twelve. When placed under the microscope, these eggs are seen to be covered with a raised pattern, a number of bold ribs running from top to bottom, while a series of raised transverse lines pass over both ribs and interstices. These eggs may generally be found by examining the leaves of the cabbages in May and August, the butterfly being double-brooded.

The chrysalis is suspended by a thread round the middle to any convenient object, such as a post or wall.

The Smaller White Butterfly, now by far the more abundant of the two insects, is, in one way, even more destructive than the preceding insect, for the larva bores into and devours the very heart of the cabbage, while that of the larger white is contented with the outer leaves. Consequently, it is not so readily detected and destroyed, being concealed from the view of most of its enemies.

To this group also belongs the beautiful Brimstone Butterfly, which is so common throughout the year in most parts of the country.

The next family, that of the Nymphalida, comprises a considerable number of species, comparatively few of which, however, possess special interest to any but an entomologist. Among these exceptions, however, we may mention the insects of the genus Vanessa, which includes the beautiful Peacock and Scarlet Admiral Butterflies, the Tortoiseshell, and some others.

Most of the Vanessas feed, while in the larval state, upon the leaves of the common stinging-nettle, which they roll up in order to form a hiding place where they may be concealed from the view of their enemies. The chrysalis is always suspended by the tail, and is, in all the species of the genus, a very beautiful object, being plentifully adorned with gilded spots, looking like so many patches of gold leaf.

These butterflies emerge from the chrysalis at the beginning of autumn, and pass the winter, in a state of torpor, in some sheltered retreat. Old barns, church towers, hollow trees, etc., are usually resorted to by numbers of hibernating butterflies, which remain motionless until the warmer days of spring tempt them once more to exercise their wings.

But their demeanour is then entirely changed. During the few weeks which intervened between their release from the pupal shell and their retreat to their winter's hiding-place, their occupation was solely that of seeking amusement, or extracting the sweet juices from various honey-bearing flowers. But as soon as spring arrives, they enter upon the serious business of their lives, viz., that of providing for a future progeny, and the female butterflies, worn and battered in their passage through life, may then be seen flitting slowly along the roadsides in search of some convenient spot in which to deposit their eggs. This done, their work is over, and they seldom survive the operation for more than a few hours.

The Scarlet Admiral Butterfly is perhaps the most generally known of these insects, its broad black wings, with their stripes of brightest red, being familiar to almost every resident in the country. It may sometimes be seen in countless numbers, resting upon the blossoms of the ivy-bushes on a sunny morning, in company with others of the same genus. In such a case, the effect of the mass of black and crimson wings, slowly opening and closing as their possessors. regale upon the sweet juices, is almost magical.

One of the families of butterflies, the Erycinide, al

family of the Lycanida. In these all six legs are fully developed in both sexes. In common with the Skipper Butterflies (Hesperida), these do not possess any great general interest, and we will therefore proceed to the second great section of the Lepidoptera, which includes those insects popularly known as Moths.

In these insects there is far more variety in shape and size, at least as far as our British species are concerned, than in the butterflies. Indeed, the antennæ alone of the insects comprising the group are sufficiently diverse in form to have earned the title of Heterocera, i.e., 'varied horned,' for the section.

The first family of these insects, the Sphingida, consists of the large Hawk-moths, which form a very distinct group. The scientific title was derived from the fanciful likeness supposed to exist between the larvæ of these moths and the Egyptian sphinx.

Álthough this resemblance is not very striking to an ordinary eye, the larvæ are yet very interesting creatures, furnishing, on account of their large size, very suitable objects in which to watch the life-history of the Lepidoptera. They nearly all possess upon the upper surface of the twelfth segment of the body a curious curved horn, the object of which is altogether unknown. That it cannot be intended as a weapon either of defence or offence is evident enough, for the substance of which it is composed is not sufficiently firm to allow it to cause the slightest harm, even to the most sensitive skin.

