu Latin, Notodtmta ziczac, from this zig-zag or contorted position; the tail being raised, and also the front segments, which are bent twice. The head is large aud brown; the body purplish brown, with dark patches on the first three segments, which have light margins ; there aro also several oblique lines on the sides of several segments, the hinder are yellowish, with brown inarkinge. There are three humps, two of which point backwards ; tho one at the extremity of the body is directed forwards. Like some others of this family the caterpillars ef the Pebble Prominent are frequently attacked by parasitic enemies, and a majority of those taken nearly fullgrown will be found thus affected. The cocoon is slight, being of silk, mixed with a little earth. The Swallow Prominent (Notodonta dictica), which seems generally distributed throughout England, is to be found in tho caterpillar state in September. This is usually seen on poplar or sallow, and it endeavours to escape notice by placing itself on a twig, and drawing the body close thereto. There is one hump at the anal extremity of the body, marked with a black line at its top. The head is rather large and pale green; the surface of the body is whitish, sometimes a dull green, and sometimes brown; along each side runs a broad and a narrow stripe, the latter being yellowish usually. In some instances these stripes are entirely wanting. The cocoon is large, and attached to a dead leaf.

Many a collector of catoqiillars hae sought without success for that of the Peach-blossom (Thijalira batie). It feeds on the common bramble, a plant not convenient for beating into the net or umbrella, and not very agreeable for a hand search. Like certain Prominent caterpillars, the Peach-blossom in that state rests with both extremities slightly raised. It has also humps, which sometimes vary in number. The most conspicuous one is on the third segment, cleft in the centre, and bending forwards over the head; the others are smaller, and that on the twelfth segment is reduced to a point; the general colour' is reddish brown, chequered with grey. The bead is largish, bent downwards, and slightly notched. The cocoon is spun upon the podplant in August or September.

On reeds, in the fen districts of Cambridgeshire, and rather conspicuous in its colouring, we may detect the caterpillar of the Powdered Wainscot (¡iimyra venosa); directly it is touched, however, it fails, coiled up in a ring. On each segment of the body there is a circle of warts, from which proceed small brushes of hairs. The head is black, with several whitish marks; the body variesmuch in colour; there is always a broad grey stripe along each side, which is really made up of very small black and white spots; down tho back is a broadish black stripe, with a narrow creamcoloured one on each side of it; the legs are glossy, and the claspers greenish and semi-transparent. When of full size, this caterpillar forms its cocoon with great care, and chops up several leaves into fragments, connecting them so as to form a roof-like cover, to secure it from the changes of winter: the moth does not come out until midsummer. In various counties of England the caterpillar of the Brown Moth (¡ladena Pin) is now found feeding on broom and other plants; in some years it occurs plentifully on the brake fern about London, and feeding in companies. It

is picked up by non-entomologists sometimes and taken to naturalists as a supposed rarity. The general colour is green or brown, and on each side there are two yellow lines—the lower one including the spiracles or breathing pores. It is smooth and flattened, and crawls with great rapidity. Tho chrysalis is buried and enclosed in an earthen cocoon. Iu heathy places, and occasionally on the hedgerows, the adult caterpillar of the Emperor Moth {Saturnia Carpini) is now to be discovered. This is one of our largest British caterpillars, and though not displaying а great variety of colours is yet a beautiful creature. The body is delicate green, tapering towards the head, the segments showing very distinctly. Each of these is surrounded with a series of pinkish tubercles set iu black rings, and giving off short black bristles. The cocoon, which is composed entirely of silk, has often been figured, and is constructed with a double trap-door, to secure the chrysalis from disturbance.

Thongh not by any means of large 6ize, the caterpillars of the Shark Moths are so conspicuously coloured (mostly) as to render them an easy prey to entomologists—and to birds. On the "blanket plant," or mullein, we may probably find a colony of the Mullein Shark (Cucullia Verband). These individuals are greenish white, with a transverse yellow band on each segment, besides which there eight or ten large black spots on each; the skin is glossy, and the head yellowish. On golden-rod feeds the striped caterpillar of C. a»teris, preferring the flowers to the leaves. The Chamomile Shark (C. Chamomilla) feeds upon different species of that and allied genera. This caterpillar is pale yellow, having light red transverse bands, and an olivegreen line down the back. The commonest caterpillar of the Sharks is that of C. umbrática, which is of a black huo, marked regularly with orange blotches. This feeds upon the common sow-thistle, remaining concealed during the day.

