On the party returning to the library, Mr. Seymour expressed a wish that, before they suspended their morning’s recreations, they should take into consideration a peculiar property of matter, which they had not yet discussed.
“And what may that be?” asked Louisa.
“Elasticity,” replied her father; “and I wish to hear whether Tom can explain to us the meaning of the term.”
Tom very well knew what was meant by Elasticity; but he was like many a merchant with a bill of exchange, who, although well acquainted with its value, has not sufficient small change to cash it. Tom wanted words to enable him to furnish a clear definition; his father, therefore, kindly relieved his embarrassment, by informing him that “it was a property inherent in certain bodies, by which they possessed a disposition to have their form altered by force or pressure, and to recover it on the removal of that pressure, throwing off the striking body with some degree of force: for example,” continued he, “the cane which I hold in my hand can be bent to a certain extent, and then, if I let it go, it will immediately return to its former condition with considerable force.”
Louisa inquired whether bending and pressing upon a body were the same thing. Mr. Seymour replied, that the form of an elastic body might be altered either by compression or distension, and that bending was, in fact, only a combination of these two methods; “For,” said he, “when a straight body, like my cane, is bent, those particles of it which are on the one side are compressed, while those on the other are distended. But let us proceed with the subject. I have said that elastic bodies, on returning to their original form, throw off the striking body with some degree of force. I have here,” continued Mr. Seymour, taking out of his pocket a wooden image of a cat, “a toy which I intend as a gift to John; it will serve to illustrate our subject. The tail, you perceive is movable, one of its ends being tied to a piece of catgut, which is a highly elastic substance. When I bend the tail under the body of the animal, I necessarily twist the string; and by pressing the other end of the wooden tail upon a piece of wax, I can retain it for a few seconds in that situation.”
Mr. Seymour having fixed the tail in the manner above described, placed the wooden image on the ground, when, in a few seconds, it suddenly sprang forward, to the great delight of the younger children.
“Can you explain this action?” asked Mr. Seymour.
“The wax,” answered Tom, “was incapable of holding the end of the tail longer than a few seconds; and as soon as it was let loose, the elasticity of the catgut enabled it to return to its former condition; in doing which the tail struck with force against the ground, which threw off the body of the cat and produced the leap.”
“Very well explained; and you, no doubt, will readily perceive that the operation of steel springs depends upon the same principle of elasticity: a piece of wire or steel, coiled up, may be made to set a machine in motion by the endeavour it makes to unbend itself. This is the principle of the spring in a watch. When our watches are what is termed down, this steel has uncoiled itself; and the operation of winding them up, is nothing more than that of bending it again for action.[(11)] If the elasticity of a body be perfect,” added Mr. Seymour, “it will restore itself with a force equal to that with which it was compressed. As I have given John a toy, it is but fair that I should reward you, Tom: open that box, and examine the gift which it contains.”
Tom received the present from his father, and proceeded to open the lid, when, to his great astonishment, the figure of an old witch suddenly sprang upwards. Mr. Seymour explained its mechanism, by stating “that the figure contained a wire coiled up like a corkscrew, and which, upon the removal of the pressure of the lid which confined it, immediately regained its original form.”[(12)]