The popular term of Hawk' moth is appropriate enough, for, both in form and flight, these insects strongly resemble the birds after which they are named. With such remarkable speed, indeed, do they pass through the air that the eye cannot follow their movements, the insects seeming to vanish and reappear as if by magic. Most of them are nocturnal in their habits, and may be seen hovering over flowers at dusk.

Our largest British moth, the Death's-head Hawk, is the most prominent member of this family, and attains to a very considerable size, a large specimen being often as much as five inches in expanse of wing. The caterpillar too is of very great size, and is found in potato-fields, that plant constituting its food.

It is, however, often found upon the jessamine, and, as a rule, a caterpillar which is found upon jessamine will not eat potato, and vice versa.

Owing to the curious skull-like mark upon the thorax, from which the insect derives its name, the Death's-head Moth has been, and still is, regarded with superstitious views by the uneducated portion of the community. Its advent is thought to presage death or misfortune, and the insect is consequently regarded with great awe and terror. Its peculiar faculty of uttering a shrill squeak when alarmed is almost invariably considered as a proof of its supernatural attributes.

The method in which this sound is produced has never been satisfactorily cleared up. Some observers have thought that it is caused by the friction of the palpi with the proboscis, others by the movement of the head against the thorax, and so on. Neither of these theories, however, will hold good when we take into consideration the fact that the sound is produced, not by the perfect insect alone, but also by the larva, and, more curious still, the pupa. The modus operandi, therefore, of producing the squeak employed by the insect during the various stages of its existence, has still to be deter

mined.

It is currently reported in the south of France, where the insect is abundant, that the death's-head moth is in the habit of making its way into the bee-hives in order to feast upon the honey contained in the cells.

Want of space permits us to mention one more moth of the hawk tribe only, and we will therefore select the aptlynamed Hummingbird Hawk-moth.

to deceive any but an experienced entomologist. The larvæ are all internal' feeders, in other words, they live inside the roots, stems, twigs, or even the solid wood itself, of various plants and trees.

The next family, the Zeuzerida, is a very small one in this country, consisting of two species only. Both are wood-boring insects while in the larval state, the more familiar of the two being especially interesting.

This is the well-known Goat-moth (Cossus ligniperda), so-called from the odour proceeding from the burrows of the larva, which somewhat resembles the scent of the he-goat. The perfect insect, although common, is not very often seen, being of a very sluggish temperament, and seldom stirring forth until after dark.

The larva feeds upon the solid wood of various trees, such as the oak and the willow, frequently causing great damage by its network of burrows. Its jaws are admirably fitted for their duties, for they are so powerful that scarcely any substance short of iron or tin will withstand their attacks. These caterpillars have even been known to perforate sheet-lead, so it may be easily imagined how rapidly the wood upon. which they feed is cut away by their powerful mandibles.

By a curious provision, the pupa of this insect, as well as that of its congener, is able to travel along its tunnels with considerable speed. The edge of each segment is furnished with a row of little hooks, by the aid of which the pupa can wriggle itself along almost as rapidly as when in the caterpillar state. Just before the perfect insect emerges, the pupa travels in this manner to the mouth of the tunnel, in order that the moth may not be incommoded by want of space.

This insect so closely resembles, both in form and habits, the little bird whose name it bears, that even residents in the tropics, who have been acquainted with humming-birds from their earliest childhood, have been deceived by the wonderful likeness borne to those birds by the hawk-moths. This resemblance is still further increased by the habit of the hawk-moth of hovering over the flower while extracting its juices, the humming-birds themselves feeding in exactly the

same manner.

This is one of the swiftest of the tribe when upon the wing, vanishing and re-appearing with the rapidity of lightning. It is common in most parts of the country, and differs from nearly all the members of the family in that it flies by day instead of by night.

Next to the hawk-moths most authors place the Sesiida, or Clearwings, insects whose wings are only partly covered with scales. One of our leading authorities, however, considers that they are out of place in their present position, and removes them to a totally different group.

