cure and read it. It cannot fail to add a great
and additional zest to his contemplation of the
most marvellous object in the celestial vault.
NOTE. It is necessary to observe, with refer-
ence to the first paragraph of this article, on p.
505, 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 :-In 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 " simulta-
neously though."

ELECTRICITY-ITS THEORY, SOURCES,
AND APPLICATION.

BY J. T. SPRAGUE.*

(Continued from page 482.)

MEASUREMENT.-It has been already 155. marked 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 the 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 fcurrent. The reason of this is to be found in sec. 35 and the diagrams Hence if over a magnetic accompanying it. ncedle 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. end 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 it enters 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.

galvanometers) to make its working parts mov-
able around a fixed centre which carries the
needle, as this permits of exact adjustment in the
rod or strong tube is fitted with three branching
true magnetic N. and S. line; to effect this a brass
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 the graduated card and cover, but it will
be steadier if it tapers away from this to the ring.
For ordinary use the 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
line on 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 a 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
61° into 60 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 a
current about half the value of the same deflec-
tion produced by the first four only; several such
series may be useful.

The needle itself

the top of the fixed tube, the point for the need
being in the middle of this plate. Greater d
cacy may be obtained by suspending the
nected with the fixed stand, or from the rin
by a fibre of unspun silk, either from a pointe
self, where this is not made capable of we
but for ordinary purposes the other pa
simplest, particularly if the needle is fisted
an agate centre to diminish friction. Fies
(see next page), will, with this descriptie koe
the construction of the instrument easy, T
so perfect an instrument is not needs, is easy
to adopt only the essential parts, a lar
ing ring around a small needle.

158. The accompanying table will, Ely

11.96

13:07

66 2.2460

141-834

15:09
15'87

16.75

17.70

18.82

21:25

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 made in the battery. Hence the reaction of such a galvanometer is very complicated and can only be thoroughly traced out by an amount of cal157.-It may be well here to make some reculation unsuited to these pages, and of little in-marks as to the needles, applicable to all kinds terest except to pure mathematicians.

of galvanometers. They are usually made far too The practical result is, however, that no de- heavy, and nothing answers better than watchfinite value can be given to the deflections of an spring, softened to cut and form it into shape, ordinary galvanometer by any means except direct and then hardened again. It is light, susceptible measurement. Most people think only of the of the very highest magnetic charge, and easily degrees of the deflection, and suppose a deflection obtained and worked. I have compared the of 60 to be double that of 30°, which is entirely action of this simple needle, lin. long, with the erroneous. There are only two forms of galvano-aluminum extension to 3in., on an ordinary galmeter whose teachings can be at once valued, as vanometer, fitted with a purchased 3in. needle. 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.

It deflects to a higher degree, and 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 156. THE TANGENT GALVANOMETER is of most acquired by its motion. It is, however, not geneconsequence, because the simplest; it is an in- rally known, or at all events little practised in strument which should be possessed by every one instruments made for sale, that these vibrations, who has any desire for real knowledge or accurate which render experiments so tedious, may be observations: I will therefore go fully into the very greatly reduced by placing a stout copper principles and construction of the instrument. plate under and above the needle. The reason is The fundamental principle, however, it is not that a moving magnet generates an electric curnecessary to demonstrate mathematically, it may rent in neighbouring conductors. This both be assumed as a known fact. It is that if a means force absorbed to produce the current, and magnetic needle is placed at the centre of a a reacting tendency to stop the magnet-therecircular electric current, to whose diameter it fore, all galvanometers should be provided with a bears a very small ratio, the tangents of the stout copper plate under the needle, and of someangles of its deflections will be exactly propor- what larger radius, and the graduation may be tional to the quantity of electricity circulating. either on this plate or on a paper pasted on it. The larger the circle and the shorter the needle, Additional effect may be gained by cementing a the more absolutely true this is; however, a needle small circle of copper on the glass cover, if this lin. long in a circle of a foot diameter is correct is flat and close to the needle, or else by fixing for all ordinary purposes; this is the proportion one over the needle, in any convenient manner. of the instrument used in all the experiments in It must be remembered, if a movable arm is emthese papers, and the table and particulars to ployed as before described, and this is of metal, follow will show how nearly true it is, but a circle when it is moved it will for the same reason disof 18in. or more would be better. turb the needle, and tend to pull it after it, and therefore its motion should take place below a stout copper plate. This may be effected by causing it to move freely round the fixed central tube, and fixing the copper plate by a socket on to

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

* "Sigma."

