To apply this also to the ascent of bodies projected directly upwards, with a given velocity.

Again, the moving force of bodies being equal to the mass multiplied into the velocity: How a small body, moving with a great velocity, may produce the same effect as a large body with a small one-as a small shot killing a bird-—a large weight crushing it to death.

Interesting observations of a simple kind might be made on the strength of timber-weights suspended on beams between supports, such as the walls of a building-these coming under the principle of the lever, &c.; also such simple things as the following might be asked: Why is it easier to break a two-foot rule flat-ways than edgeways; and why joists are now always made thin and laid edgeways?-which our forefathers did not understand. Although the reasons are sufficiently simple, very few even amongst the tolerably well educated can give a satisfactory explanation of them. The usual answer, that it breaks more easily because it is thinner, will not do.

Wood, and all fibrous matter, is much stronger in the direction of the fibre than across it, and the strength varies as the square of the dimension in direction of the pressure, multiplied into the dimension transverse to it, when the length breadth x depth 2

is given, or generally as the

length

It is a curious fact, but completely proved by experiment, that hollow tubes are stronger than solid ones of the same quantity of material-how beautiful this provision of nature, as shown in the structure of the bones of animals, more particularly in those of birds and the larger quadrupeds, giving them the greatest strength, and encumbering them with the least possible weight.*

* As a means of testing with accuracy and of forming some definite idea of the strength of the hollow stems of plants, &c., the following simple experiment, which I witnessed, by Professor Cowper, of King's College, London, is very instructive :

He placed a length of one inch of wheat straw in a vertical position in a hole bored in the lower of two parallel boards, held together by a hinge of the same height, one inch, and then brought down the upper board upon it. This he loaded with a load of sixteen pounds, without any appearance of breaking, and stated that he had known a straw bear as much as 35 lbs. placed in this position, before it broke.

NATURAL PHILOSOPHY.

Nature herself seems to give a very instructive hint on this part of education, in the amusements of early childhood. We see a child as soon as it can use its hands, trying to move, or to lift anything which it can, placing it first in one position, then in another, and trying it in all the various ways which its senses admit of-in fact, making a variety of experiments with it, and this is generally looked upon as mere amusement; but children when thus employed, are, as has been observed by Dr. Reid, "acquiring the habits of observation, and by merely indulging an undetermined curiosity, are making themselves acquainted with surrounding objects. If some new effect occurs from any of their little plays, they are eager to repeat it. When a child has for the first time thrown down a spoon from the table, and is pleased with the jingling noise upon the floor, if another or the same is again given to him, he is sure to throw it down, expecting the same noise to occur; but if a piece of wood is given, he very soon finds out that the same effect does not take place, and is no longer anxious to repeat the experiment. So long as the noise goes on, the child has pleasure in repeating it, and if two objects are given, one of which produces a noise when thrown down in this way, and the other not, he very soon finds out the difference, and acts accordingly, and this is, in fact, the method of induction. The child is thoroughly persuaded that a jingling noise is sure to follow his throwing down the spoon, and goes on repeating it until he is tired.

"Such," observes the same philosopher, " is the education of kind Nature, who, from the beginning to the end of our lives, makes the play of her scholars their most instructive lessons, and has implanted in our mind the curiosity and the inductive propensity by which we are enabled and disposed to learn them.'

It is an observation of the late Professor Daniel, in some of his works," that the principles of natural philosophy are the principles of common sense," and from my own experience here in introducing this kind of teaching into the school, I am confident that, with those who have been able to remain to an age to profit from it, it has given an interest in what they are learning, and a kind of practical character to it, which no other teaching could.

I recollect many years ago, going into a school in Germany, and a German gentleman, with whom I was, observed of something they were teaching, "das ist kein practicables ding," that is no practicable thing-the impression made at the time has remained on my mind ever since. We look upon the Germans, in some respects, as a people fond of theories, but at all events this was a sensible remark.

The following hints are intended to show to our schoolmasters, of the class for which this book is intended, the importance of being so far instructed in subjects of this nature, as to be able to point out, in a common-sense way, some of those results in science which bear more immediately on the occupations of life; these will be found not only interesting and instructive to the children while at school, but may be most useful to them after they have left it.

As a class, no doubt at the present day the far greater number of our schoolmasters are not qualified to give this instruction, but there are many, and that number, I hope, increasing, who are ;-to such, although the following pages may not add much to their knowledge, they may perhaps suggest something in the way of imparting it, and in bringing it to bear upon their teaching. They will also point out to others some things with which they may easily make themselves acquainted, and a few simple experiments which are easily tried.