The greater number of these clearwing-moths bear so close a resemblance to bees, wasps, gnats, etc., as

The development of this insect extends over a considerable period of time, the larva alone requiring four years in order to complete its growth. The egg from which it springs is deposited by the parent insect deeply in the bark of the tree by means of her long ovipositor, the little caterpillar travelling into the wood immediately it is hatched. From the time it is hatched until it attains the pupal condition, it never seems to cease feeding. Consequently, a tree attacked by these larvæ is so weakened by the combined attacks of the caterpillars, sixty or seventy of which are often found in the same trunk, that it dies as surely as if the woodman's axe had laid it low. So plentiful and widely-spread is the insect, that it is hardly possible to find a row of willow-trees several of which do not exhibit signs of the presence of the destroyer, while many a noble tree is so riddled by the tunnels that scarcely a cubic inch of sound wood is to be found in the trunk.

(To be continued.)

How I Teach Elementary Science.'

BY RICHARD BALCHIN,

Head Master of the Gloucester Road Board School, London. FOURTH-SCHEDULE SUBJECTS:

IN

'MECHANICS.'

N this article I will reproduce one of the lessons upon force, work, energy, and machinery. There seems to be a little vagueness in the use of some of these terms by even our leading men of science. Professor Guthrie, for instance, uses the term 'energy' not quite in the sense that Tyndall uses it. In most of the little text-books read in our schools there is, however, very considerable confusion, and other words are introduced, such as 'power,' 'strength,' etc., to which no very definite meaning is attached. All this tends greatly to the bewilderment of both scholars and teachers. In my lessons I endeavour to keep to the sense in which Grove, Tyndall, and Huxley use them.

Look, boys! I have taken my knife, and am making a point to this piece of chalk. What do you see is happening to the particles of chalk? Ans.-They are falling to the ground. Where were they before they fell? Ans. On to the piece of chalk. What kept them there? Ans. The attraction of cohesion. What made them come away from the stick of chalk? Ans. The knife. The knife? Ans.-Yes, sir. Then what did I do? Ans.-You moved the knife up and down. (A boy)-It was you, sir, and not the knife, that made them come off. (Another boy)-It was the knife, because you could not cut them off, without a knife. (The previous boy)-Yes; but the knife could not cut them off without you. Well, Jones, you seem to be thinking very deeply, what do you say? Ans.-Well, sir, I was thinking that it was not you nor yet the knife that made them come off: it was the power you put into the knife. (A boy)-It was the force of your arm, sir. Quite right, Howell, my boy. Now what do we call that force? Ans.-Muscular force. What was it, therefore, that this muscular force did? Ans.-Moved the knife up and down. And what resulted from that? Ans.-The particles of chalk came off. Then what did the muscular force accomplish in the end? Ans.-Cutting away the chalk. Yes: we will call that the 'work' that was done. Now let us go over this again. What is the work to be done? Ans.-Cutting away the chalk. What is the force employed? Ans.-Muscular force. Now what duty does the knife perform in all this? Answers-It is the thing you use. It is the instrument. It comes between the force and the work. Very good, Smith. (A boy, Jones)-It connects the force applied with the work to be done. That's capital, Jones: is that sentence your own? Ans.-No, sir: I read it in a book. (A boy)-Please, sir, what Jones says is what a machine does. Indeed! is it? Ans. (same boy)Yes, sir: it says, 'A machine connects the force applied with the work to be done.' Well, don't you agree with that? Ans.-No, sir: a knife is not a machine. Indeed! why not? Ans.-Because it has no wheels, and bands, and things of that sort, and it does not go by steam. Well, let us stop a minute or two to see what a machine really is. Here we have a picture of the 'striking mechanism of a clock.' Now tell me what is the work to be done in this case?

