CONVERSATION XIII.

Of the Echo.

FATHER. Let us turn our attention to another curious subject relating to sound, and which depends on the air; I mean the echo.

Emma. I have often been de

lighted to hear my own words repeated, and I once asked Charles how it happened, that if I stood in a particular spot in the garden, and shouted loud, my words were distinctly repeated; whereas if I moved a few yards nearer to the wall I had no an

swer. He told me that he knew nothing more than this, that in a part of Ovid's Metamorphoses, ECHO is represented as having been a nymph of the woods, but that, pining away in love, her voice was all that was left of her.

Charles.

I did; and you shall

hear a translation of the whole pas

sage:

So wondrous are the effects of restless pain, That nothing but her voice and bones remain, Nay, e'en the very bones at last are gone, And metamorphos'd to a thoughtless stone; Yet still the voice does in the wood survive ; The form's departed, but the sound's alive.

Emma. But these lines say no. thing of Echo being a nymph. Charles. Well, then, here are others applied immediately

Echo:

to

A nymph she was, though only now a sound, Yet of her tongue no other use was found, Than now she has; which never could be more Than to repeat what she had heard before.

Father. I doubt this will give your sister but little satisfaction respecting the cause of the echo, which she has often heard, and which she may still hear in the garden.

Emma. No, I cannot conceive why a nymph of the woods should take up her residence in our garden, and the more so as I never saw her.

Father. If she is a mere sound, you cannot see her: I will endeavour to explain the subject. When you throw a pebble into a small pool of water, what happens to the waves when they reach the margin?

Charles. They are thrown back

Father. The same happens with regard to the undulations in the air, which are the cause of sound. They strike against any surface fitted for the purpose, as the side of a house, a brick wall, a hill, or even against trees, and are reflected or beat back. again: this is the cause of an echo.

Emma. I wonder then that we do not hear echoes more frequently. Father. There must be several concurring circumstances before an echo can be produced. For an echo to be heard, the ear must be in the line of reflection.

Charles. I do not know what you mean by the line of reflection.

Father. I cannot always avoid using terms that have not been previously explained. This is an instance. I will, however, explain

VOL. IV.

M

what is meant by the line of incidence, and the line of reflection. When you come to Optics, these subjects will be made very familiar to you. You can play at marbles? Charles. Yes, and so can Emma. Father. It is not a very common amusement for girls; however, as it happens, I shall find my advantage in it, as she will the more readily enter into my explanation.

Suppose you were to shoot a. marble against the wainscot, what would happen?

Charles. That depends on the direction in which I shoot it: if I stand directly opposite to the wainscot, the marble will, if I shoot it strong enough, return to my hand.

Father. The line which the marble describes in going to the wall, is