57-2899 3283-
Infinite Infinite

studied until fully understood, furnish a con,
comprehension of the principles of measurer.
&c.
it, because I have never yet met with a wi
I have expended a great deal of labour
which those principles are thoroughly exp»
and put before readers in a practical manne
has already been stated, that in the tangent
vanometer the force of the current is proport
to the tangents of the angles of dete
Column I. is the angles; II. the natural tan

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.

cult, and at 60 one degree is equivalent to three possible. The mirror should be full sized and
of the lower degrees; still, results are obtainable double (concave on one side, plane on the other),
quite near enough for practical purposes. Thus and should be capable of movement in all direc-
at 45°, 317 grains of copper were deposited in tions, as well as of adjustment nearer to or
fifteen hours; this is equal to 6-666 units in ten further from the stage. It is convenient if the
hours, and the table gives 6-537 or 310 grains in mirror be carried by a jointed arm, as a more
oblique illumination may be thus obtained.
the time; a difference of 21 per cent.
(To be continued.)

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

A

ARRANGEMENTS FOR ALTERING THE FOCUS.Every microscope should have a coarse and fine 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 the thickness of an object approximately obtained. The milled heads of all these adjustments should be so placed as to be conveniently accessible, and they must work smoothly or they are utterly worthless.

S the optical principles of the microscope are essentially the same as those of the telescope, and have been so frequently treated upon in the 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 the 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 be either " simple or compound." The former class is commonly repreSECONDLY, THE OPTICAL ARRANGEMENT. - A sented by the single lens of the botanist, but a student's microscope is usually furnished with simple microscope may consist of a combination two eyepieces, called A and B, being, as nearly of several lenses, arranged so as to act as a single as possible, in the following ratios, 1, 2; and one. Of the latter class are "doublets" and with two object-glasses-or, to use henceforward "triplets." The compound microscope neces- the correct technical term-objectives of 1" and sarily consists of at least two lenses, one of which" focus respectively (i. e. these lenses have is called the eye-glass, the other the object-glass, the same magnifying power as simple lenses of 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 amplification than is possible with a simple lens, inasmuch 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 achromatic microscope there are two essential parts-the mechanical and the optical-i.e., the stand and the lenses.

those foci). The range of these powers is about as follows:-55, 90, 210, 350 diameters. These should be 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 Lankester 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 be obtained better at home. English makers will beat the world for quality, and now-thanks to the 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 £15 with two eyepieces. The price of objectives will vary with different makers. A fair English inch may be purchased for £2 10s., and a good quarter for about £3. German lenses (Grundlach, of Berlin) of these foci will not cost more than 17s. 6d. and 21s. respectively, and are about equal in quality. Of course the first-class lenses of our best makers are unequalled by those of any continental maker; but Messrs. Beck's first-class" costs £5 5s.-a sum as large as many can afford to spend on the whole affair. To such we commend the German 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 sufficiently large to admit either edge of a glass slide, 2" in diameter, to be brought under the object-glass." The aperture in the stage should not be less than 14" or 2" in diameter, and the stage should be thin to allow the oblique pencil to be thrown by the mirror upon any object on the stage. The stage may be either simple or mechanical. If the former be chosen, either the "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 it is advisable to screw this down tightly, as otherwise it has an 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 as to be grasped by the thumb and forefinger of the right hand. This stage is extremely useful to the working microscopist, 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 a diaphragm, It will be seen that the values of the deflections which should be furnished with a series of holes, in are approximately proportionate to the degrees up order that a variety of apertures may be availto 15 or 16, then the value of each degree begins able, and the whole arrangement should be to increase more and more, and thus, after about capable of being easily turned aside. Mr. Col- remains truly in the centre of the field of each 30 degrees, accurate observation becomes diffi- lins's " "duated diaphragm" is perhaps the best power, and that there is no "twist'

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., VI., and VII., are corresponding deflections of other galvanometers, and their object will be seen as I describe the other forms of the instrument.

gro

HOW TO CHOOSE A MICROSCOPE.-It will have been seen in what we have said that the essentials of a stand are steadiness in all positions of the body, ample stage room, and proper adaptation for the reception of extra apparatus. The appearance of an instrument is of secondary importance. To test the steadiness of the instrument use the " 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 promptly to the slightest movement of the milled heads in either direction. Use the 1" and 4" objectives, and also a 3", and see that a small object or sideway

"answer

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

microscope. We have but learned to make a
plaything of it, or at best to look at bought slides.
Again, we have but learned the use of a very
simple and unadorned compound microscope.
We will, accordingly, if our readers be not
a-weary, glance at the use of a few accessories,
and then try to learn how to use the microscope
whether in its simplest or most complete form.
H. POCKLINGTON, Hull.

