Louisa was quite delighted with this simple and satisfactory application of philosophy, and observed, that she should not herself mind a thorough soaking, if it were afterwards rewarded by a scientific discovery.
“I will give you, then, another illustration of the same law of motion,” said Mr. Seymour, “which, instead of explaining an accident, may, perhaps, have the effect of preventing one. If, while you are sitting quietly on your horse, the animal starts forward, you will be in danger of falling off backward; but if, while you are galloping along, it should stop suddenly, you will inevitably be thrown forward over the head of the animal.”
“I clearly perceive,” said Louisa, “that such would be my fate under the circumstances you state.”
“Now, then, my dear children, since our friend the vicar cannot attend us at present, suppose we retire to the library, where I have an interesting experiment to perform, and a new toy ready for your inspection.”
In compliance with their father’s wishes, the children cheerfully returned to the library, when Mr. Seymour presented Louisa with a Bandilor. Most of our readers are, doubtless, acquainted with this elegant toy. It consists of two discs of wood, united to each other by a small axis, upon which a piece of string is affixed. When this string is wound round the axis, and the bandilor is suffered to run down from the hand, the end of the string being held by a loop on the fore finger, its momentum winds up the string again, and thus it will continue for any length of time to descend from, and ascend to, the hand. It affords a good example of the operation of vis inertiæ, or what may, with equal propriety, be termed the momentum of rotatory motion. Its action may be compared to that of a wheel, which, running down a hill, acquires sufficient momentum to carry it up another. There are several toys which owe their operation to the same principle, of which we may particularize the windmill, whose fliers are pulled round by a string affixed to the axis of the sails. In playing with the bandilor, a certain address is required to prevent the sudden check which the toy would otherwise receive when it arrived at the end of the string, and which would necessarily so destroy its momentum as to prevent its winding itself up again. Mr. Seymour now informed his young pupils that he had an experiment to exhibit, which would further illustrate, in a very pleasing manner, the truth of the doctrine of vis inertiæ. He accordingly inverted a wine-glass, and placed a shilling on its foot; and, having pushed it suddenly along the table, the coin flew off, towards the operator, or in a direction opposite to that in which the glass was moving. He then replaced the shilling, and imparted to the glass a less sudden motion; and, when it had acquired sufficient velocity, he checked it, and the coin darted forward, leaving the glass behind it.
Louisa, upon witnessing this experiment, observed that she felt satisfied of the correctness of her father’s statement, when he told her that, if the horse suddenly started forward, when she was at rest, she would be thrown off behind, and that if it should suddenly stop on the gallop, she would be precipitated over its head. The children now arranged themselves around the table, in order to consider several curious toys which Mr. Seymour had collected for the purpose of explaining the nature of the Centre of Gravity.
“But, in the first place,” said Mr. Seymour, “can you tell me, Tom, what is meant by The Centre of Gravity?”
“Its central point,” answered the boy.
“Certainly not; the central point is termed its centre of magnitude, not that of gravity; and it is only when a body is of uniform density, and regular figure, that these centres of magnitude and gravity coincide, or fall in the same spot.”
“I now remember,” cried Tom, “that the centre of gravity is that point, about which all the parts of a body exactly balance each other.”