What was the force

Ans.-The hammer striking against the bell. And what is the force employed to accomplish this? Ans. -That weight going down. Yes; but what causes the iron weight to go down? Ans.-The attraction of gravitation. Is that a force? Ans.-Yes, sir. Why? Because it causes motion. Well, now, what comes between this force and the hammer? Ans.— All those wheels and things. Exactly so; and what do we call all those wheels? Ans.-The machinery. Here, again, is a picture of what we call the 'mechanical powers.' We shall have a great deal to say about them next year. You see the representation of a man lifting a log of wood? Ans.-Yes, sir. What is the force he uses to do this work? Ans. The strength of his arms. Yes; muscular force. You see he does not put his arms round the log and lift it that way: can you tell me why? Ans. He is not strong enough. Yes; or, in other words, he has not sufficient muscular force. Well, what is he using? Ans.-A bar of iron, a crow-bar. Yes: that is called a 'lever.' Now what is it that connects the force applied, which is muscular force, with the work? Ans.-A lever. What, therefore, is a lever? Ans.-A machine. Yes; it is called a 'simple machine;' and you see, Smith, it has no wheels nor bands, and does not go by steam. Last Saturday I saw at the gas-works, Kent Road, a steam-engine at work, raising immense boxes of gravel and sand from a great depth. (A boy)— Yes, sir: my father works there, and they got up some tremendous tusks and bones of elephants. Yes, I know they were bones of the 'mammoth ;' but we won't go into that now. employed here? Ans.-Steam. (A boy putting up his hand.) Well? Please will you give us a lesson some day about those bones, how they got in the gravel? Yes, I will; but never mind about that now. You say that steam was the force employed. That is only partly right. Ans.-The expansive force of steam. Ah, that is better. But what caused the expansion? Ans.-Heat. Therefore, what was really the force employed? Ans.-Heat. Yes; and you have heard me say that the heat force in the coal came first of all from the sun; but we won't go back so far as that. We will say that the force applied was the expansive force of steam. What is the work to be done? Ans.-Lifting up the gravel. And what connects the force with the work? Ans.-The machinery. Yes. Here, Smith, we have wheels, and bands, and things of that sort. Do you think we could apply this force without anything coming between? No, sir. Now just think. Could we use steam, without the machinery, to send all this gravel to the top? (A boy)-Yes, sir; we could. How? We might put the boiler down the hole underneath a box of gravel, then make the water so hot that the boiler burst, and the gravel and all the other things would all be blown up to the top. (A boy)-Please, sir, that wouldn't do. No: that would be very inconvenient, still it would be getting the gravel to the to. But do you know of any instance where the force is applied without anything coming between? No answer. Well, how do they get the great blocks of stone from the rock in the quarry? Ans.-By blasting. What is blasting? Ans.-A hole is bored in the rock, some gunpowder is put in and then lighted, and the great blocks of stone are blown offay, and Kirkcudand when force is applied in this Luce and Wigtown applied 'direct.' This is, howe

case; generally something connects the force with the work, and then we say the force is applied indirect.'

Have any of you ever seen a water-wheel? I see not many of you have. Well, here is a picture of a water-wheel and a flour-mill. What do you think is going on in that little house? Ans.-Grinding corn. Yes; that is the work. Now what is the force employed here? Ans.-The force of that water coming down the mountain. But what gives the water this force you speak of? Ans.-Coming such a long way down the hill. Then if it did not come such a long way wouldn't it have so much force? Ans.-No, sir. Just explain what you mean. Ans. If you had the mill much higher up the mountain, just where the stream begins, there would be very little force, not enough to turn the wheel. Well? Ans.-And if you took the mill nearer to the bottom of the mountain there would be ever so much more force. (A boy) Please, sir, that would be because there would be ever so much more water. How would there be more water? Ans. Because other streams would join it. (The previous boy)-What would be the use of having ever so much more water if it didn't run? you might have ever so much water at the top, but if it didn't run it would do nothing. Now, Jones, don't get angry let us try to get at the truth about it. You have said there is a force in the water; that the farther the water has run down the hill, and the more there is of it, the greater the force is. Let us think about this. (A boy holding up his hand.) Well? Ans. Please, sir, there is not any force in the water at all. Oh, indeed! then you disagree with all that has been said? Ans.-Yes, sir. It's the force of gravitation acting upon the water and making it come to the bottom: it's the weight of the water. (Another boy)-It is not only because there is more water at the bottom that makes it have more force. What makes you think that? Ans.-Because if there were only the same quantity of water running at the bottom as near the top, it would still have greater force. Why? Ans. Because it has been running down a greater distance. (A boy)-Please, sir, in the last lesson you told us about a hoop running down a hill is this about water the same? But what did I tell you that the hoop running down a hill was an instance of? Ans.-Accelerated velocity. (A boy putting up his hand.) Well? Ans.-Can you have accelerated force? Yes, you can; and that is a very sensible question. But at present I shall say nothing about that we will go into the subject of accelerated force, as you call it, when we have a lesson upon 'momentum.' What is really the force which causes that water to run down the mountain? Ans.-The attraction of gravitation. Yes; that is the force which is applied to do the work of grinding the corn.

