“If the depth of the well be considerable, such a plan will not answer the purpose, since, in that case, there must necessarily be a perceptible interval between the fall of the stone and the sound produced by it, as you have just seen exemplified by the woodman, which, unless taken into account,(4) will vitiate the result.” Tom observed that he had not thought of that difficulty, and did not know how he could get over it. His father told him, that he must look at the surface of the water, and mark the moment it was disturbed by the stone. “Now, Mr. Twaddleton,” exclaimed Mr. Seymour, “are you ready to count the seconds?” “Quite ready.” “Then drop the stone.” “One,--two,--three,--four--” “There,” said Tom, “it touched the water.” “And there, there,” cried several voices, “what a noise it made!” “_Facilis descensus Averni_,” exclaimed the vicar; “the stone descended in four seconds.” “Now, my boy, make your calculation.” Mr. Seymour furnished pencil and paper, and Tom proceeded;--“_Sixteen_ feet for the first second,--I put that down.”-- “Well,” said his father, “and _three_ times _sixteen_ for the second?” “_Forty-eight_,” cried Tom.-- “Put it down.” “_Five times sixteen_, for the third?” “_Eighty._”-- “Down with it.” “And _seven times sixteen_, for the fourth?” “_One hundred and twelve._” “Now, cast up these numbers,” said Mr. Seymour. “_Two hundred and fifty-six feet_,” cried Tom, “is the depth of the well.” A shout of delight, from the whole juvenile party, announced the satisfaction which they felt at the success of their first experiment in NATURAL PHILOSOPHY. Louisa observed, that she could not distinguish any interval between the actual contact of the stone with the water and the sound which it produced. “At so small a distance as two hundred and fifty-six feet,” said her father, “the interval could not have exceeded in duration the fourth part of a second, and was, consequently, imperceptible: we might therefore, in the present instance, have accepted the sound as a signal of the stone’s arrival at the water, without prejudice to the result of the experiment.” Mr. Seymour told his son, that the method which he had pursued was unobjectionable when the experiment did not extend beyond a few seconds: but that, if a case occurred in which a greater space of time were consumed, he would find his plan tedious: “Now,” continued he, “I will give you a general rule that will enable you to obtain the answer in a shorter time without the details of addition. ‘_The spaces described by a falling body increase as the squares of the times increase._’ I conclude that you already know that the _square_ of a number is the sum obtained by multiplying the number into itself.” “Certainly,” answered Tom; “the square of 4 is 16; that of 3, 9, and so on.”