ELEMENTARY SCIENCE.

APPLICATION X.

INCLINED PLANE.
(Continued from page 508.)

the plate X. So delicate is the measurement of the spherometer, that it will indicate the increase of thickness in a plate of glass when examined a second time after having been slightly warmed by the contact of a finger.

SCREW SUB-APP. II.-THE DIVIDING MACHINE. -The micrometer screw is the basis of the invaluable machine by means of which rules, scales, verniers, &c., can be divided most delicately and most exactly. Theoretically, the dividing machine is a very simple apparatus; but practically, as DOW perfected, and especially as adapted for divisa of a curved line, it is considerably complicatel

simple form employed for dividing a straight he
A few words will describe the machine. 4
micrometer screw set in a frame, moves, by
of its nut, either the holder of the "graving
or the table on which rests the rule, &c.,
divided. When one division has been m
the number of degrees on the micrometer "
shows how much the screw should be
before again applying the "graving tool" to
the second division, and so on to the third, fo
nth division. In Fig. 93, AB is a fr

power of 1" should not be less than 30 diameters BY THE REV. E. KERNAN, CLONGOWES COLLEGE. For the present, only the explanation of th 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 radial section of fir. The glandular markings in this should be well shown, and be free from colour with the C eyepiece. For flatness of field the section of an Echinus spine is useful. For definition the pollen of mallow. For "a good test for definition is the scale of the Podura or the frustules of Pleurosigma 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 4-10th" to 1-5th" 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.

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 out 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 having 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 " 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 mirror 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 " is essentially the same, but 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 the mirror. We 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 be obtained from the use of a candle, even if it be protected by a glass shade, and it be not more than 10in. or 15in. 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, we think, gone through our A B C. We have seen what our tool is what are its essentials, and how to put it through its A B C. But that is not learning how to use the

I am indebted for many valuable hints on this matter to Dr. Carpenter's valuable "Microscope and its Revelations," and Professor Beale's "How to Work with the Microscope."

CREW SUB-APP. I.-THE SPHEROMETER.-This
instrument differs in nothing essential from
the micrometer just explained. The original object
was to measure the curves of lenses, hence the
name "Spherometer," given to it by the inventor,
Laroue.
spherical surface, the feet of the instrument were
placed over the lens L, Fig. 91, and the space

To ascertain the curvature of the

FIG. 91

of metal, the top edges planed level. On these edges slides the table T, which holds the measure m, &c., to be divided. To the back of the frame is joined the tool-holder C, which has a horizontal motion by which the tool (a) cuts the divisions, and a perpendicular motion to lift off the tool, when the table must be moved for the next division. When the work is circular, for instance the dividing of a glass tube or the like, the work (the tube, &c.) is moved round under the graving tool. With this simple form of the machine, much is left to the operator,-the number of turns, the short strokes on the measure, the long. the medium strokes, &c. All this is done by the machine itself in the perfected arrangement. which shall be explained in the next section o "parallel forces."

SCREW APP. IV. THE SCREW PROPELLE The moving of ships by what is called a peller" has proved to be so successful an cation of the screw that it must be treat th exceptional attention. Without, however, into too many particulars, it can be fully explained in two points-1st. What the propeller is. How its action is that of a screw. The f point is easily settled-it requires but a plan statement of fact. The second, of itself not at al clear, becomes quite evident by means of a fe principles and practical considerations.

through which the micrometer screw A should be
moved so as just to touch the top of the curve, 1st. The propeller has no appearance of bein!
gave the measure of the curve. The medium of a screw. Hence the early notions of it in the
comparison used was some perfectly plane sur- minds of many were very far from the reality.
face. For a concave lens, the spherometer feet A most learned professor at
conveyed
and screw being made perfectly level on the plane, to his audience the impression that the propeller
and the height
of the micrometer accurately was a long screw, extending from stem to stern
noted from the divided bar A, the amount of of the ship; and exemplified his explanation by
depression of the screw required to just touch
(when the instrument was transferred to the
lens) was indicated by the bar A and the micro-
meter 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 spherometer 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 are placed upon a plane surface (Fig. 92), and under the 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. The screw 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