Now suppose somebody had a spite against the man at the mill, and so went higher up the mountain, and placed a quantity of stones and earth in the bed of the stream, so as to block it up: what would happen? Answers The water-wheel would stop. The stream would not come to the mill. The miller would come out and see what was the matter. And what would he find was the matter? Ans. That the force had stopped. Indeed! what force? Ans.-The force of the water. But what have we just said that really Ans. The force of gravitation. Then do you mean to say that the force of gravitation had stopped acting? Ans.-No, sir: it is still acting. Upon what?

-

Ans. Upon the water on the other side of the stones. Now I want you to think carefully. Tell me what is the difference between the action of this force now that the stream is blocked up, and before, when it caused the water to turn the mill. (A pause, during which every boy is thinking. Surely a moment of delight in the life of a teacher, whose own psychic force is reflected from every eye.) Answers-The force is acting, but it is not grinding the corn. Before, the force did work now it does nothing. There is more and more force getting in the water on the other side of the wall. (A boy raises his hand.) Well? Ans.If the stones and things were knocked away, and all the water came rushing down, it would do more work than it did before. You think it would? Yes, sir : it would make the wheel go round very fast. (Another boy)-I don't think it would do any work at all: it would wash the mill and the wheel all down to the bottom of the hill. Very well; now I will take one or two of your answers, and see what we can make of them. It was you, Johnson, I think, who said there was more and more force getting in the water on the other side of the stones: what makes you think so? Ans. Because the water is getting higher and higher. (A boy)—The man must keep on bringing more and more stones. Why? Ans.-If he did not, the water would soon have power enough to break down the wall altogether. How did the water get this power that you speak of? Ans.-By the force of gravitation. Now what word would you use when speaking of more and more things collected together, such, for instance, as more and more stones? Ans.-A heap; an accumulation; a collection. Just so. Now what is there an accumulation of in the mass of water? AnsForce. And what is force able to accomplish? Ans. -Work. I will now write on the board a name for such an accumulation of force, or such a store of power to do work-'Energy;' and this is its meaning, 'power to do work.' (A boy)-Please, sir, is it called 'energy' when it does work? Ans.-Yes, certainly. (Same boy)-Then energy is the same as force. (Another boy)-No; it is not the same: energy is a lot of force that has been stored up, and then all coming out. (A boy)-It is energy before it does come out. The fact is we want words to describe all this. It is not quite right to talk about energy coming out;' still you have, I think, the correct idea. If the energy exists, but is not in operation, like the energy in the mass of water still on the other side of the wall, we say it, that is the water, has 'potential energy,' which I will write down on the board; but if we knock down the wall, and allow the energy to operate in performing work, we then say the energy is 'actual.' Now, boys, our lesson has been rather a long one. Just take out your exercise books

and write these definitions :-Force is that which produces or tends to produce motion, or changes or stops motion. Work is the result of the application of force. Machinery is the apparatus for conveniently connecting, and passing on, the force applied with or to the work to be done. Energy is the power to perform work, stored-up force. If the energy is in operation it is termed 'actual energy:' if it exists, but is not in operation, it is called 'potential energy.'