Among the more striking of these things will be such as the following: the elastic and other properties of air—the nature of aeriform fluids-of water-how the pressure of fluid bodies differs from that of solids-how these properties enable man to turn them to useful purposes, such as windmills, watermills, &c.

Civilized man is able to take advantage of these properties, and avail himself of them as motive powers in the business of life; the savage, on the contrary, observes the trees torn up by the winds, stones and rubbish carried down by mountain torrents, but is unable to turn this observation to any useful purpose.*

* Archbishop Whateley, in his Introductory Lectures on Political Economy,' observes: "Many of the commonest arts, which are the most universal among mankind, and which appear the simplest, and require but a very humble degree of intelligence for their exercise, are yet such that

And first of the atmosphere-a sphere of air surrounding the earth-has substance and weight, but is invisible-elastic, can be squeezed into a less space by pressure-expands again when the pressure is removed-expands by heat and contracts by cold. This may be easily made intelligible to them in the following way :

Take a tumbler and invert it—or better, take a jar used for gases, with an air-tight stopper, and placing its mouth horizontally on the surface of the water, in a pneumatic trough, or in any vessel of sufficient depth, having a shelf for support, show them, by letting them feel it, the difficulty of pressing the jar down-it offers resistance-increase the pressure, the air occupies less and less space, but the water inside the glass does not rise so high as on the outside ;-.difference owing to what ?-point out. Diminish the pressure, it again expands, showing its elasticity. Of course the attention of the children must be called to the surface of the water inside and outside the jar.

Take out the stopper, the jar sinks by its own weight, proving clearly that the resistance was offered by the air.

Again, allow the jar to fill with water, put in the stopper, and raise the jar nearly to the surface of the water in the trough-explain why the column of water is supported, and would be supported if the jar were 33 feet high at the ordi

we must suppose various accidents to have occurred, and to have been noted-many observations to have been made and combined—and many experiments to have been tried—in order to their being originally invented.

"And the difficulty must have been much greater, before the invention and the familiar use of writing had enabled each generation to record for the use of the next, not only its discoveries, but its observations and incomplete experiments. It has often occurred to me that the longevity of the antediluvians may have been a special provision to meet this difficulty in those early ages which most needed such help. Even now that writing is in use, a single individual, if he live long enough to follow up a train of experiments, has a great advantage in respect of discoveries over a succession of individuals; because he will recollect, when the occasion arises, many of his former observations, and of the ideas that had occurred to his mind, which, at the time, he had not thought worth recording. But previous to the use of writing, the advantage of being able to combine in one's own person the experience of several centuries, must have been of immense importance; and it was an advantage which the circumstances of the case seemed to require."

nary pressure of the atmosphere-take out the stopper, the water immediately falls;-or while the column of water remains, show how the jar may be filled with air, by carrying down successive tumblers of it until the jar is filled.*

From this the method first used of taking down barrels of air into a diving-bell is easily understood.

Air expands by heat. Experiment: A half-blown bladder placed before the fire, the wrinkles disappear, the air expanding it; remove it, the air again contracts.

Place the same under the receiver of an air-pump, it expands from diminished pressure.

Air has weight. A bottle exhausted of the air is lighter than when full-difference, the weight of a volume of air equal to the contents of the bottle-this means air at the ordinary temperature and pressure of the atmosphere-100 cubic inches dry pure air weigh 31-0117 grains, being for a cubic yard 4 oz.

Drinking through a straw. The teacher, taking a straw and a basin of water, shows them, if the mouth or orifice of the straw is not wholly immersed, or under water, the water will not rise; wholly covered-when they begin to draw out the air the water immediately rises, and why?-What takes place if a hole is made in the straw above the surface of the water? Water does not rise.-What if you plunge it deeper, so that the hole made in the straw is below the surface?-It immediately rises again.-Reasons for all this, which, if they comprehend, they will at once understand the barometer and common pump.

Again, a piece of wet leather with a string attached, called a sucker-press it with the foot against a stone-removes the air between the leather and the stone-leather, say a square piece three inches on a side, ought to support 9 × 15 pounds, only supports, say 80lbs.-reason why? The vacuum not complete. Then take a circular piece, three inches diameter, let them find the area, and calculate how much it ought to support. This is the principle on which a fly is able to walk along a pane of glass, or across the ceiling.

The common syringe. The popgun they are in the habit of making out of a piece of the elder tree-how, by pressing

* Why is it necessary to have a vent-peg in a barrel-or how does it happen that the teapot sometimes will not pour, &c.