*All rights reserved.

means of the "Archimedean screw" (water-lifting screw), saying, "it turns, and turns." The real propeller is represented in Fig. 94. A the

centre or "boss." BB projecting arms, the back turning it with one hand and marking it with the to this hump, then bends to the spiracle on the edges of which a a are slanted away from the thumb nail of the other. Having produced a few eighth segment, and turning up again slightly, lane of the paper, which presents an end view. turns, begin again, but at a point on the end of terminates at the anal horns. This stripe forms a side view, Fig. 95, shows the slant. The the cylinder diametrically opposite to the start- boundary of the two shades of ground colour; number of arms varies; there must be two (this ing point of the first spiral. Trace as before. above it the body is whitish, streaked with is a very common sort); there may be 3, 4, 5, or Examination proves that the second spiral no- purplish brown; beneath it all is yellowish green, more. The shape, both as to outline and slant, is where meets the first. In the same way, any except a blotch on the eighth segment of a brown undergoing changes every day. Of these changes other starting point may be taken to begin the hue, which sometimes connects itself with the a good idea may be had from the excellent series spiral, and as to the number of spirals or white stripe, and is sometimes altogether absent. in the ENGLISH MECHANIC, Vol. IX., pp. 51, 102, threads" the strength of the material is the The "leading feature" of this caterpillar's singu&c., by Mr. P. N. Burgh, C.E. only limit. A screw thus formed with 1, 2, &c., larity is, however, not at the head, but at the .. n threads, is called an Archimedean screw, tail, which bears a couple of tubular horns, which as Archimedes is said to have been the first to in- have a bristly surface, and within these are vestigate the properties of a many-threaded contained rose-coloured filaments, which can be screw. To these conclusions one more may be protruded or withdrawn at the will of the cateradded, which having been fully explained need pillar, until it is very near its change into the only be recalled. And this conclusion is that chrysalis, when it ceases to exercise this power. each portion of the screw acts independent of When annoyed the caterpillar of the " Puss" the rest; a small portion will act exactly as the bends these horns forward, and displays the inner whole screw, provided in that portion the "ele- horns, but does not, as far as we have observed, ments" of the plan are not changed. attempt to strike with them. In fact, it is evident that they have no power of wounding, nor (To be continued.) 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

The 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 (or stiff paper) into the form of Fig. 96. 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 a' is to be pushed in, or vice versa. A little delicate manipulation of the arms will give them a graceful slant, which, however, differs from the strict notion of the propeller, inasmuch as the slant of the paper model varies from the boss to the point a a'. In the propeller, strictly speak

SEPTEMBER.

ing, the slant on the "boss" is the same as at CHAPTERS ON CURIOUS CATERPILLARS. a myth. The real defensive apparatus is to be the points. Practically, the paper model mounted on a shaft will convey to the eye an excellent idea of the propeller, and by the knowledge of its deficiency all danger of error is avoided of con

sidering the curves of the paper to represent

exactly the curves of the propeller.

66

2nd. To see 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 consequences," 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. The 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 proportions of a shaft, i.e., as compared to the "breadth of a plane. Thus in Fig. 97

By J. R. S. CLIFFORD.

SE
HOME of our most remarkable native cater-
pillars are comprehended in the family of the
Cuspidates, formerly classed with the Bombyces,
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
vinula) has been recorded in black and white.
Fancy the worthy angler, sitting expectant under
the 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 1 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 the appearance of this devourer of
willow leaves. He is marked 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 the living caterpillar is
to be found through part of September on willow
and poplar. Friend Izaak's description, we must

