LECTURE X.

MEANS AND METHODS IN ELEMENTARY PHYSICS.

B

BY PROF. I. J. OSBUN.

EFORE beginning to teach the experimental sciences, the teacher ought to realize that there is a science of experi

ment, per se.

Ingenuity in devising apparatus to illustrate the principles of natural philosophy and chemistry is by no means its fundamental principle, though I am inclined to think that many judge this to be the case. It is in reality a minor qualification. Skill in the use of apparatus, home-made or other, which enables the teacher to present in their most striking aspect the principles involved, is certainly to be desired, but it is far from being the most desirable power for which to seek. To be able to invent apparatus and to use it skilfully, and this alone, is to be able to present physics and chemistry as merely artificial, and not as natural sciences." I dare say that not a few persons who have been

taught natural philosophy by means of experiment would find, upon reflection, an impression that "if Atwood had not invented his machine, we had not known that falling bodies move with a varying velocity." As a matter of fact, we all know this so early and so well that we should call that boy stupid indeed who would stand under a tall appletree while another shook apples from its limbs; and we should say the same of the boy in the tree were he to let himself fall from one of the high branches, though he might drop without hurt from a branch near the ground, while an apple might fall from a limb a foot above the first boy's head and not cause pain. It is true the boy has never thought the matter over in the following

manner:

An apple falling upon my head gives pain by the force of its blow. The force of its blow is momentum. Now, the elements of momentum are velocity and weight. An apple falling from a low limb causes little pain. An apple falling from a high limb causes more pain, because of greater momentum. Since the apples may have the same weight, the increased momentum must be due to increased velocity. Therefore, an apple falling from a limb has a greater velocity the farther it falls.

If a teacher were at hand, he might lead the boy through this mental process. He should first direct the boy's powers of observation to those phenomena

in the falling apples that were pertinent to the desired end, and then lead his mind to the necessary inference.

The construction of a beautiful machine, with its perfect electrical escapement and annunciator, reveals a model of inventive ingenuity. Accurate results in the experiments with the delicate apparatus, such as inspire a feeling of perfect confidence in the teacher and the machine, are a sign of skill in the experimenter. But after all, the laws obtained from the machine must suffer the painful process of translation into Nature's g of a body falling in an artificial vacuum,—a kind of process that is very undesirable in elementary teaching.

Nature's great principles are at work immediately about us, in the commonest phenomena that we observe; and he alone is the great teacher by experiment who has the ability to present common, every-day things in such a way that the phenomena observed shall lead to a knowledge of the beautiful principles involved.

Clerk Maxwell, in his excellent treatise on the "Theory of Heat," presents the student with the picture of a teaspoon plunged into a cup of hot tea. By a careful consideration of the facts which one may observe at the tea-table, he develops ideas of some of the principles underlying the theory of heat.

Faraday, in his beautiful lecture to a juvenile

audience at the Royal Institution, gives expression to thoughts like the following: Having occasion to make use of a little rubber bag filled with

air, - an object from which he desires to get much knowledge, he calls attention to the little balloon and says, "This is a very beautiful thing, although it is very common (most beautiful things are common)." Again, in illustrating the centre of gravity, he performs the very common experiment of balancing a cork on a pointed stick with the aid of two pointed sticks stuck into the sides of the cork; and says, "Do not refer to your toybooks and say, I have seen that before. Answer me, rather, if I ask you, Have you understood it before?"

Tyndall, addressing an audience of older and wiser heads at the same great institution, would have them look at a sheet of snow and ice and water as it slides down a roof and bends down over the eaves, on a sunny afternoon of a winter's day. Studying this phenomenon, he would lead them to a perfect knowledge of regelation and the way in which an Alpine glacier moves and bends down through a crooked mountain gorge. Such is the method of model teachers.

Another decidedly necessary qualification for teaching by experiment is ability to make much. from little. Fill a common wide-mouthed bottle with peas, then add sand, and finally pour in warm

water until it overflows. Set the bottle aside for a few hours. Go to it at the end of that time, and you will find the strong bottle burst into many fragments. We infer that the peas must have swollen. This is a simple experiment, a very plain observation, and an inference very easily drawn. But let us reconsider. We go back to a lesson where we learned about the constitution of matter in its three forms. We remember that it is made up of very small parts called molecules, and that these molecules do not touch each other.

We return to the broken bottle, and we see that in the swelling of the peas the molecules must have separated by their own effort: this thought leads us to the beautiful principle of spontaneous molecular motion. Regard the broken bottle again: it was very strong. Moving molecules broke it : they did it with their momentum; they must have moved with great force. But small bodies, to move with great force, must move with great velocity; and we then add to the idea of spontaneous molecular motion the idea of immense velocity. again bodies moving with a great velocity, in the course of an hour will move through a great distance. Look at the bottle and the peas: none of the molecules have moved outside the limits of each swollen pea; the little molecules must have gone forward and then back. But to accomplish the great distance through which their enormous

Think