THE PBOG11ESSION OF LITE.«

IN some of those minnte organisms which the microscope reveals to us, as in the yeast plant, for instance, we have mere aggregations of cells, these cells themselves being to all appearance specks of a structureless, transparent substance. In such forms, so little is the relation between the parts, so little bond of union exists between them, that we scarce know whether to speak of them as individual organisms or not. Each part enjoys a life and activity of its own. A limb of a man cut otf is a mere dead thing, but a portion of one of these has that principle of vitality within it whereby it can exist apart from the community to which it is joined. Advancing another step we come to organisms in which these cells become more an integral part of the whole; but in which, nevertheless, there is a great similarity between different portions. Thus in liverworts, mushrooms, lichens, aud sea-weeds, the structure of the leaf, stem, and root are all the same; that is to say, portions performing different functions in the economy of the plant are little differentiated from any or all of the other parts. None of those individual cells, or groups of them, are set apart for any specified ends. These are termed Thallogens. Now, in ferns and mosses this is not the case. The root is different from the froud, as the stem is from both. The structure is no longer cellular but vascular, no longer indefinite but definite. There are vessels forming organs of nutrition and reproduction, spores and eporecases, &c. They grow only at the summit, hence are termed Acrogens. These two great divisions arc flowerless. In the higher orders of flowering plants we bave a still greater complexity of structure. The Coniferic, or cone-bearing trees—the fir-tribe—are in a measure intermediate between flowering and flowerless plants. "Their organs of fructification are reduced to the simplest form they can possess, while still maintaining the character which distinguishes the reproductive process in flowering plants." The stem-like processes which represent leaves are repetitions of one another, and give the tree a monotonous aspect. In Endógena, the lower division of flowering plants, the leaves are more varied, though the veins are parallel; the parts of the flowers are arranged in threes; the seeds have but one cotyledon; and the fibres of the wood present to ns a simpler arrangement than in the higher division. Grass, blies, and jviluis may be taken as representatives of this subkiugdom. Iu Exogene, such as our oaks, elms, beeches, Ac, we have the most complex arrangement of parta that the vegetable kingdom presents to us. The seeds have two cotyledons: the leaves are intersected with veins; the different tissues of which they are composed are more distinctly marked; the fibres of the wood are more variously,

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and at the same time more definitely arranged. We have, as in the oak tree, a whole, a complete organism, in which the structure of different parts varies considerably, winch parts have many different qualities, properties, aud aspects. We hove organs, to which are assigned, or which severaUy perform, certain peculiar duties or functieiis, thereby rendering the economy of the plant more complex, thus giving to it a fuller, completer life than is attained by those whose structure is less involved. We may, therefore, conclude that "the difference between the higher and lower divisions of the vegetable kingdom, as a whole, is in the main analogous to that which distinguishes the more civilized, or more highly developed social organism from the earlier, the uncivilized, or less developed one."

Now, if society, as a whole, has had a continuous and progressive history, so, too, has the vegetable kingdom. We find no oak-trees in the primary rocks. In the first strata which succeed those which the action of fire has fused more or less into a homogeneous mass, it is simple cellular growth— confervie, sea-weeds, and hchens—which everywhere predominate. After the lapse of ages, ferns, clubmosses, and Equiseto; are found in abundance— that is to say, the strata nearer our own are characterized still by flowerless plants, but of these it is the higher division, Acrogens, which is noticeable.

That exuberant vegetation, which formed what is now our coal-bod. is made up nearly entirely of plants allied to our conifers, cycads, yews, and the like, which, as we noticed, formed a link between flowerless and flowering plants. Later epochs still saw the preponderance of grasses, lilies, palms, Ac —that is to say, Endogens attain to significance in the economy of the growing world; and. finally, the last geologic epoch—the strata contemporaneous with or immediately preceding the advent of man —is remarkable for the relative preponderance of those complex forms which as Exogens cover the face of the earth at present.

But the character of this progression must not bo misunderstood. If Exogens flourish to-day, so do vast numbers of individuals and species niongins to lower classes. To say that the nineteenth century is distinguished from preceding ones by the lúgher character of the civilization which it presents to ns, is not to affirm thajthe lower forms are absent. In like manner to say Exogeus as the highest vegetable organisms characterize the vegetation of the present does not imply that lower ones do not exist.

The analogy between the progression of the human race and that of vegetable existence as a whole holds good also in tins, that it is the "relative predominance" of certain types which characterizes an epoch, not the entire absence of lower or higher forms. Tim«, in saying that Thallogens, Acrogens, Conifers, Endogen-, &c, successively distinguish the strata which disclose to »- the order of the world's development ianot to uni mi that structures having mauv points of resemblance to the highest types as they exist at later periods were not coeval with the lowest. At-the present time the difference between what we term the highest vegetable organism and the lowest, as between the civilization of England and the barbarism of the African tribes, is immense; but as we gobaak in time the difference between these two extremes is in both caaes lessened—that is to say, that the higher forms of both have arisen by a gradual and insensible evolution from lower ones. The epochs of the geologist are arbitrary. The rocks of one strata are not separated from those of another by any gulf or blank interval. Between these different divisions of the botanist there are many forms which cannot be accurately placed in any" category. These classes, families, &c, of plants, with all their special characteristics, have arisen by an insensible gradation from simpler, less definite structures.

The oak-tree ivas not always such as it is now. We know how the influences of soil, climate, temperature, or cultivation may change the form of a plant until we can scarcely recognize its ancestors— in some cases even not at all. So the varied and different influences which the world in its development has exercised on its vegetation, as a whole, has moulded and fashioned the individual forms which compose it; given unto them those more pronounced and divergent characters which make the difference between existing organisms and those forme which have preceded them, and from which thev are derived.

And now let us turn to the animal kingdom Here, as among vegetables, we recognize a common plan of structure between vast numbers of individuals, and again, great differences between the groups into which by such resemblances they are classified.

Tho simplest form iu winch animal life presents itself is a microscopical speck of "mixed jelly or sarcode, comparable in its properties to white of egg.'* It is scarcely to be termed a structure er an individual, so indefinite the substance of which it is made, so little defined its form from the medium in which it lives. When wee «me to a creature whose body is a determinate cell, one therefore in which the parts are more obviously related to one another, and interdependent, there is evidently an advance

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