there is no change made in the screw A by cut-
ing away the cylinder-dotted line-and produc-
ing a broad thread a a a winding round a shaft B.
The inclined plane is not changed, it is only made
wider; none of its "elements" have been affected
in their action. In the same way the "thread
may be increased in breadth to the dimensions of
a "vane" without making any change in the in-
clined plane. The screw 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 modi- remark, applies specially to the full-grown cater-
fication of the pitch can affect the essentials of pillar; when young, it is almost sooty in hue, and
the screw the "elements" of the inclined gradually, through successive changes of skin,
plane are changed (in their direction, &c.), but assumes its more ornamental appearance. On the
not destroyed; their action but not their ex- head, when young, there are two little prominences,
istence is affected. The "pitch," therefore, may which look like ears, and when in a position of
be given according to the required motion. It repose they look remarkably like lilliputian black
follows (3) that two, three, &c. ...n inclined planes cats, hence probably was suggested the vernacular
may be wound round the cylinder without in any name. After the last change of skin we find that
way interfering one with another. This very impor- the head is brown, shading into black at the sides.
tant conclusion, apparently perhaps difficult to un- The second segment forms a sort of recess, into
derstand, is seen by a simple experiment. Hold in which the head can be drawn back; this is pink,
one hand a cylinder of wood, say a pencil or pen- with a spot on each side, which, though eyelike, is
holder, and trace upon it a spiral, commencing not an organ of vision, but merely a surface
from the end of the cylinder. This spiral may mark. Behind this the body slopes upward to
be traced in many ways; it is done very simply the fourth segment, which is decidedly humped.
by holding the cylinder lightly in both hands, A white Stripe passes on each side, from the head

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 of experience, that the caterpillar can not only distance; and we are assured by an entomologist manage to direct this at its pleasure, but also, when annoyed by an inquisitive biped, it takes aim at his eye, as he (the observer) had experienced more than once to his sorrow! Removed from the Puss caterpillar loses this remarkable means its original habitat, and fed up in confinement, 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 Sphingina. The head and the front segments are raised in the air, the centre of the tail and its appendages also elevated. The cocoon body fixed by means of the eight claspers, and the 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.

much less in size, are the species called the Closely connected with the preceding, though Poplar and the Sallow Kittens. This is one of the singular instances which occur amongst Lepidoptera, wherein two species have very different each other as to be with difficulty isolated. Of caterpillars, and yet the moths so nearly resemble 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 the 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 Puss, is crowned with a hump. The anal horns are green, with two brown rings, the 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, the latter rising into a ridge, and giving the caterpillar a singular appearance. The ground colour is white on the back, and an apple green on the sides, striped longitudinally with purple and a deeper green; there are also numerous purplish dots, with a white point in the centre of each; on the second segment are two brownish blotches, and there 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 the 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 (Platypteryx unguicula) is not unfrequent 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. The head is pale brown, the body a darker brown; a pale stripe

one."

function.

and progressive history, so, too, has the vegetable Now, if society, as a whole, has had a continuos kingdom. We find no oak-trees in the prim rocks. In the first strata which succeed thus which the action of fire has fused more or less intor homogeneous mass, it is simple cellular growthconfervæ, sea-weeds, and lichens-which everywhe predominate. After the lapse of ages, ferus, clas mosses, and Equisetæ are found in abundancethat is to say, the strata nearer our own are cla the higher division, Acrogens, which is noticeable. racterized still by flowerless plants, but of these it is

That exuberant vegetation, which formed what is now our coal-bed, 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, &c.

rans from the head to the sixth segment, from is picked up by non-entomologists sometimes and and at the same time more definitely arranged. We which another light brown stripe passes to the taken to naturalists as a supposed rarity. The have, as in the oak tree, a whole, a complet tenth segment. The under-side, legs and claspers, general colour is green or brown, and on each organism, in which the structure of different parts are of a pale green. When of full size, this side there are two yellow lines-the lower one varies considerably, which parts have many dif caterpillar spins a web between two leaves, and including the spiracles or breathing pores. It is ferent qualities, properties, and aspects. We have therein changes to a chrysalis. Of its habits Mr. smooth and flattened, and crawls with great organs, to which are assigned, or which severally Newman observes that when not feeding "it rests rapidity. The chrysalis is buried and enclosed thereby rendering the economy of the plant more perform, certain peculiar duties or in a nearly straight position, but with both ex- in an earthen cocoon. In heathy places, and complex, thus giving to it a fuller, completer lif tremities slightly raised, and not touching the occasionally on the hedgerows, the adult cater than is attained by those whose structure is less inobject on which it may be. When roughly touched, pillar of the Emperor Moth (Saturnia Carpini) is volved. We may, therefore, conclude that "th: or picked off, it very frequently hangs by a now to be discovered. This is one of our largest difference between the higher and lower divisions of thread, and thus suspended, begins twirling British caterpillars, and though not displaying a the vegetable kingdom, as a whole, is in the mar round and round, at first slowly, and afterwards great variety of colours is yet a beautiful creature. analogous to that which distinguishes the mos with great rapidity." A species very partial to The body is delicate green, tapering towards the civilized, or more highly developed social organsa woods is that called the Iron Prominent (Noto-head, the segments showing very distinctly. Each from the earlier, the uncivilized, or less develops. donta dromedarius), the caterpillar feeding on of these is surrounded with a series of pinkish birch, and sometimes on oak. The head is broad, tubercles set in black rings, and giving off short and cleft above; it is shining and brown. Along black bristles. The cocoon, which is composed the back there are five humps, four of which point entirely of silk, has often been figured, and is backwards, the last one only being erect; the constructed with a double trap-door, to secure body is of a yellowish green colour, chequered the chrysalis from disturbance. with purplish brown. The cocoon is formed on Though not by any means of large size, the the ground, and the caterpillar usually attaches caterpillars of the Shark Moths are so conone side of it to a fallen leaf; the chrysalis state spicuously coloured (mostly) as to render them lasting until the following June. Still more re- an easy prey to entomologists-and to birds. markable, from the attitude it assumes in repose, On the "blanket plant," or mullein, we may which gives it an extraordinary appearance, is the probably find a colony of the Mullein Shark caterpillar of the Pebble Prominent, called in (Cucullia Verbasci). These individuals are Latin, Notodonta ziczac, from this zig-zag or greenish white, with a transverse yellow band contorted position; the tail being raised, and on each segment, besides which there eight or also the front segments, which are bent twice. ten large black spots on each; the skin is glossy, The head is large and brown; the body purplish and the head yellowish. On golden-rod feeds brown, with dark patches on the first three seg- the striped caterpillar of C. asteris, preferring the that is to say, Endogens attain to significance in ments, which have light margins; there are also flowers to the leaves. The Chamomile Shark (C. the economy of the growing world; and, finally, the several oblique lines on the sides of several seg-Chamomille) feeds upon different species of that with or immediately preceding the advent of man last geologic epoch-the strata contemporaneous ments, the hinder are yellowish, with brown and allied genera. This caterpillar is pale yellow, is remarkable for the relative preponderance of markings. There are three humps, two of which having light red transverse bands, and an olive-those complex forms which as Exogens cover the point backwards; the one at the extremity of the green line down the back. The commonest cater- face of the earth at present. body is directed forwards. Like some others of pillar of the Sharks is that of C. umbratica, this family the caterpillars of the Pebble Promi- which is of a black hue, marked regularly with nent are frequently attacked by parasitic enemies, orange blotches. This feeds upon the common and a majority of those taken nearly full- sow-thistle, remaining concealed during the day. grown will be found thus affected. The cocoon is slight, being of silk, mixed with a little earth. The Swallow Prominent (Notodonta dictea), which seems generally distributed throughout England, is to be found in the caterpillar state microscope reveals to us, or in the your plate in September. This is usually seen on poplar or sallow, and it endeavours to escape notice by for instance, we have mere aggregations of cells, placing itself on a twig, and drawing the body these cells themselves being to all appearance human race and that of vegetable existence as specks of a structureless, transparent substance. close thereto. There is one hump at the anal such forms, so little is the relation between extremity of the body, marked with a black line the parts, so little boud of union exists between at its top. The head is rather large and pale them, that we scarce know whether to speak of green; the surface of the body is whitish, some- them as individual organisms or not. Each part times a dull green, and sometimes brown; along enjoys a life and activity of its own. A limb of a each side runs a broad and a narrow stripe, the man cut off is a mere dead thing, but a portion of latter being yellowish usually. In some instances one of these has that principle of vitality within it these stripes are entirely wanting. The cocoon is whereby it can exist apart from the community to large, and attached to a dead leaf. 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, and 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.

Many a collector of caterpillars has sought without success for that of the Peach-blossom (Thyatira batis). 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 head is largish, bent downwards, and slightly notched. The cocoon is spun upon the podplant in August or September.

THE PROGRESSION OF LIFE.*

N some of those minute organisms which the

The

But the character of this progression must not be misunderstood. If Exogens flourish to-day, so do vast numbers of individuals and species belonging to lower classes. To say that the nineteenth century is distinguished from preceding ones by the higher character of the civilization which it presents to us, is not to affirm that the lower forms are absent. In like manner to sy Exogens as the highest vegetable organisms characterize the vege

tation of the present does not imply that lower thes do not exist.

The analogy between the progression of the whole holds good also in this, that it is the "relative predominance" of certain types which characterizes an epoch, not the entire absence of lower or higher forms. Thus, in saying that Thallogens. Acroges Conifers, Endogens, &c., successively distinguier strata which disclose to us the order of the we development is not to affirm that structures b many points of resemblance to the highest res they exist at later periods were not coeval z lowest. At the present time the difference is what we term the highest vegetable organz the lowest, as between the civilization of Exand the barbarism of the African tribes, is inure but as we go back in time the difference bet is to say, that the higher forms of both have arsa these two extremes is in both cases lessened-2 by a gradual and insensible evolution from lo

The epochs of the geologist are arbitra The rocks of one strata are not separated in those of another by any gulf or blank interva Between these different divisions of the botsust 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 simpler, less definite structures.

fre

ones. None of those individual cells, or groups of them, are set apart for any specified ends. These are termed Thallogens. Now, in ferns and from the frond, as the stem is from both. mosses this is not the case. The root is different structure is no longer cellular but vascular, no longer indefinite but definite. There are vessels forming organs of nutrition and reproduction, spores and sporecases, &c. They grow only at the summit, hence are termed Acrogens. These two The oak-tree was not always such as it is now great divisions are flowerless. In the higher orders We know how the influences of soil, climate, temOn reeds, in the fen districts of Cambridgeshire, of flowering plants we have a still greater comperature, or cultivation may change the form of a and rather conspicuous in its colouring, we may plexity of structure. The Conifera, or cone-bearing plant until we can scarcely recognize its ancestors detect the caterpillar of the Powdered Wainscot trees-the fir-tribe-are in a measure intermediate in some cases even not at all. So the varied and (Simyra venosa); directly it is touched, however, between flowering and flowerless plants. Their different influences which the world in its developit falls, coiled up in a ring. On each segment of organs of fructification are reduced to the simplest ment has exercised on its vegetation, as a whole, form they can possess, while still maintaining the has moulded and fashioned the individual forms the body there is a circle of warts, from which character which distinguishes the reproductive which compose it; given unto them those more proproceed small brushes of hairs. The head is process in flowering plants." The stem-like pro- difference between existing organisms and those nounced and divergent characters which make the black, with several whitish marks; the body cesses which represent leaves are repetitions of one varies much in colour; there is always a broad grey another, and give the tree a monotonous aspect. forms which have preceded them, and from which stripe along each side, which is really made up of In Endogens, the lower division of flowering they are derived. And now let us turn to the animal kingdom very small black and white spots; down the back plants, the leaves are more varied, though the veins is a broadish black stripe, with a narrow creamare parallel; the parts of the flowers are arranged Here, as among vegetables, we recognize a common coloured one on each side of it; the legs are in threes; the seeds have but one cotyledon; and the plan of structure between vast numbers of indiviglossy, and the claspers greenish and semi-trans- fibres of the wood present to us a simpler arrange-duals, and again, great differences between the parent. When of full size, this caterpillar forms ment than in the higher division. Grass, lilies, and groups into which by such resemblances they are its cocoon with great care, and chops up several palms may be taken as representatives of this sub-classified. The simplest form in which animal life presents In Exogens, such as our oaks, elms, itself is a microscopical speck of "mixed jelly or kingdom. leaves into fragments, connecting them so as to beeches, &c., we have the most complex arrange sarcode, comparable in its properties to white of form a roof-like cover, to secure it from the ment of parts that the vegetable kingdom presents It is scarcely to be termed a structure or an changes of winter: the moth does not come out to us. The seeds have two cotyledons; the leaves until midsummer. In various counties of England are intersected with veins; the different tissues of individual, so indefinite the substance of which it is the caterpillar of the Brown Moth (Hadena Pisi) which they are composed are more distinctly made, so little defined its form from the medium in is now found feeding on broom and other plants; marked; the fibres of the wood are more variously, which it lives. When we come to a creature whose body is a determinate cell, one therefore in which in some years it occurs plentifully on the brake fern about London, and feeding in companies. It in St. Cuthbert's Magazine. From a treatise entitled "Force v. Organization," the parts are more obviously related to one another, and interdependent, there is evidently an advance